____ _ _
| _ \| |_| |__
| |_) | __| '_ \ ``Only those who attempt
| __/| |_| | | | the absurd can achieve
|_| \__|_| |_| the impossible.''
Pth is a very portable POSIX/ANSI-C based library for Unix platforms which
provides non-preemptive priority-based scheduling for multiple threads of
execution (aka `multithreading') inside event-driven applications. All threads
run in the same address space of the application process, but each thread has
its own individual program counter, run-time stack, signal mask and `errno'
variable.
The thread scheduling itself is done in a cooperative way, i.e., the threads
are managed and dispatched by a priority- and event-driven non-preemptive
scheduler. The intention is that this way both better portability and run-time
performance is achieved than with preemptive scheduling. The event facility
allows threads to wait until various types of internal and external events
occur, including pending I/O on file descriptors, asynchronous signals,
elapsed timers, pending I/O on message ports, thread and process termination,
and even results of customized callback functions.
Pth also provides an optional emulation API for POSIX.1c threads
(`Pthreads') which can be used for backward compatibility to existing
multithreaded applications. See Pth's pthread(3) manual page for
details.
Threading Background
When programming event-driven applications, usually servers, lots of
regular jobs and one-shot requests have to be processed in parallel.
To efficiently simulate this parallel processing on uniprocessor
machines, we use `multitasking' --- that is, we have the application
ask the operating system to spawn multiple instances of itself. On
Unix, typically the kernel implements multitasking in a preemptive and
priority-based way through heavy-weight processes spawned with fork(2).
These processes usually do not share a common address space. Instead
they are clearly separated from each other, and are created by direct
cloning a process address space (although modern kernels use memory
segment mapping and copy-on-write semantics to avoid unnecessary copying
of physical memory).
The drawbacks are obvious: Sharing data between the processes is
complicated, and can usually only be done efficiently through shared
memory (but which itself is not very portable). Synchronization is
complicated because of the preemptive nature of the Unix scheduler
(one has to use atomic locks, etc). The machine's resources can be
exhausted very quickly when the server application has to serve too many
long-running requests (heavy-weight processes cost memory). And when
each request spawns a sub-process to handle it, the server performance
and responsiveness is horrible (heavy-weight processes cost time to
spawn). Finally, the server application doesn't scale very well with the
load because of these resource problems. In practice, lots of tricks
are usually used to overcome these problems - ranging from pre-forked
sub-process pools to semi-serialized processing, etc.
One of the most elegant ways to solve these resource- and data-sharing
problems is to have multiple light-weight threads of execution
inside a single (heavy-weight) process, i.e., to use multithreading.
Those threads usually improve responsiveness and performance of the
application, often improve and simplify the internal program structure,
and most important, require less system resources than heavy-weight
processes. Threads are neither the optimal run-time facility for all
types of applications, nor can all applications benefit from them. But
at least event-driven server applications usually benefit greatly from
using threads.
The World of Threading
Even though lots of documents exists which describe and define the world
of threading, to understand Pth, you need only basic knowledge about
threading. The following definitions of thread-related terms should at
least help you understand thread programming enough to allow you to use
Pth.
oprocess vs. thread
A process on Unix systems consists of at least the following fundamental
ingredients: virtual memory table, program code, program
counter, heap memory, stack memory, stack pointer, file
descriptor set, signal table. On every process switch, the kernel
saves and restores these ingredients for the individual processes. On
the other hand, a thread consists of only a private program counter,
stack memory, stack pointer and signal table. All other ingredients, in
particular the virtual memory, it shares with the other threads of the
same process.
okernel-space vs. user-space threading
Threads on a Unix platform traditionally can be implemented either
inside kernel-space or user-space. When threads are implemented by the
kernel, the thread context switches are performed by the kernel without
the application's knowledge. Similarly, when threads are implemented in
user-space, the thread context switches are performed by an application
library, without the kernel's knowledge. There also are hybrid threading
approaches where, typically, a user-space library binds one or more
user-space threads to one or more kernel-space threads (there usually
called light-weight processes - or in short LWPs).
User-space threads are usually more portable and can perform faster
and cheaper context switches (for instance via swapcontext(2) or
setjmp(3)/longjmp(3)) than kernel based threads. On the other hand,
kernel-space threads can take advantage of multiprocessor machines and
don't have any inherent I/O blocking problems. Kernel-space threads are
usually scheduled in preemptive way side-by-side with the underlying
processes. User-space threads on the other hand use either preemptive or
non-preemptive scheduling.
opreemptive vs. non-preemptive thread scheduling
In preemptive scheduling, the scheduler lets a thread execute until a
blocking situation occurs (usually a function call which would block)
or the assigned timeslice elapses. Then it detracts control from the
thread without a chance for the thread to object. This is usually
realized by interrupting the thread through a hardware interrupt
signal (for kernel-space threads) or a software interrupt signal (for
user-space threads), like `SIGALRM' or `SIGVTALRM'. In non-preemptive
scheduling, once a thread received control from the scheduler it keeps
it until either a blocking situation occurs (again a function call which
would block and instead switches back to the scheduler) or the thread
explicitly yields control back to the scheduler in a cooperative way.
oconcurrency vs. parallelism
Concurrency exists when at least two threads are in progress at the
same time. Parallelism arises when at least two threads are executing
simultaneously. Real parallelism can be only achieved on multiprocessor
machines, of course. But one also usually speaks of parallelism or
high concurrency in the context of preemptive thread scheduling
and of low concurrency in the context of non-preemptive thread
scheduling.
oresponsiveness
The responsiveness of a system can be described by the user visible
delay until the system responses to an external request. When this delay
is small enough and the user doesn't recognize a noticeable delay,
the responsiveness of the system is considered good. When the user
recognizes or is even annoyed by the delay, the responsiveness of the
system is considered bad.
oreentrant, thread-safe and asynchronous-safe functions
A reentrant function is one that behaves correctly if it is called
simultaneously by several threads and then also executes simultaneously.
Functions that access global state, such as memory or files, of course,
need to be carefully designed in order to be reentrant. Two traditional
approaches to solve these problems are caller-supplied states and
thread-specific data.
Thread-safety is the avoidance of data races, i.e., situations
in which data is set to either correct or incorrect value depending
upon the (unpredictable) order in which multiple threads access and
modify the data. So a function is thread-safe when it still behaves
semantically correct when called simultaneously by several threads (it
is not required that the functions also execute simultaneously). The
traditional approach to achieve thread-safety is to wrap a function body
with an internal mutual exclusion lock (aka `mutex'). As you should
recognize, reentrant is a stronger attribute than thread-safe, because
it is harder to achieve and results especially in no run-time contention
between threads. So, a reentrant function is always thread-safe, but not
vice versa.
Additionally there is a related attribute for functions named
asynchronous-safe, which comes into play in conjunction with signal
handlers. This is very related to the problem of reentrant functions. An
asynchronous-safe function is one that can be called safe and without
side-effects from within a signal handler context. Usually very few
functions are of this type, because an application is very restricted in
what it can perform from within a signal handler (especially what system
functions it is allowed to call). The reason mainly is, because only a
few system functions are officially declared by POSIX as guaranteed to
be asynchronous-safe. Asynchronous-safe functions usually have to be
already reentrant.
User-Space Threads
User-space threads can be implemented in various way. The two
traditional approaches are:
1.
Matrix-based explicit dispatching between small units of execution:
Here the global procedures of the application are split into small
execution units (each is required to not run for more than a few
milliseconds) and those units are implemented by separate functions.
Then a global matrix is defined which describes the execution (and
perhaps even dependency) order of these functions. The main server
procedure then just dispatches between these units by calling one
function after each other controlled by this matrix. The threads are
created by more than one jump-trail through this matrix and by switching
between these jump-trails controlled by corresponding occurred events.
This approach gives the best possible performance, because one can
fine-tune the threads of execution by adjusting the matrix, and the
scheduling is done explicitly by the application itself. It is also very
portable, because the matrix is just an ordinary data structure, and
functions are a standard feature of ANSI C.
The disadvantage of this approach is that it is complicated to write
large applications with this approach, because in those applications
one quickly gets hundreds(!) of execution units and the control flow
inside such an application is very hard to understand (because it is
interrupted by function borders and one always has to remember the
global dispatching matrix to follow it). Additionally, all threads
operate on the same execution stack. Although this saves memory, it is
often nasty, because one cannot switch between threads in the middle of
a function. Thus the scheduling borders are the function borders.
2.
Context-based implicit scheduling between threads of execution:
Here the idea is that one programs the application as with forked
processes, i.e., one spawns a thread of execution and this runs from the
begin to the end without an interrupted control flow. But the control
flow can be still interrupted - even in the middle of a function.
Actually in a preemptive way, similar to what the kernel does for the
heavy-weight processes, i.e., every few milliseconds the user-space
scheduler switches between the threads of execution. But the thread
itself doesn't recognize this and usually (except for synchronization
issues) doesn't have to care about this.
The advantage of this approach is that it's very easy to program,
because the control flow and context of a thread directly follows
a procedure without forced interrupts through function borders.
Additionally, the programming is very similar to a traditional and well
understood fork(2) based approach.
The disadvantage is that although the general performance is increased,
compared to using approaches based on heavy-weight processes, it is decreased
compared to the matrix-approach above. Because the implicit preemptive
scheduling does usually a lot more context switches (every user-space context
switch costs some overhead even when it is a lot cheaper than a kernel-level
context switch) than the explicit cooperative/non-preemptive scheduling.
Finally, there is no really portable POSIX/ANSI-C based way to implement
user-space preemptive threading. Either the platform already has threads,
or one has to hope that some semi-portable package exists for it. And
even those semi-portable packages usually have to deal with assembler
code and other nasty internals and are not easy to port to forthcoming
platforms.
So, in short: the matrix-dispatching approach is portable and fast, but
nasty to program. The thread scheduling approach is easy to program,
but suffers from synchronization and portability problems caused by its
preemptive nature.
The Compromise of Pth
But why not combine the good aspects of both approaches while avoiding
their bad aspects? That's the goal of Pth. Pth implements
easy-to-program threads of execution, but avoids the problems of
preemptive scheduling by using non-preemptive scheduling instead.
This sounds like, and is, a useful approach. Nevertheless, one has to
keep the implications of non-preemptive thread scheduling in mind when
working with Pth. The following list summarizes a few essential
points:
o
Pth provides maximum portability, but NOT the fanciest features.
This is, because it uses a nifty and portable POSIX/ANSI-C approach for
thread creation (and this way doesn't require any platform dependent
assembler hacks) and schedules the threads in non-preemptive way (which
doesn't require unportable facilities like `SIGVTALRM'). On the other
hand, this way not all fancy threading features can be implemented.
Nevertheless the available facilities are enough to provide a robust and
full-featured threading system.
o
Pth increases the responsiveness and concurrency of an event-driven
application, but NOT the concurrency of number-crunching applications.
The reason is the non-preemptive scheduling. Number-crunching
applications usually require preemptive scheduling to achieve
concurrency because of their long CPU bursts. For them, non-preemptive
scheduling (even together with explicit yielding) provides only the old
concept of `coroutines'. On the other hand, event driven applications
benefit greatly from non-preemptive scheduling. They have only short
CPU bursts and lots of events to wait on, and this way run faster under
non-preemptive scheduling because no unnecessary context switching
occurs, as it is the case for preemptive scheduling. That's why Pth
is mainly intended for server type applications, although there is no
technical restriction.
o
Pth requires thread-safe functions, but NOT reentrant functions.
This nice fact exists again because of the nature of non-preemptive
scheduling, where a function isn't interrupted and this way cannot be
reentered before it returned. This is a great portability benefit,
because thread-safety can be achieved more easily than reentrance
possibility. Especially this means that under Pth more existing
third-party libraries can be used without side-effects than its the case
for other threading systems.
o
Pth doesn't require any kernel support, but can NOT
benefit from multiprocessor machines.
This means that Pth runs on almost all Unix kernels, because the
kernel does not need to be aware of the Pth threads (because they
are implemented entirely in user-space). On the other hand, it cannot
benefit from the existence of multiprocessors, because for this, kernel
support would be needed. In practice, this is no problem, because
multiprocessor systems are rare, and portability is almost more
important than highest concurrency.
The life cycle of a thread
To understand the Pth Application Programming Interface (API), it
helps to first understand the life cycle of a thread in the Pth
threading system. It can be illustrated with the following directed
graph:
NEW
|
V
+---> READY ---+
| ^ |
| | V
WAITING <--+-- RUNNING
|
: V
SUSPENDED DEAD
When a new thread is created, it is moved into the NEW queue of the
scheduler. On the next dispatching for this thread, the scheduler picks
it up from there and moves it to the READY queue. This is a queue
containing all threads which want to perform a CPU burst. There they are
queued in priority order. On each dispatching step, the scheduler always
removes the thread with the highest priority only. It then increases the
priority of all remaining threads by 1, to prevent them from `starving'.
The thread which was removed from the READY queue is the new
RUNNING thread (there is always just one RUNNING thread, of
course). The RUNNING thread is assigned execution control. After
this thread yields execution (either explicitly by yielding execution
or implicitly by calling a function which would block) there are three
possibilities: Either it has terminated, then it is moved to the DEAD
queue, or it has events on which it wants to wait, then it is moved into
the WAITING queue. Else it is assumed it wants to perform more CPU
bursts and immediately enters the READY queue again.
Before the next thread is taken out of the READY queue, the
WAITING queue is checked for pending events. If one or more events
occurred, the threads that are waiting on them are immediately moved to
the READY queue.
The purpose of the NEW queue has to do with the fact that in Pth
a thread never directly switches to another thread. A thread always
yields execution to the scheduler and the scheduler dispatches to the
next thread. So a freshly spawned thread has to be kept somewhere until
the scheduler gets a chance to pick it up for scheduling. That is for
what the NEW queue is for.
The purpose of the DEAD queue is to support thread joining. When a
thread is marked to be unjoinable, it is directly kicked out of the
system after it terminated. But when it is joinable, it enters the
DEAD queue. There it remains until another thread joins it.
Finally, there is a special separated queue named SUSPENDED, to where
threads can be manually moved from the NEW, READY or WAITING
queues by the application. The purpose of this special queue is to
temporarily absorb suspended threads until they are again resumed by
the application. Suspended threads do not cost scheduling or event
handling resources, because they are temporarily completely out of the
scheduler's scope. If a thread is resumed, it is moved back to the queue
from where it originally came and this way again enters the schedulers
scope.
APPLICATION PROGRAMMING INTERFACE (API)
In the following the PthApplication Programming Interface (API)
is discussed in detail. With the knowledge given above, it should be
now easy to understand how to program threads with this API. In good
Unix tradition, Pth functions use special return values (`NULL'
in pointer context, `FALSE' in boolean context and `-1' in integer
context) to indicate an error condition and set (or pass through) the
`errno' system variable to pass more details about the error to the
caller.
Global Library Management
The following functions act on the library as a whole. They are used to
initialize and shutdown the scheduler and fetch information from it.
int pth_init(void);
This initializes the Pth library. It has to be the first Pth API
function call in an application, and is mandatory. It's usually done at
the begin of the main() function of the application. This implicitly
spawns the internal scheduler thread and transforms the single execution
unit of the current process into a thread (the `main' thread). It
returns `TRUE' on success and `FALSE' on error.
int pth_kill(void);
This kills the Pth library. It should be the last Pth API function call
in an application, but is not really required. It's usually done at the end of
the main function of the application. At least, it has to be called from within
the main thread. It implicitly kills all threads and transforms back the
calling thread into the single execution unit of the underlying process. The
usual way to terminate a Pth application is either a simple
``pth_exit(0);'' in the main thread (which waits for all other threads to
terminate, kills the threading system and then terminates the process) or a
``pth_kill(); exit(0)'' (which immediately kills the threading system and
terminates the process). The pth_kill() return immediately with a return
code of `FALSE' if it is called not from within the main thread. Else
kills the threading system and returns `TRUE'.
long pth_ctrl(unsigned long query, ...);
This is a generalized query/control function for the Pth library. The
argument query is a bitmask formed out of one or more `PTH_CTRL_'XXXX
queries. Currently the following queries are supported:
`PTH_CTRL_GETTHREADS'
This returns the total number of threads currently in existence. This query
actually is formed out of the combination of queries for threads in a
particular state, i.e., the `PTH_CTRL_GETTHREADS' query is equal to the
OR-combination of all the following specialized queries:
`PTH_CTRL_GETTHREADS_NEW' for the number of threads in the
new queue (threads created via pth_spawn(3) but still not
scheduled once), `PTH_CTRL_GETTHREADS_READY' for the number of
threads in the ready queue (threads who want to do CPU bursts),
`PTH_CTRL_GETTHREADS_RUNNING' for the number of running threads
(always just one thread!), `PTH_CTRL_GETTHREADS_WAITING' for
the number of threads in the waiting queue (threads waiting for
events), `PTH_CTRL_GETTHREADS_SUSPENDED' for the number of
threads in the suspended queue (threads waiting to be resumed) and
`PTH_CTRL_GETTHREADS_DEAD' for the number of threads in the new queue
(terminated threads waiting for a join).
`PTH_CTRL_GETAVLOAD'
This requires a second argument of type ``float *'' (pointer to a floating
point variable). It stores a floating point value describing the exponential
averaged load of the scheduler in this variable. The load is a function from
the number of threads in the ready queue of the schedulers dispatching unit.
So a load around 1.0 means there is only one ready thread (the standard
situation when the application has no high load). A higher load value means
there a more threads ready who want to do CPU bursts. The average load value
updates once per second only. The return value for this query is always 0.
`PTH_CTRL_GETPRIO'
This requires a second argument of type ``pth_t'' which identifies a
thread. It returns the priority (ranging from `PTH_PRIO_MIN' to
`PTH_PRIO_MAX') of the given thread.
`PTH_CTRL_GETNAME'
This requires a second argument of type ``pth_t'' which identifies a
thread. It returns the name of the given thread, i.e., the return value of
pth_ctrl(3) should be casted to a ``char *''.
`PTH_CTRL_DUMPSTATE'
This requires a second argument of type ``FILE *'' to which a summary
of the internal Pth library state is written to. The main information
which is currently written out is the current state of the thread pool.
The function returns `-1' on error.
long pth_version(void);
This function returns a hex-value `0xVRRTLL' which describes the
current Pth library version. V is the version, RR the revisions,
LL the level and T the type of the level (alphalevel=0, betalevel=1,
patchlevel=2, etc). For instance Pth version 1.0b1 is encoded as 0x100101.
The reason for this unusual mapping is that this way the version number is
steadily increasing. The same value is also available under compile time as
`PTH_VERSION'.
Thread Attribute Handling
Attribute objects are used in Pth for two things: First stand-alone/unbound
attribute objects are used to store attributes for to be spawned threads.
Bounded attribute objects are used to modify attributes of already existing
threads. The following attribute fields exists in attribute objects:
`PTH_ATTR_PRIO' (read-write) [`int']
Thread Priority between `PTH_PRIO_MIN' and `PTH_PRIO_MAX'.
The default is `PTH_PRIO_STD'.
`PTH_ATTR_NAME' (read-write) [`char *']
Name of thread (up to 40 characters are stored only), mainly for debugging
purposes.
`PTH_ATTR_JOINABLE' (read-write> [`int']
The thread detachment type, `TRUE' indicates a joinable thread, `FALSE'
indicates a detached thread. When a the is detached after termination it is
immediately kicked out of the system instead of inserted into the dead queue.
The thread cancellation state, i.e., a combination of `PTH_CANCEL_ENABLE' or
`PTH_CANCEL_DISABLE' and `PTH_CANCEL_DEFERRED' or
`PTH_CANCEL_ASYNCHRONOUS'.
The thread start function.
This can be queried only when the attribute object is bound to a thread.
`PTH_ATTR_START_ARG' (read-only) [`void *']
The thread start argument.
This can be queried only when the attribute object is bound to a thread.
`PTH_ATTR_STATE' (read-only) [`pth_state_t']
The scheduling state of the thread, i.e., either `PTH_STATE_NEW',
`PTH_STATE_READY', `PTH_STATE_WAITING', or `PTH_STATE_DEAD'
This can be queried only when the attribute object is bound to a thread.
`PTH_ATTR_EVENTS' (read-only) [`pth_event_t']
The event ring the thread is waiting for.
This can be queried only when the attribute object is bound to a thread.
`PTH_ATTR_BOUND' (read-only) [`int']
Whether the attribute object is bound (`TRUE') to a thread or not (`FALSE').
The following API functions exists to handle the attribute objects:
pth_attr_t pth_attr_of(pth_t tid);
This returns a new attribute object bound to thread tid. Any queries on
this object directly fetch attributes from tid. And attribute modifications
directly change tid. Use such attribute objects to modify existing threads.
pth_attr_t pth_attr_new(void);
This returns a new unbound attribute object. An implicit pth_attr_init() is
done on it. Any queries on this object just fetch stored attributes from it.
And attribute modifications just change the stored attributes. Use such
attribute objects to pre-configure attributes for to be spawned threads.
int pth_attr_init(pth_attr_t attr);
This initializes an attribute object attr to the default values:
`PTH_ATTR_PRIO' := `PTH_PRIO_STD', `PTH_ATTR_NAME' := ``unknown'',
`PTH_ATTR_JOINABLE' := `TRUE', `PTH_ATTR_CANCELSTATE' :=
`PTH_CANCEL_DEFAULT', `PTH_ATTR_STACK_SIZE' := 64*1024 and
`PTH_ATTR_STACK_ADDR' := `NULL'. All other `PTH_ATTR_*' attributes are
read-only attributes and don't receive default values in attr, because they
exists only for bounded attribute objects.
int pth_attr_set(pth_attr_t attr, int field, ...);
This sets the attribute field field in attr to a value
specified as an additional argument on the variable argument
list. The following attribute fields and argument pairs can
be used:
PTH_ATTR_PRIO int
PTH_ATTR_NAME char *
PTH_ATTR_JOINABLE int
PTH_ATTR_CANCEL_STATE unsigned int
PTH_ATTR_STACK_SIZE unsigned int
PTH_ATTR_STACK_ADDR char *
int pth_attr_get(pth_attr_t attr, int field, ...);
This retrieves the attribute field field in attr and stores its
value in the variable specified through a pointer in an additional
argument on the variable argument list. The following fields and
argument pairs can be used:
PTH_ATTR_PRIO int *
PTH_ATTR_NAME char **
PTH_ATTR_JOINABLE int *
PTH_ATTR_CANCEL_STATE unsigned int *
PTH_ATTR_STACK_SIZE unsigned int *
PTH_ATTR_STACK_ADDR char **
PTH_ATTR_TIME_SPAWN pth_time_t *
PTH_ATTR_TIME_LAST pth_time_t *
PTH_ATTR_TIME_RAN pth_time_t *
PTH_ATTR_START_FUNC void *(**)(void *)
PTH_ATTR_START_ARG void **
PTH_ATTR_STATE pth_state_t *
PTH_ATTR_EVENTS pth_event_t *
PTH_ATTR_BOUND int *
int pth_attr_destroy(pth_attr_t attr);
This destroys a attribute object attr. After this attr is no
longer a valid attribute object.
Thread Control
The following functions control the threading itself and form the main API of
the Pth library.
This spawns a new thread with the attributes given in attr (or
`PTH_ATTR_DEFAULT' for default attributes - which means that thread priority,
joinability and cancel state are inherited from the current thread) with the
starting point at routine entry. This entry routine is called as
`pth_exit(entry(arg))' inside the new thread unit, i.e., entry's
return value is fed to an implicit pth_exit(3). So the thread usually can exit
by just returning. Nevertheless the thread can also exit explicitly at any
time by calling pth_exit(3). But keep in mind that calling the POSIX function
exit(3) still terminates the complete process and not just the current thread.
There is no Pth-internal limit on the number of threads one can spawn,
except the limit implied by the available virtual memory. Pth internally
keeps track of thread in dynamic data structures. The function returns
`NULL' on error.
int pth_once(pth_once_t *ctrlvar, void (*func)(void *), void *arg);
This is a convenience function which uses a control variable of type
`pth_once_t' to make sure a constructor function func is called only once
as `func(arg)' in the system. In other words: Only the first call to
pth_once(3) by any thread in the system succeeds. The variable referenced via
ctrlvar should be declared as ``pth_once_t'variable-name =
`PTH_ONCE_INIT';' before calling this function.
pth_t pth_self(void);
This just returns the unique thread handle of the currently running thread.
This handle itself has to be treated as an opaque entity by the application.
It's usually used as an argument to other functions who require an argument of
type `pth_t'.
int pth_suspend(pth_t tid);
This suspends a thread tid until it is manually resumed again via
pth_resume(3). For this, the thread is moved to the SUSPENDED queue
and this way is completely out of the scheduler's event handling and
thread dispatching scope. Suspending the current thread is not allowed.
The function returns `TRUE' on success and `FALSE' on errors.
int pth_resume(pth_t tid);
This function resumes a previously suspended thread tid, i.e. tid
has to stay on the SUSPENDED queue. The thread is moved to the
NEW, READY or WAITING queue (dependent on what its state was
when the pth_suspend(3) call were made) and this way again enters the
event handling and thread dispatching scope of the scheduler. The
function returns `TRUE' on success and `FALSE' on errors.
int pth_raise(pth_t tid, int sig)
This function raises a signal for delivery to thread tid only. When one
just raises a signal via raise(3) or kill(2), its delivered to an arbitrary
thread which has this signal not blocked. With pth_raise(3) one can send a
signal to a thread and its guarantees that only this thread gets the signal
delivered. But keep in mind that nevertheless the signals action is still
configured process-wide. When sig is 0 plain thread checking is
performed, i.e., ``pth_raise(tid, 0)'' returns `TRUE' when thread tid
still exists in the PTH system but doesn't send any signal to it.
int pth_yield(pth_t tid);
This explicitly yields back the execution control to the scheduler thread.
Usually the execution is implicitly transferred back to the scheduler when a
thread waits for an event. But when a thread has to do larger CPU bursts, it
can be reasonable to interrupt it explicitly by doing a few pth_yield(3) calls
to give other threads a chance to execute, too. This obviously is the
cooperating part of Pth. A thread has not to yield execution, of
course. But when you want to program a server application with good response
times the threads should be cooperative, i.e., when they should split their CPU
bursts into smaller units with this call.
Usually one specifies tid as `NULL' to indicate to the scheduler that it
can freely decide which thread to dispatch next. But if one wants to indicate
to the scheduler that a particular thread should be favored on the next
dispatching step, one can specify this thread explicitly. This allows the
usage of the old concept of coroutines where a thread/routine switches to a
particular cooperating thread. If tid is not `NULL' and points to a new
or ready thread, it is guaranteed that this thread receives execution
control on the next dispatching step. If tid is in a different state (that
is, not in `PTH_STATE_NEW' or `PTH_STATE_READY') an error is reported.
The function usually returns `TRUE' for success and only `FALSE' (with
`errno' set to `EINVAL') if tid specified and invalid or still not
new or ready thread.
int pth_nap(pth_time_t naptime);
This functions suspends the execution of the current thread until naptime
is elapsed. naptime is of type `pth_time_t' and this way has theoretically
a resolution of one microsecond. In practice you should neither rely on this
nor that the thread is awakened exactly after naptime has elapsed. It's
only guarantees that the thread will sleep at least naptime. But because
of the non-preemptive nature of Pth it can last longer (when another thread
kept the CPU for a long time). Additionally the resolution is dependent of the
implementation of timers by the operating system and these usually have only a
resolution of 10 microseconds or larger. But usually this isn't important for
an application unless it tries to use this facility for real time tasks.
int pth_wait(pth_event_t ev);
This is the link between the scheduler and the event facility (see below for
the various pth_event_xxx() functions). It's modeled like select(2), i.e., one
gives this function one or more events (in the event ring specified by ev)
on which the current thread wants to wait. The scheduler awakes the thread
when one ore more of them occurred after tagging them as occurred. The ev
argument is a pointer to an event ring which isn't changed except for the
tagging. pth_wait(3) returns the number of occurred events and the application
can use pth_event_occurred(3) to test which events occurred.
int pth_cancel(pth_t tid);
This cancels a thread tid. How the cancellation is done depends on the
cancellation state of tid which the thread can configure itself. When its
state is `PTH_CANCEL_DISABLE' a cancellation request is just made pending.
When it is `PTH_CANCEL_ENABLE' it depends on the cancellation type what is
performed. When its `PTH_CANCEL_DEFERRED' again the cancellation request is
just made pending. But when its `PTH_CANCEL_ASYNCHRONOUS' the thread is
immediately canceled before pth_cancel(3) returns. The effect of a thread
cancellation is equal to implicitly forcing the thread to call
``pth_exit(PTH_CANCELED)'' at one of his cancellation points. In Pth
thread enter a cancellation point either explicitly via pth_cancel_point(3) or
implicitly by waiting for an event.
int pth_abort(pth_t tid);
This is the cruel way to cancel a thread tid. When it's already dead and
waits to be joined it just joins it (via ``pth_join('tid`, NULL)'') and
this way kicks it out of the system. Else it forces the thread to be not
joinable and to allow asynchronous cancellation and then cancels it via
``pth_cancel('tid`)''.
int pth_join(pth_t tid, void **value);
This joins the current thread with the thread specified via tid. It first
suspends the current thread until the tid thread has terminated. Then it is
awakened and stores the value of tid's pth_exit(3) call into *value (if
value and not `NULL') and returns to the caller. A thread can be joined
only when it was not spawned with `PTH_FLAG_NOJOIN'. A thread can only be
joined once, i.e., after the pth_join(3) call the thread tid is removed
from the system.
void pth_exit(void *value);
This terminates the current thread. Whether it's immediately removed from the
system or inserted into the dead queue of the scheduler depends on its join
type which was specified at spawning time. When it was spawned with
`PTH_FLAG_NOJOIN' it's immediately removed and value is ignored.
Else the thread is inserted into the dead queue and value remembered
for a pth_join(3) call by another thread.
Utilities
The following functions are utility functions.
int pth_fdmode(int fd, int mode);
This switches the non-blocking mode flag on file descriptor fd. The
argument mode can be `PTH_FDMODE_BLOCK' for switching fd into blocking
I/O mode, `PTH_FDMODE_NONBLOCK' for switching fd into non-blocking I/O
mode or `PTH_FDMODE_POLL' for just polling the current mode. The current mode
is returned (either `PTH_FDMODE_BLOCK' or `PTH_FDMODE_NONBLOCK') or
`PTH_FDMODE_ERROR' on error. Keep in mind that since Pth 1.1 there is no
longer a requirement to manually switch a file descriptor into non-blocking
mode in order to use it. This is automatically done temporarily inside Pth.
Instead when you now switch a file descriptor explicitly into non-blocking
mode, pth_read(3) or pth_write(3) will never block the current thread.
pth_time_t pth_time(long sec, long usec);
This is a constructor for a `pth_time_t' structure which is a convenient
function to avoid temporary structure values. It returns a pth_time_t
structure which holds the absolute time value specified by sec and usec.
pth_time_t pth_timeout(long sec, long usec);
This is a constructor for a `pth_time_t' structure which is a convenient
function to avoid temporary structure values. It returns a pth_time_t
structure which holds the absolute time value calculated by adding sec and
usec to the current time.
Sfdisc_t *pth_sfiodisc(void);
This functions is always available, but only reasonably usable when Pth
was built with Sfio support (`--with-sfio' option) and `PTH_EXT_SFIO' is
then defined by `pth.h'. It is useful for applications which want to use the
comprehensive Sfio I/O library with the Pth threading library. Then this
function can be used to get an Sfio discipline structure (`Sfdisc_t')
which can be pushed onto Sfio streams (`Sfio_t') in order to let this
stream use pth_read(3)/pth_write(2) instead of read(2)/write(2). The benefit
is that this way I/O on the Sfio stream does only block the current thread
instead of the whole process. The application has to free(3) the `Sfdisc_t'
structure when it is no longer needed. The Sfio package can be found at
http://www.research.att.com/sw/tools/sfio/.
Cancellation Management
Pth supports POSIX style thread cancellation via pth_cancel(3) and the
following two related functions:
void pth_cancel_state(int newstate, int *oldstate);
This manages the cancellation state of the current thread. When oldstate
is not `NULL' the function stores the old cancellation state under the
variable pointed to by oldstate. When newstate is not `0' it sets the
new cancellation state. oldstate is created before newstate is set. A
state is a combination of `PTH_CANCEL_ENABLE' or `PTH_CANCEL_DISABLE' and
`PTH_CANCEL_DEFERRED' or `PTH_CANCEL_ASYNCHRONOUS'.
`PTH_CANCEL_ENABLE|PTH_CANCEL_DEFERRED' (or `PTH_CANCEL_DEFAULT') is the
default state where cancellation is possible but only at cancellation points.
Use `PTH_CANCEL_DISABLE' to complete disable cancellation for a thread and
`PTH_CANCEL_ASYNCHRONOUS' for allowing asynchronous cancellations, i.e.,
cancellations which can happen at any time.
void pth_cancel_point(void);
This explicitly enter a cancellation point. When the current cancellation
state is `PTH_CANCEL_DISABLE' or no cancellation request is pending, this has
no side-effect and returns immediately. Else it calls
``pth_exit(PTH_CANCELED)''.
Event Handling
Pth has a very flexible event facility which is linked into the scheduler
through the pth_wait(3) function. The following functions provide the handling
of event rings.
pth_event_t pth_event(unsigned long spec, ...);
This creates a new event ring consisting of a single initial event. The type
of the generated event is specified by spec. The following types are
available:
`PTH_EVENT_FD'
This is a file descriptor event. One or more of `PTH_UNTIL_FD_READABLE',
`PTH_UNTIL_FD_WRITEABLE' or `PTH_UNTIL_FD_EXECPTION' have to be OR-ed into
spec to specify on which state of the file descriptor you want to wait. The
file descriptor itself has to be given as an additional argument. Example:
``pth_event(PTH_EVENT_FD|PTH_UNTIL_FD_READABLE, fd)''.
`PTH_EVENT_SELECT'
This is a multiple file descriptor event modeled directly after the select(2)
call (actually it is also used to implement pth_select(3) internally). It's a
convenient way to wait for a large set of file descriptors at once and at each
file descriptor for a different type of state. Additionally as a nice
side-effect one receives the number of file descriptors which causes the event
to be occurred (using BSD semantics, i.e., when a file descriptor occurred in
two sets it's counted twice). The arguments correspond directly to the
select(2) function arguments except that there is no timeout argument (because
timeouts already can be handled via `PTH_EVENT_TIME' events).
Example: ``pth_event(PTH_EVENT_SELECT, &rc, nfd, rfds, wfds, efds)'' where
`rc' has to be of type ``int *'', `nfd' has to be of type ``int'' and
`rfds', `wfds' and `efds' have to be of type ``fd_set *'' (see
select(2)). The number of occurred file descriptors are stored in `rc'.
`PTH_EVENT_SIGS'
This is a signal set event. The two additional arguments have to be a pointer
to a signal set (type ``sigset_t *'') and a pointer to a signal number
variable (type ``int *''). This event waits until one of the signals in
the signal set occurred. As a result the occurred signal number is stored in
the second additional argument. Keep in mind that the Pth scheduler doesn't
block signals automatically. So when you want to wait for a signal with this
event you've to block it via sigprocmask(2) or it will be delivered without
your notice. Example: ``sigemptyset(&set); sigaddset(&set, SIGINT);
pth_event(PTH_EVENT_SIG, &set, &sig);''.
`PTH_EVENT_TIME'
This is a time point event. The additional argument has to be of type
`pth_time_t' (usually on-the-fly generated via pth_time(3)). This events
waits until the specified time point has elapsed. Keep in mind that the value
is an absolute time point and not an offset. When you want to wait for a
specified amount of time, you've to add the current time to the offset
(usually on-the-fly achieved via pth_timeout(3)). Example:
``pth_event(PTH_EVENT_TIME, pth_timeout(2,0))''.
`PTH_EVENT_MSG'
This is a message port event. The additional argument has to be of type
`pth_msgport_t'. This events waits until one or more messages were received
on the specified message port. Example: ``pth_event(PTH_EVENT_MSG, mp)''.
`PTH_EVENT_TID'
This is a thread event. The additional argument has to be of type `pth_t'.
One of `PTH_UNTIL_TID_NEW', `PTH_UNTIL_TID_READY', `PTH_UNTIL_TID_WAITING'
or `PTH_UNTIL_TID_DEAD' has to be OR-ed into spec to specify on which
state of the thread you want to wait. Example:
``pth_event(PTH_EVENT_TID|PTH_UNTIL_TID_DEAD, tid)''.
`PTH_EVENT_FUNC'
This is a custom callback function event. Three additional arguments
have to be given with the following types: ``int (*)(void *)'',
``void *'' and ``pth_time_t''. The first is a function pointer to
a check function and the second argument is a user-supplied context
value which is passed to this function. The scheduler calls this
function on a regular basis (on his own scheduler stack, so be very
careful!) and the thread is kept sleeping while the function returns
`FALSE'. Once it returned `TRUE' the thread will be awakened. The
check interval is defined by the third argument, i.e., the check
function is polled again not until this amount of time elapsed. Example:
``pth_event(PTH_EVENT_FUNC, func, arg, pth_time(0,500000))''.
unsigned long pth_event_typeof(pth_event_t ev);
This returns the type of event ev. It's a combination of the describing
`PTH_EVENT_XX' and `PTH_UNTIL_XX' value. This is especially useful to know
which arguments have to be supplied to the pth_event_extract(3) function.
int pth_event_extract(pth_event_t ev, ...);
When pth_event(3) is treated like sprintf(3), then this function is
sscanf(3), i.e., it is the inverse operation of pth_event(3). This means that
it can be used to extract the ingredients of an event. The ingredients are
stored into variables which are given as pointers on the variable argument
list. Which pointers have to be present depends on the event type and has to
be determined by the caller before via pth_event_typeof(3).
To make it clear, when you constructed ev via ``ev =
pth_event(PTH_EVENT_FD, fd);'' you have to extract it via
``pth_event_extract(ev, &fd)'', etc. For multiple arguments of an event the
order of the pointer arguments is the same as for pth_event(3). But always
keep in mind that you have to always supply pointers to variables and
these variables have to be of the same type as the argument of pth_event(3)
required.
This concatenates one or more additional event rings to the event ring ev
and returns ev. The end of the argument list has to be marked with a
`NULL' argument. Use this function to create real events rings out of the
single-event rings created by pth_event(3).
pth_event_t pth_event_isolate(pth_event_t ev);
This isolates the event ev from possibly appended events in the event ring.
When in ev only one event exists, this returns `NULL'. When remaining
events exists, they form a new event ring which is returned.
pth_event_t pth_event_walk(pth_event_t ev, int direction);
This walks to the next (when direction is `PTH_WALK_NEXT') or previews
(when direction is `PTH_WALK_PREV') event in the event ring ev and
returns this new reached event. Additionally `PTH_UNTIL_OCCURRED' can be
OR-ed into direction to walk to the next/previous occurred event in the
ring ev.
int pth_event_occurred(pth_event_t ev);
This checks whether the event ev occurred. This is a fast operation because
only a tag on ev is checked which was either set or still not set by the
scheduler. In other words: This doesn't check the event itself, it just checks
the last knowledge of the scheduler.
int pth_event_free(pth_event_t ev, int mode);
This deallocates the event ev (when mode is `PTH_FREE_THIS') or all
events appended to the event ring under ev (when mode is
`PTH_FREE_ALL').
Key-Based Storage
The following functions provide thread-local storage through unique keys
similar to the POSIX Pthread API. Use this for thread specific global data.
int pth_key_create(pth_key_t *key, void (*func)(void *));
This created a new unique key and stores it in key. Additionally func
can specify a destructor function which is called on the current threads
termination with the key.
int pth_key_delete(pth_key_t key);
This explicitly destroys a key key.
int pth_key_setdata(pth_key_t key, const void *value);
This stores value under key.
void *pth_key_getdata(pth_key_t key);
This retrieves the value under key.
Message Port Communication
The following functions provide message ports which can be used for efficient
and flexible inter-thread communication.
This returns a pointer to a new message port with name name. The name
can be used by other threads via pth_msgport_find(3) to find the message port
in case they do not know directly the pointer to the message port.
void pth_msgport_destroy(pth_msgport_t mp);
This destroys a message port mp. Before all pending messages on it are
replied to their origin message port.
pth_msgport_t pth_msgport_find(const char *name);
This finds a message port in the system by name and returns the pointer to
it.
int pth_msgport_pending(pth_msgport_t mp);
This returns the number of pending messages on message port mp.
int pth_msgport_put(pth_msgport_t mp, pth_message_t *m);
This puts (or sends) a message m to message port mp.
pth_message_t *pth_msgport_get(pth_msgport_t mp);
This gets (or receives) the top message from message port mp. Incoming
messages are always kept in a queue, so there can be more pending messages, of
course.
int pth_msgport_reply(pth_message_t *m);
This replies a message m to the message port of the sender.
Thread Cleanups
The following functions provide per-thread cleanup functions.
int pth_cleanup_push(void (*handler)(void *), void *arg);
This pushes the routine handler onto the stack of cleanup routines for the
current thread. These routines are called in LIFO order when the thread
terminates.
int pth_cleanup_pop(int execute);
This pops the top-most routine from the stack of cleanup routines for the
current thread. When execute is `TRUE' the routine is additionally called.
Process Forking
The following functions provide some special support for process forking
situations inside the threading environment.
This function declares forking handlers to be called before and after
pth_fork(3), in the context of the thread that called pth_fork(3). The
prepare handler is called before fork(2) processing commences. The
parent handler is called after fork(2) processing completes in the parent
process. The child handler is called after fork(2) processing completed in
the child process. If no handling is desired at one or more of these three
points, the corresponding handler can be given as `NULL'. Each handler is
called with arg as the argument.
The order of calls to pth_atfork_push(3) is significant. The parent and
child handlers are called in the order in which they were established by
calls to pth_atfork_push(3), i.e., FIFO. The prepare fork handlers are
called in the opposite order, i.e., LIFO.
int pth_atfork_pop(void);
This removes the top-most handlers on the forking handler stack which were
established with the last pth_atfork_push(3) call. It returns `FALSE' when no
more handlers couldn't be removed from the stack.
pid_t pth_fork(void);
This is a variant of fork(2) with the difference that the current thread only
is forked into a separate process, i.e., in the parent process nothing changes
while in the child process all threads are gone except for the scheduler and
the calling thread. When you really want to duplicate all threads in the
current process you should use fork(2) directly. But this is usually not
reasonable. Additionally this function takes care of forking handlers as
established by pth_fork_push(3).
Synchronization
The following functions provide synchronization support via mutual exclusion
locks (mutex), read-write locks (rwlock), condition variables (cond)
and barriers (barrier). Keep in mind that in a non-preemptive threading
system like Pth this might sound unnecessary at the first look, because a
thread isn't interrupted by the system. Actually when you have a critical code
section which doesn't contain any pth_xxx() functions, you don't need any
mutex to protect it, of course.
But when your critical code section contains any pth_xxx() function the chance
is high that these temporarily switch to the scheduler. And this way other
threads can make progress and enter your critical code section, too. This is
especially true for critical code sections which implicitly or explicitly use
the event mechanism.
int pth_mutex_init(pth_mutex_t *mutex);
This dynamically initializes a mutex variable of type ``pth_mutex_t''.
Alternatively one can also use static initialization via ``pth_mutex_t
mutex = PTH_MUTEX_INIT''.
int pth_mutex_acquire(pth_mutex_t *mutex, int try, pth_event_t ev);
This acquires a mutex mutex. If the mutex is already locked by another
thread, the current threads execution is suspended until the mutex is unlocked
again or additionally the extra events in ev occurred (when ev is not
`NULL'). Recursive locking is explicitly supported, i.e., a thread is allowed
to acquire a mutex more than once before its released. But it then also has be
released the same number of times until the mutex is again lockable by others.
When try is `TRUE' this function never suspends execution. Instead it
returns `FALSE' with `errno' set to `EBUSY'.
int pth_mutex_release(pth_mutex_t *mutex);
This decrements the recursion locking count on mutex and when it is zero it
releases the mutex mutex.
int pth_rwlock_init(pth_rwlock_t *rwlock);
This dynamically initializes a read-write lock variable of type
``pth_rwlock_t''. Alternatively one can also use static initialization
via ``pth_rwlock_t rwlock = PTH_RWLOCK_INIT''.
int pth_rwlock_acquire(pth_rwlock_t *rwlock, int op, int try, pth_event_t ev);
This acquires a read-only (when op is `PTH_RWLOCK_RD') or a read-write
(when op is `PTH_RWLOCK_RW') lock rwlock. When the lock is only locked
by other threads in read-only mode, the lock succeeds. But when one thread
holds a read-write lock, all locking attempts suspend the current thread until
this lock is released again. Additionally in ev events can be given to let
the locking timeout, etc. When try is `TRUE' this function never suspends
execution. Instead it returns `FALSE' with `errno' set to `EBUSY'.
int pth_rwlock_release(pth_rwlock_t *rwlock);
This releases a previously acquired (read-only or read-write) lock.
int pth_cond_init(pth_cond_t *cond);
This dynamically initializes a condition variable variable of type
``pth_cond_t''. Alternatively one can also use static initialization via
``pth_cond_t cond = PTH_COND_INIT''.
int pth_cond_await(pth_cond_t *cond, pth_mutex_t *mutex, pth_event_t ev);
This awaits a condition situation. The caller has to follow the semantics of
the POSIX condition variables: mutex has to be acquired before this
function is called. The execution of the current thread is then suspended
either until the events in ev occurred (when ev is not `NULL') or
cond was notified by another thread via pth_cond_notify(3). While the
thread is waiting, mutex is released. Before it returns mutex is
reacquired.
int pth_cond_notify(pth_cond_t *cond, int broadcast);
This notified one or all threads which are waiting on cond. When
broadcast is `TRUE' all thread are notified, else only a single
(unspecified) one.
int pth_barrier_init(pth_barrier_t *barrier, int threshold);
This dynamically initializes a barrier variable of type ``pth_barrier_t''.
Alternatively one can also use static initialization via ``pth_barrier_t
barrier = PTH_BARRIER_INIT('threadhold`)''.
int pth_barrier_reach(pth_barrier_t *barrier);
This function reaches a barrier barrier. If this is the last thread (as
specified by threshold on init of barrier) all threads are awakened.
Else the current thread is suspended until the last thread reached the barrier
and this way awakes all threads. The function returns (beside `FALSE' on
error) the value `TRUE' for any thread which neither reached the barrier as
the first nor the last thread; `PTH_BARRIER_HEADLIGHT' for the thread which
reached the barrier as the first thread and `PTH_BARRIER_TAILLIGHT' for the
thread which reached the barrier as the last thread.
Generalized POSIX Replacement API
The following functions are generalized replacements functions for the POSIX
API, i.e., they are similar to the functions under `Standard POSIX
Replacement API' but all have an additional event argument which can be used
for timeouts, etc.
int pth_sigwait_ev(const sigset_t *set, int *sig, pth_event_t ev);
This is equal to pth_sigwait(3) (see below), but has an additional event
argument ev. When pth_sigwait(3) suspends the current threads execution it
usually only uses the signal event on set to awake. With this function any
number of extra events can be used to awake the current thread (remember that
ev actually is an event ring).
This is equal to pth_connect(3) (see below), but has an additional event
argument ev. When pth_connect(3) suspends the current threads execution it
usually only uses the I/O event on s to awake. With this function any
number of extra events can be used to awake the current thread (remember that
ev actually is an event ring).
int pth_accept_ev(int s, struct sockaddr *addr, socklen_t *addrlen, pth_event_t ev);
This is equal to pth_accept(3) (see below), but has an additional event
argument ev. When pth_accept(3) suspends the current threads execution it
usually only uses the I/O event on s to awake. With this function any
number of extra events can be used to awake the current thread (remember that
ev actually is an event ring).
This is equal to pth_select(3) (see below), but has an additional event
argument ev. When pth_select(3) suspends the current threads execution it
usually only uses the I/O event on rfds, wfds and efds to awake. With
this function any number of extra events can be used to awake the current
thread (remember that ev actually is an event ring).
int pth_poll_ev(struct pollfd *fds, unsigned int nfd, int timeout, pth_event_t ev);
This is equal to pth_poll(3) (see below), but has an additional event argument
ev. When pth_poll(3) suspends the current threads execution it usually only
uses the I/O event on fds to awake. With this function any number of extra
events can be used to awake the current thread (remember that ev actually
is an event ring).
This is equal to pth_read(3) (see below), but has an additional event argument
ev. When pth_read(3) suspends the current threads execution it usually only
uses the I/O event on fd to awake. With this function any number of extra
events can be used to awake the current thread (remember that ev actually
is an event ring).
This is equal to pth_readv(3) (see below), but has an additional event
argument ev. When pth_readv(3) suspends the current threads execution it
usually only uses the I/O event on fd to awake. With this function any
number of extra events can be used to awake the current thread (remember that
ev actually is an event ring).
This is equal to pth_write(3) (see below), but has an additional event argument
ev. When pth_write(3) suspends the current threads execution it usually
only uses the I/O event on fd to awake. With this function any number of
extra events can be used to awake the current thread (remember that ev
actually is an event ring).
This is equal to pth_writev(3) (see below), but has an additional event
argument ev. When pth_writev(3) suspends the current threads execution it
usually only uses the I/O event on fd to awake. With this function any
number of extra events can be used to awake the current thread (remember that
ev actually is an event ring).
This is equal to pth_recv(3) (see below), but has an additional event
argument ev. When pth_recv(3) suspends the current threads execution it
usually only uses the I/O event on fd to awake. With this function any
number of extra events can be used to awake the current thread (remember that
ev actually is an event ring).
This is equal to pth_recvfrom(3) (see below), but has an additional event
argument ev. When pth_recvfrom(3) suspends the current threads execution it
usually only uses the I/O event on fd to awake. With this function any
number of extra events can be used to awake the current thread (remember that
ev actually is an event ring).
This is equal to pth_send(3) (see below), but has an additional event
argument ev. When pth_send(3) suspends the current threads execution it
usually only uses the I/O event on fd to awake. With this function any
number of extra events can be used to awake the current thread (remember that
ev actually is an event ring).
This is equal to pth_sendto(3) (see below), but has an additional event
argument ev. When pth_sendto(3) suspends the current threads execution it
usually only uses the I/O event on fd to awake. With this function any
number of extra events can be used to awake the current thread (remember that
ev actually is an event ring).
Standard POSIX Replacement API
The following functions are standard replacements functions for the POSIX API.
The difference is mainly that they suspend the current thread only instead of
the whole process in case the file descriptors will block.
int pth_usleep(unsigned int usec);
This is a variant of the 4.3BSD usleep(3) function. It suspends the current
threads execution until usec microseconds (= usec*1/1000000 sec)
elapsed. The thread is guaranteed to not awakened before this time, but
because of the non-preemptive scheduling nature of Pth, it can be awakened
later, of course. The difference between usleep(3) and pth_usleep(3) is that
that pth_usleep(3) suspends only the execution of the current thread and not
the whole process.
unsigned int pth_sleep(unsigned int sec);
This is a variant of the POSIX sleep(3) function. It suspends the current
threads execution until sec seconds elapsed. The thread is guaranteed to
not awakened before this time, but because of the non-preemptive scheduling
nature of Pth, it can be awakened later, of course. The difference between
sleep(3) and pth_sleep(3) is that that pth_sleep(3) suspends only the
execution of the current thread and not the whole process.
pid_t pth_waitpid(pid_t pid, int *status, int options);
This is a variant of the POSIX waitpid(2) function. It suspends the
current threads execution until status information is available for a
terminated child process pid. The difference between waitpid(2) and
pth_waitpid(3) is that that pth_waitpid(3) suspends only the execution of the
current thread and not the whole process. For more details about the
arguments and return code semantics see waitpid(2).
int pth_system(const char *cmd);
This is a variant of the POSIX system(3) function. It executes the
shell command cmd with Bourne Shell (`sh') and suspends the current
threads execution until this command terminates. The difference between
system(3) and pth_system(3) is that that pth_system(3) suspends only
the execution of the current thread and not the whole process. For more
details about the arguments and return code semantics see system(3).
int pth_sigmask(int how, const sigset_t *set, sigset_t *oset)
This is the Pth thread-related equivalent of POSIX sigprocmask(2) respectively
pthread_sigmask(3). The arguments how, set and oset directly relate
to sigprocmask(2), because Pth internally just uses sigprocmask(2) here. So
alternatively you can also directly call sigprocmask(2), but for consistency
reasons you should use this function pth_sigmask(3).
int pth_sigwait(const sigset_t *set, int *sig);
This is a variant of the POSIX.1c sigwait(3) function. It suspends the current
threads execution until a signal in set occurred and stores the signal
number in sig. The important point is that the signal is not delivered to a
signal handler. Instead it's caught by the scheduler only in order to awake
the pth_sigwait() call. The trick and noticeable point here is that this way
you get an asynchronous aware application that is written completely
synchronously. When you think about the problem of asynchronous safe
functions you should recognize that this is a great benefit.
int pth_connect(int s, const struct sockaddr *addr, socklen_t addrlen);
This is a variant of the 4.2BSD connect(2) function. It establishes a
connection on a socket s to target specified in addr and addrlen.
The difference between connect(2) and pth_connect(3) is that that
pth_connect(3) suspends only the execution of the current thread and not the
whole process. For more details about the arguments and return code semantics
see connect(2).
int pth_accept(int s, struct sockaddr *addr, socklen_t *addrlen);
This is a variant of the 4.2BSD accept(2) function. It accepts a connection on
a socket by extracting the first connection request on the queue of pending
connections, creating a new socket with the same properties of s and
allocates a new file descriptor for the socket (which is returned). The
difference between accept(2) and pth_accept(3) is that that pth_accept(3)
suspends only the execution of the current thread and not the whole process.
For more details about the arguments and return code semantics see accept(2).
This is a variant of the 4.2BSD select(2) function. It examines the I/O
descriptor sets whose addresses are passed in rfds, wfds, and efds to
see if some of their descriptors are ready for reading, are ready for writing,
or have an exceptional condition pending, respectively. For more details
about the arguments and return code semantics see select(2).
int pth_poll(struct pollfd *fds, unsigned int nfd, int timeout);
This is a variant of the SysV poll(2) function. It examines the I/O
descriptors which are passed in the array fds to see if some of them are
ready for reading, are ready for writing, or have an exceptional condition
pending, respectively. For more details about the arguments and return code
semantics see poll(2).
This is a variant of the POSIX read(2) function. It reads up to nbytes
bytes into buf from file descriptor fd. The difference between read(2)
and pth_read(2) is that that pth_read(2) suspends execution of the current
thread until the file descriptor is ready for reading. For more details about
the arguments and return code semantics see read(2).
ssize_t pth_readv(int fd, const struct iovec *iovec, int iovcnt);
This is a variant of the POSIX readv(2) function. It reads data from
file descriptor fd into the first iovcnt rows of the iov vector. The
difference between readv(2) and pth_readv(2) is that that pth_readv(2)
suspends execution of the current thread until the file descriptor is ready for
reading. For more details about the arguments and return code semantics see
readv(2).
This is a variant of the POSIX write(2) function. It writes nbytes bytes
from buf to file descriptor fd. The difference between write(2) and
pth_write(2) is that that pth_write(2) suspends execution of the current
thread until the file descriptor is ready for writing. For more details about
the arguments and return code semantics see write(2).
ssize_t pth_writev(int fd, const struct iovec *iovec, int iovcnt);
This is a variant of the POSIX writev(2) function. It writes data to
file descriptor fd from the first iovcnt rows of the iov vector. The
difference between writev(2) and pth_writev(2) is that that pth_writev(2)
suspends execution of the current thread until the file descriptor is ready for
reading. For more details about the arguments and return code semantics see
writev(2).
This is a variant of the POSIX pread(3) function. It performs the same action
as a regular read(2), except that it reads from a given position in the file
without changing the file pointer. The first three arguments are the same as
for pth_read(3) with the addition of a fourth argument offset for the
desired position inside the file.
This is a variant of the POSIX pwrite(3) function. It performs the same
action as a regular write(2), except that it writes to a given position in the
file without changing the file pointer. The first three arguments are the same
as for pth_write(3) with the addition of a fourth argument offset for the
desired position inside the file.
ssize_t pth_recv(int fd, void *buf, size_t nbytes, int flags);
This is a variant of the SUSv2 recv(2) function and equal to
``pth_recvfrom(fd, buf, nbytes, flags, NULL, 0)''.
This is a variant of the SUSv2 recvfrom(2) function. It reads up to
nbytes bytes into buf from file descriptor fd while using
flags and from/fromlen. The difference between recvfrom(2) and
pth_recvfrom(2) is that that pth_recvfrom(2) suspends execution of the
current thread until the file descriptor is ready for reading. For more
details about the arguments and return code semantics see recvfrom(2).
ssize_t pth_send(int fd, const void *buf, size_t nbytes, int flags);
This is a variant of the SUSv2 send(2) function and equal to
``pth_sendto(fd, buf, nbytes, flags, NULL, 0)''.
This is a variant of the SUSv2 sendto(2) function. It writes nbytes
bytes from buf to file descriptor fd while using flags and
to/tolen. The difference between sendto(2) and pth_sendto(2) is
that that pth_sendto(2) suspends execution of the current thread until
the file descriptor is ready for writing. For more details about the
arguments and return code semantics see sendto(2).
EXAMPLE
The following example is a useless server which does nothing more than
listening on TCP port 12345 and displaying the current time to the
socket when a connection was established. For each incoming connection a
thread is spawned. Additionally, to see more multithreading, a useless
ticker thread runs simultaneously which outputs the current time to
`stderr' every 5 seconds. The example contains no error checking and
is only intended to show you the look and feel of Pth.
/* the stderr time ticker thread */
static void *ticker(void *_arg)
{
time_t now;
char *ct;
float load;
for (;;) {
pth_sleep(5);
now = time(NULL);
ct = ctime(&now);
ct[strlen(ct)-1] = '\0';
pth_ctrl(PTH_CTRL_GETAVLOAD, &load);
printf("ticker: time: %s, average load: %.2f\n", ct, load);
}
}
/* the main thread/procedure */
int main(int argc, char *argv[])
{
pth_attr_t attr;
struct sockaddr_in sar;
struct protoent *pe;
struct sockaddr_in peer_addr;
int peer_len;
int sa, sw;
int port;
In this section we will discuss the canonical ways to establish the build
environment for a Pth based program. The possibilities supported by Pth
range from very simple environments to rather complex ones.
Manual Build Environment (Novice)
As a first example, assume we have the above test program staying in the
source file `foo.c'. Then we can create a very simple build environment by
just adding the following `Makefile':
This imports the necessary compiler and linker flags on-the-fly from the
Pth installation via its `pth-config' program. This approach is
straight-forward and works fine for small projects.
Autoconf Build Environment (Advanced)
The previous approach is simple but unflexible. First, to speed up
building, it would be nice to not expand the compiler and linker flags
every time the compiler is started. Second, it would be useful to
also be able to build against an uninstalled Pth, that is, against
a Pth source tree which was just configured and built, but not
installed. Third, it would be also useful to allow checking of the
Pth version to make sure it is at least a minimum required version.
And finally, it would be also great to make sure Pth works correctly
by first performing some sanity compile and run-time checks. All this
can be done if we use GNU autoconf and the `AC_CHECK_PTH' macro
provided by Pth. For this, we establish the following three files:
First we again need the `Makefile', but this time it contains autoconf
placeholders and additional cleanup targets. And we create it under the name
`Makefile.in', because it is now an input file for autoconf:
Because autoconf generates additional files, we added a canonical
`distclean' target which cleanups this, too. Second, we write
a (minimalistic) autoconf script specification in a file
`configure.in':
$ vi configure.in
| AC_INIT(Makefile.in)
| AC_CHECK_PTH(1.3.0)
| AC_OUTPUT(Makefile)
Then we let autoconf's `aclocal' program generate for us an `aclocal.m4'
file containing Pth's `AC_CHECK_PTH' macro. Then we generate the final
`configure' script out of this `aclocal.m4' file and the `configure.in'
file:
$ aclocal --acdir=`pth-config --acdir`
$ autoconf
After these steps, the working directory should look similar to this:
$ ls -l
-rw-r--r-- 1 rse users 176 Nov 3 11:11 Makefile.in
-rw-r--r-- 1 rse users 15314 Nov 3 11:16 aclocal.m4
-rwxr-xr-x 1 rse users 52045 Nov 3 11:16 configure
-rw-r--r-- 1 rse users 63 Nov 3 11:11 configure.in
-rw-r--r-- 1 rse users 4227 Nov 3 11:11 foo.c
If we now run `configure' we get a correct `Makefile' which
immediately can be used to build `foo' (assuming that Pth is already
installed somewhere, so that `pth-config' is in `$PATH'):
$ ./configure
creating cache ./config.cache
checking for gcc... gcc
checking whether the C compiler (gcc ) works... yes
checking whether the C compiler (gcc ) is a cross-compiler... no
checking whether we are using GNU C... yes
checking whether gcc accepts -g... yes
checking how to run the C preprocessor... gcc -E
checking for GNU Pth... version 1.3.0, installed under /usr/local
updating cache ./config.cache
creating ./config.status
creating Makefile
rse@en1:/e/gnu/pth/ac
$ make
gcc -g -O2 -I/usr/local/include -c foo.c
gcc -L/usr/local/lib -o foo foo.o -lpth
If Pth is installed in non-standard locations or `pth-config'
is not in `$PATH', one just has to drop the `configure' script
a note about the location by running `configure' with the option
`--with-pth='dir (where dir is the argument which was used with
the `--prefix' option when Pth was installed).
Autoconf Build Environment with Local Copy of Pth (Expert)
Finally let us assume the `foo' program stays under either a GPL or
LGPL distribution license and we want to make it a stand-alone package for
easier distribution and installation. That is, we don't want that the
end-user first has to install Pth just to allow our `foo' package to
compile. For this, it is a convenient practice to include the required
libraries (here Pth) into the source tree of the package (here `foo').
Pth ships with all necessary support to allow us to easily achieve this
approach. Say, we want Pth in a subdirectory named `pth/' and this
directory should be seamlessly integrated into the configuration and build
process of `foo'.
First we again start with the `Makefile.in', but this time it is a more
advanced version which supports subdirectory movement:
Here we provided a default value for `foo''s `--with-pth' option as the
second argument to `AC_CHECK_PTH' which indicates that Pth can be found in
the subdirectory named `pth/'. Additionally we specified that the
`--disable-tests' option of Pth should be passed to the `pth/'
subdirectory, because we need only to build the Pth library itself. And we
added a `AC_CONFIG_SUBDIR' call which indicates to autoconf that it should
configure the `pth/' subdirectory, too. The `AC_CONFIG_AUX_DIR' directive
was added just to make autoconf happy, because it wants to find a
`install.sh' or `shtool' script if `AC_CONFIG_SUBDIRS' is used.
Now we let autoconf's `aclocal' program again generate for us an
`aclocal.m4' file with the contents of Pth's `AC_CHECK_PTH' macro.
Finally we generate the `configure' script out of this `aclocal.m4'
file and the `configure.in' file.
$ aclocal --acdir=`pth-config --acdir`
$ autoconf
Now we have to create the `pth/' subdirectory itself. For this, we extract the
Pth distribution to the `foo' source tree and just rename it to `pth/':
Optionally to reduce the size of the `pth/' subdirectory, we can strip down
the Pth sources to a minimum with the striptease feature:
$ cd pth
$ ./configure
$ make striptease
$ cd ..
After this the source tree of `foo' should look similar to this:
$ ls -l
-rw-r--r-- 1 rse users 709 Nov 3 11:51 Makefile.in
-rw-r--r-- 1 rse users 16431 Nov 3 12:20 aclocal.m4
-rwxr-xr-x 1 rse users 57403 Nov 3 12:21 configure
-rw-r--r-- 1 rse users 129 Nov 3 12:21 configure.in
-rw-r--r-- 1 rse users 4227 Nov 3 11:11 foo.c
drwxr-xr-x 2 rse users 3584 Nov 3 12:36 pth
$ ls -l pth/
-rw-rw-r-- 1 rse users 26344 Nov 1 20:12 COPYING
-rw-rw-r-- 1 rse users 2042 Nov 3 12:36 Makefile.in
-rw-rw-r-- 1 rse users 3967 Nov 1 19:48 README
-rw-rw-r-- 1 rse users 340 Nov 3 12:36 README.1st
-rw-rw-r-- 1 rse users 28719 Oct 31 17:06 config.guess
-rw-rw-r-- 1 rse users 24274 Aug 18 13:31 config.sub
-rwxrwxr-x 1 rse users 155141 Nov 3 12:36 configure
-rw-rw-r-- 1 rse users 162021 Nov 3 12:36 pth.c
-rw-rw-r-- 1 rse users 18687 Nov 2 15:19 pth.h.in
-rw-rw-r-- 1 rse users 5251 Oct 31 12:46 pth_acdef.h.in
-rw-rw-r-- 1 rse users 2120 Nov 1 11:27 pth_acmac.h.in
-rw-rw-r-- 1 rse users 2323 Nov 1 11:27 pth_p.h.in
-rw-rw-r-- 1 rse users 946 Nov 1 11:27 pth_vers.c
-rw-rw-r-- 1 rse users 26848 Nov 1 11:27 pthread.c
-rw-rw-r-- 1 rse users 18772 Nov 1 11:27 pthread.h.in
-rwxrwxr-x 1 rse users 26188 Nov 3 12:36 shtool
Now when we configure and build the `foo' package it looks similar to this:
$ ./configure
creating cache ./config.cache
checking for gcc... gcc
checking whether the C compiler (gcc ) works... yes
checking whether the C compiler (gcc ) is a cross-compiler... no
checking whether we are using GNU C... yes
checking whether gcc accepts -g... yes
checking how to run the C preprocessor... gcc -E
checking for GNU Pth... version 1.3.0, local under pth
updating cache ./config.cache
creating ./config.status
creating Makefile
configuring in pth
running /bin/sh ./configure --enable-subdir --enable-batch
--disable-tests --cache-file=.././config.cache --srcdir=.
loading cache .././config.cache
checking for gcc... (cached) gcc
checking whether the C compiler (gcc ) works... yes
checking whether the C compiler (gcc ) is a cross-compiler... no
[...]
$ make
===> pth (all)
./shtool scpp -o pth_p.h -t pth_p.h.in -Dcpp -Cintern -M '==#==' pth.c
pth_vers.c
gcc -c -I. -O2 -pipe pth.c
gcc -c -I. -O2 -pipe pth_vers.c
ar rc libpth.a pth.o pth_vers.o
ranlib libpth.a
<=== pth
gcc -g -O2 -Ipth -c foo.c
gcc -Lpth -o foo foo.o -lpth
As you can see, autoconf now automatically configures the local
(stripped down) copy of Pth in the subdirectory `pth/' and the
`Makefile' automatically builds the subdirectory, too.
SYSTEM CALL WRAPPER FACILITY
Pth per default uses an explicit API, including the system calls. For
instance you've to explicitly use pth_read(3) when you need a thread-aware
read(3) and cannot expect that by just calling read(3) only the current thread
is blocked. Instead with the standard read(3) call the whole process will be
blocked. But because for some applications (mainly those consisting of lots of
third-party stuff) this can be inconvenient. Here it's required that a call
to read(3) `magically' means pth_read(3). The problem here is that such
magic Pth cannot provide per default because it's not really portable.
Nevertheless Pth provides a two step approach to solve this problem:
Soft System Call Mapping
This variant is available on all platforms and can always be enabled by
building Pth with `--enable-syscall-soft'. This then triggers some
`#define''s in the `pth.h' header which map for instance read(3) to
pth_read(3), etc. Currently the following functions are mapped: fork(2),
sleep(3), sigwait(3), waitpid(2), system(3), select(2), poll(2),
connect(2), accept(2), read(2), write(2), recv(2), send(2), recvfrom(2),
sendto(2).
The drawback of this approach is just that really all source files
of the application where these function calls occur have to include
`pth.h', of course. And this also means that existing libraries,
including the vendor's stdio, usually will still block the whole
process if one of its I/O functions block.
Hard System Call Mapping
This variant is available only on those platforms where the syscall(2)
function exists and there it can be enabled by building Pth with
`--enable-syscall-hard'. This then builds wrapper functions (for instances
read(3)) into the Pth library which internally call the real Pth
replacement functions (pth_read(3)). Currently the following functions
are mapped: fork(2), sleep(3), waitpid(2), system(3), select(2),
poll(2), connect(2), accept(2), read(2), write(2).
The drawback of this approach is that it depends on syscall(2) interface
and prototype conflicts can occur while building the wrapper functions
due to different function signatures in the vendor C header files.
But the advantage of this mapping variant is that the source files of
the application where these function calls occur have not to include
`pth.h' and that existing libraries, including the vendor's stdio,
magically become thread-aware (and then block only the current thread).
IMPLEMENTATION NOTES
Pth is very portable because it has only one part which perhaps has
to be ported to new platforms (the machine context initialization). But
it is written in a way which works on mostly all Unix platforms which
support makecontext(2) or at least sigstack(2) or sigaltstack(2) [see
`pth_mctx.c' for details]. Any other Pth code is POSIX and ANSI C
based only.
The context switching is done via either SUSv2 makecontext(2) or POSIX
make[sig]setjmp(3) and [sig]longjmp(3). Here all CPU registers, the
program counter and the stack pointer are switched. Additionally the
Pth dispatcher switches also the global Unix `errno' variable [see
`pth_mctx.c' for details] and the signal mask (either implicitly via
sigsetjmp(3) or in an emulated way via explicit setprocmask(2) calls).
The Pth event manager is mainly select(2) and gettimeofday(2) based,
i.e., the current time is fetched via gettimeofday(2) once per context
switch for time calculations and all I/O events are implemented via a
single central select(2) call [see `pth_sched.c' for details].
The thread control block management is done via virtual priority
queues without any additional data structure overhead. For this, the
queue linkage attributes are part of the thread control blocks and the
queues are actually implemented as rings with a selected element as the
entry point [see `pth_tcb.h' and `pth_pqueue.c' for details].
Most time critical code sections (especially the dispatcher and event
manager) are speeded up by inlined functions (implemented as ANSI C
pre-processor macros). Additionally any debugging code is completely
removed from the source when not built with `-DPTH_DEBUG' (see Autoconf
`--enable-debug' option), i.e., not only stub functions remain [see
`pth_debug.h' for details].
RESTRICTIONS
Pth (intentionally) provides no replacements for non-thread-safe
functions (like strtok(3) which uses a static internal buffer) or
synchronous system functions (like gethostbyname(3) which doesn't
provide an asynchronous mode where it doesn't block). When you want to
use those functions in your server application together with threads,
you've to either link the application against special third-party
libraries (or for thread-safe/reentrant functions possibly against an
existing `libc_r' of the platform vendor). For an asynchronous DNS
resolver library use the GNU adns package from Ian Jackson ( see
http://www.gnu.org/software/adns/adns.html ).
HISTORY
The Pth library was designed and implemented between February and
July 1999 by Ralf S. Engelschall after evaluating numerous (mostly
preemptive) thread libraries and after intensive discussions with
Peter Simons, Martin Kraemer, Lars Eilebrecht and Ralph
Babel related to an experimental (matrix based) non-preemptive C++
scheduler class written by Peter Simons.
Pth was then implemented in order to combine the non-preemptive
approach of multithreading (which provides better portability and
performance) with an API similar to the popular one found in Pthread
libraries (which provides easy programming).
So the essential idea of the non-preemptive approach was taken over from
Peter Simons scheduler. The priority based scheduling algorithm was
suggested by Martin Kraemer. Some code inspiration also came from
an experimental threading library (rsthreads) written by Robert
S. Thau for an ancient internal test version of the Apache webserver.
The concept and API of message ports was borrowed from AmigaOS' Exec
subsystem. The concept and idea for the flexible event mechanism came
from Paul Vixie's eventlib (which can be found as a part of
BIND v8).
BUG REPORTS AND SUPPORT
If you think you have found a bug in Pth, you should send a report as
complete as possible to [email protected]. If you can, please try to
fix the problem and include a patch, made with '`diff -u3'', in your
report. Always, at least, include a reasonable amount of description in
your report to allow the author to deterministically reproduce the bug.
For further support you additionally can subscribe to the
[email protected] mailing list by sending an Email to
[email protected] with ``subscribe pth-users'' (or
``subscribe pth-users'address' if you want to subscribe
from a particular Email address) in the body. Then you can
discuss your issues with other Pth users by sending messages to
[email protected]. Currently (as of August 2000) you can reach about
110 Pth users on this mailing list. Old postings you can find at
http://www.mail-archive.com/[email protected]/.
