The
ptrace()
system call provides a means by which a parent process may observe
and control the execution of another process,
and examine and change its core image and registers.
It is primarily used to implement breakpoint debugging and system
call tracing.
The parent can initiate a trace by calling
fork(2)
and having the resulting child do a
PTRACE_TRACEME,
followed (typically) by an
exec(3).
Alternatively, the parent may commence trace of an existing process using
PTRACE_ATTACH.
While being traced, the child will stop each time a signal is delivered,
even if the signal is being ignored.
(The exception is
SIGKILL,
which has its usual effect.)
The parent will be notified at its next
wait(2)
and may inspect and modify the child process while it is stopped.
The parent then causes the child to continue,
optionally ignoring the delivered signal
(or even delivering a different signal instead).
When the parent is finished tracing, it can terminate the child with
PTRACE_KILL
or cause it to continue executing in a normal, untraced mode
via
PTRACE_DETACH.
The value of request determines the action to be performed:
PTRACE_TRACEME
Indicates that this process is to be traced by its parent.
Any signal
(except
SIGKILL)
delivered to this process will cause it to stop and its
parent to be notified via
wait(2).
Also, all subsequent calls to
execve(2)
by this process will cause a
SIGTRAP
to be sent to it,
giving the parent a chance to gain control before the new program
begins execution.
A process probably shouldn't make this request if its parent
isn't expecting to trace it.
(pid, addr, and data are ignored.)
The above request is used only by the child process;
the rest are used only by the parent.
In the following requests, pid specifies the child process
to be acted on.
For requests other than
PTRACE_KILL,
the child process must
be stopped.
PTRACE_PEEKTEXT, PTRACE_PEEKDATA
Reads a word at the location
addr
in the child's memory, returning the word as the result of the
ptrace()
call.
Linux does not have separate text and data address spaces, so the two
requests are currently equivalent.
(The argument data is ignored.)
PTRACE_PEEKUSER
Reads a word at offset
addr
in the child's USER area,
which holds the registers and other information about the process
(see <linux/user.h> and <sys/user.h>).
The word is returned as the result of the
ptrace()
call.
Typically the offset must be word-aligned, though this might vary by
architecture.
See NOTES.
(data is ignored.)
PTRACE_POKETEXT, PTRACE_POKEDATA
Copies the word
data
to location
addr
in the child's memory.
As above, the two requests are currently equivalent.
PTRACE_POKEUSER
Copies the word
data
to offset
addr
in the child's USER area.
As above, the offset must typically be word-aligned.
In order to maintain the integrity of the kernel,
some modifications to the USER area are disallowed.
PTRACE_GETREGS, PTRACE_GETFPREGS
Copies the child's general purpose or floating-point registers,
respectively, to location data in the parent.
See <linux/user.h> for information on
the format of this data.
(addr is ignored.)
PTRACE_GETSIGINFO (since Linux 2.3.99-pre6)
Retrieve information about the signal that caused the stop.
Copies a siginfo_t structure (see
sigaction(2))
from the child to location data in the parent.
(addr is ignored.)
PTRACE_SETREGS, PTRACE_SETFPREGS
Copies the child's general purpose or floating-point registers,
respectively, from location data in the parent.
As for
PTRACE_POKEUSER,
some general
purpose register modifications may be disallowed.
(addr is ignored.)
PTRACE_SETSIGINFO (since Linux 2.3.99-pre6)
Set signal information.
Copies a siginfo_t structure from location data in the
parent to the child.
This will only affect signals that would normally be delivered to
the child and were caught by the tracer.
It may be difficult to tell
these normal signals from synthetic signals generated by
ptrace()
itself.
(addr is ignored.)
PTRACE_SETOPTIONS (since Linux 2.4.6; see BUGS for caveats)
Sets ptrace options from data in the parent.
(addr is ignored.)
data is interpreted
as a bit mask of options, which are specified by the following flags:
PTRACE_O_TRACESYSGOOD (since Linux 2.4.6)
When delivering syscall traps, set bit 7 in the signal number
(i.e., deliver (SIGTRAP | 0x80)
This makes it easy for the tracer to tell the difference
between normal traps and those caused by a syscall.
(PTRACE_O_TRACESYSGOOD
may not work on all architectures.)
PTRACE_O_TRACEFORK (since Linux 2.5.46)
Stop the child at the next
fork(2)
call with SIGTRAP | PTRACE_EVENT_FORK << 8 and automatically
start tracing the newly forked process,
which will start with a
SIGSTOP.
The PID for the new process can be retrieved with
PTRACE_GETEVENTMSG.
PTRACE_O_TRACEVFORK (since Linux 2.5.46)
Stop the child at the next
vfork(2)
call with SIGTRAP | PTRACE_EVENT_VFORK << 8 and automatically start
tracing the newly vforked process, which will start with a
SIGSTOP.
The PID for the new process can be retrieved with
PTRACE_GETEVENTMSG.
PTRACE_O_TRACECLONE (since Linux 2.5.46)
Stop the child at the next
clone(2)
call with SIGTRAP | PTRACE_EVENT_CLONE << 8 and automatically start
tracing the newly cloned process, which will start with a
SIGSTOP.
The PID for the new process can be retrieved with
PTRACE_GETEVENTMSG.
This option may not catch
clone(2)
calls in all cases.
If the child calls
clone(2)
with the
CLONE_VFORK
flag,
PTRACE_EVENT_VFORK
will be delivered instead
if
PTRACE_O_TRACEVFORK
is set; otherwise if the child calls
clone(2)
with the exit signal set to
SIGCHLD,
PTRACE_EVENT_FORK
will be delivered
if
PTRACE_O_TRACEFORK
is set.
PTRACE_O_TRACEEXEC (since Linux 2.5.46)
Stop the child at the next
execve(2)
call with SIGTRAP | PTRACE_EVENT_EXEC << 8.
PTRACE_O_TRACEVFORKDONE (since Linux 2.5.60)
Stop the child at the completion of the next
vfork(2)
call with SIGTRAP | PTRACE_EVENT_VFORK_DONE << 8.
PTRACE_O_TRACEEXIT (since Linux 2.5.60)
Stop the child at exit with SIGTRAP | PTRACE_EVENT_EXIT << 8.
The child's exit status can be retrieved with
PTRACE_GETEVENTMSG.
This stop will be done early during process exit when registers
are still available, allowing the tracer to see where the exit occurred,
whereas the normal exit notification is done after the process
is finished exiting.
Even though context is available, the tracer cannot prevent the exit from
happening at this point.
PTRACE_GETEVENTMSG (since Linux 2.5.46)
Retrieve a message (as an
unsigned long)
about the ptrace event
that just happened, placing it in the location data in the parent.
For
PTRACE_EVENT_EXIT
this is the child's exit status.
For
PTRACE_EVENT_FORK,
PTRACE_EVENT_VFORK
and
PTRACE_EVENT_CLONE
this
is the PID of the new process.
Since Linux 2.6.18, the PID of the new process is also available
for
PTRACE_EVENT_VFORK_DONE.
(addr is ignored.)
PTRACE_CONT
Restarts the stopped child process.
If data is non-zero and not
SIGSTOP,
it is interpreted as a signal to be delivered to the child;
otherwise, no signal is delivered.
Thus, for example, the parent can control
whether a signal sent to the child is delivered or not.
(addr is ignored.)
PTRACE_SYSCALL, PTRACE_SINGLESTEP
Restarts the stopped child as for
PTRACE_CONT,
but arranges for
the child to be stopped at the next entry to or exit from a system call,
or after execution of a single instruction, respectively.
(The child will also, as usual, be stopped upon receipt of a signal.)
From the parent's perspective, the child will appear to have been
stopped by receipt of a
SIGTRAP.
So, for
PTRACE_SYSCALL,
for example, the idea is to inspect
the arguments to the system call at the first stop,
then do another
PTRACE_SYSCALL
and inspect the return value of
the system call at the second stop.
(addr is ignored.)
PTRACE_SYSEMU, PTRACE_SYSEMU_SINGLESTEP (since Linux 2.6.14)
For
PTRACE_SYSEMU,
continue and stop on entry to the next syscall,
which will not be executed.
For
PTRACE_SYSEMU_SINGLESTEP,
do the same
but also singlestep if not a syscall.
This call is used by programs like
User Mode Linux that want to emulate all the child's system calls.
(addr and data are ignored;
not supported on all architectures.)
PTRACE_KILL
Sends the child a
SIGKILL
to terminate it.
(addr and data are ignored.)
PTRACE_ATTACH
Attaches to the process specified in
pid,
making it a traced "child" of the calling process;
the behavior of the child is as if it had done a
PTRACE_TRACEME.
The calling process actually becomes the parent of the child
process for most purposes (e.g., it will receive
notification of child events and appears in
ps(1)
output as the child's parent), but a
getppid(2)
by the child will still return the PID of the original parent.
The child is sent a
SIGSTOP,
but will not necessarily have stopped
by the completion of this call; use
wait(2)
to wait for the child to stop.
(addr and data are ignored.)
PTRACE_DETACH
Restarts the stopped child as for
PTRACE_CONT,
but first detaches
from the process, undoing the reparenting effect of
PTRACE_ATTACH,
and the effects of
PTRACE_TRACEME.
Although perhaps not intended, under Linux a traced child can be
detached in this way regardless of which method was used to initiate
tracing.
(addr is ignored.)
RETURN VALUE
On success,
PTRACE_PEEK*
requests return the requested data,
while other requests return zero.
On error, all requests return -1, and
errno
is set appropriately.
Since the value returned by a successful
PTRACE_PEEK*
request may be -1, the caller must check
errno
after such requests to determine whether or not an error occurred.
ERRORS
EBUSY
(i386 only) There was an error with allocating or freeing a debug
register.
EFAULT
There was an attempt to read from or write to an invalid area in
the parent's or child's memory,
probably because the area wasn't mapped or accessible.
Unfortunately, under Linux, different variations of this fault
will return
EIO
or
EFAULT
more or less arbitrarily.
EINVAL
An attempt was made to set an invalid option.
EIO
request is invalid, or an attempt was made to read from or
write to an invalid area in the parent's or child's memory,
or there was a word-alignment violation,
or an invalid signal was specified during a restart request.
EPERM
The specified process cannot be traced.
This could be because the
parent has insufficient privileges (the required capability is
CAP_SYS_PTRACE);
non-root processes cannot trace processes that they
cannot send signals to or those running
set-user-ID/set-group-ID programs, for obvious reasons.
Alternatively, the process may already be being traced, or be
init(8)
(PID 1).
ESRCH
The specified process does not exist, or is not currently being traced
by the caller, or is not stopped (for requests that require that).
CONFORMING TO
SVr4, 4.3BSD.
NOTES
Although arguments to
ptrace()
are interpreted according to the prototype given,
glibc currently declares
ptrace()
as a variadic function with only the request argument fixed.
This means that unneeded trailing arguments may be omitted,
though doing so makes use of undocumented
gcc(1)
behavior.
init(8),
the process with PID 1, may not be traced.
The layout of the contents of memory and the USER area are quite OS- and
architecture-specific.
The offset supplied, and the data returned,
might not entirely match with the definition of
struct user.
The size of a "word" is determined by the OS variant
(e.g., for 32-bit Linux it is 32 bits, etc.).
Tracing causes a few subtle differences in the semantics of
traced processes.
For example, if a process is attached to with
PTRACE_ATTACH,
its original parent can no longer receive notification via
wait(2)
when it stops, and there is no way for the new parent to
effectively simulate this notification.
When the parent receives an event with
PTRACE_EVENT_*
set,
the child is not in the normal signal delivery path.
This means the parent cannot do
ptrace(PTRACE_CONT)
with a signal or
ptrace(PTRACE_KILL).
kill(2)
with a
SIGKILL
signal can be used instead to kill the child process
after receiving one of these messages.
This page documents the way the
ptrace()
call works currently in Linux.
Its behavior differs noticeably on other flavors of Unix.
In any case, use of
ptrace()
is highly OS- and architecture-specific.
The SunOS man page describes
ptrace()
as "unique and arcane", which it is.
The proc-based debugging interface
present in Solaris 2 implements a superset of
ptrace()
functionality in a more powerful and uniform way.
BUGS
On hosts with 2.6 kernel headers,
PTRACE_SETOPTIONS
is declared
with a different value than the one for 2.4.
This leads to applications compiled with such
headers failing when run on 2.4 kernels.
This can be worked around by redefining
PTRACE_SETOPTIONS
to
PTRACE_OLDSETOPTIONS,
if that is defined.
This page is part of release 3.14 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
