The
popen ();
function
``opens''
a process by creating a bidirectional pipe
forking,
and invoking the shell.
Any streams opened by previous
popen ();
calls in the parent process are closed in the new child process.
Historically,
popen ();
was implemented with a unidirectional pipe;
hence many implementations of
popen ();
only allow the
Fa type
argument to specify reading or writing, not both.
Since
popen ();
is now implemented using a bidirectional pipe, the
Fa type
argument may request a bidirectional data flow.
The
Fa type
argument is a pointer to a null-terminated string
which must be
`r'
for reading,
`w'
for writing, or
`r+'
for reading and writing.
The
Fa command
argument is a pointer to a null-terminated string
containing a shell command line.
This command is passed to
/bin/sh
using the
-c
flag; interpretation, if any, is performed by the shell.
The return value from
popen ();
is a normal standard
I/O
stream in all respects
save that it must be closed with
pclose ();
rather than
fclose (.);
Writing to such a stream
writes to the standard input of the command;
the command's standard output is the same as that of the process that called
popen (,);
unless this is altered by the command itself.
Conversely, reading from a
``popened''
stream reads the command's standard output, and
the command's standard input is the same as that of the process that called
popen (.);
Note that output
popen ();
streams are fully buffered by default.
The
pclose ();
function waits for the associated process to terminate
and returns the exit status of the command
as returned by
wait4(2).
RETURN VALUES
The
popen ();
function returns
NULL
if the
fork(2)
or
pipe(2)
calls fail,
or if it cannot allocate memory.
The
pclose ();
function
returns -1 if
Fa stream
is not associated with a
``popened''
command, if
Fa stream
already
``pclosed''
or if
wait4(2)
returns an error.
ERRORS
The
popen ();
function does not reliably set
errno
A
popen ();
and a
pclose ();
function appeared in
AT&T System
v7 .
Bidirectional functionality was added in
Fx 2.2.6 .
BUGS
Since the standard input of a command opened for reading
shares its seek offset with the process that called
popen (,);
if the original process has done a buffered read,
the command's input position may not be as expected.
Similarly, the output from a command opened for writing
may become intermingled with that of the original process.
The latter can be avoided by calling
fflush(3)
before
popen (.);
Failure to execute the shell
is indistinguishable from the shell's failure to execute command,
or an immediate exit of the command.
The only hint is an exit status of 127.
The
popen ();
function
always calls
sh(1),
never calls
csh(1).
