NAME
pclose — close a
pipe stream to or from a process
SYNOPSIS
int pclose(FILE *stream);
DESCRIPTION
The pclose () function shall close a
stream that was opened by popen(3), wait for the command
to terminate, and return the termination status of the process that was
running the command language interpreter. However, if a call caused the
termination status to be unavailable to pclose (),
then pclose () shall return -1 with
errno set to [ECHILD] to report this situation. This can
happen if the application calls one of the following functions:
- wait(3)
- waitpid(3) with a pid argument less than or equal to 0 or equal to the process ID of the command line interpreter
- Any other function not defined in this volume of POSIX.1-2017 that could do one of the above
In any case, pclose () shall not return
before the child process created by popen(3) has
terminated.
If the command language interpreter cannot be executed,
the child termination status returned by pclose ()
shall be as if the command language interpreter terminated using
exit (127) or
_exit
(127).
The pclose () function shall not affect
the termination status of any child of the calling process other than the
one created by popen(3) for the associated stream.
If the argument stream to
pclose () is not a pointer to a stream created by
popen(3), the result of pclose ()
is undefined.
If a thread is canceled during execution of
pclose (), the behavior is undefined.
RETURN VALUE
Upon successful return, pclose () shall
return the termination status of the command language interpreter.
Otherwise, pclose () shall return -1 and set
errno to indicate the error.
ERRORS
The pclose () function shall fail if:
- [ECHILD]
- The status of the child process could not be obtained, as described above.
EXAMPLES
None.
APPLICATION USAGE
None.
RATIONALE
There is a requirement that pclose () not
return before the child process terminates. This is intended to disallow
implementations that return [EINTR] if a signal is received while waiting.
If pclose () returned before the child terminated,
there would be no way for the application to discover which child used to be
associated with the stream, and it could not do the cleanup itself.
If the stream pointed to by stream was not
created by popen(3), historical implementations of
pclose () return -1 without setting
errno. To avoid requiring pclose
() to set errno in this case, POSIX.1-2017 makes the
behavior unspecified. An application should not use
pclose () to close any stream that was not created
by popen(3).
Some historical implementations of pclose
() either block or ignore the signals SIGINT, SIGQUIT, and SIGHUP while
waiting for the child process to terminate. Since this behavior is not
described for the pclose () function in
POSIX.1-2017, such implementations are not conforming. Also, some historical
implementations return [EINTR] if a signal is received, even though the
child process has not terminated. Such implementations are also considered
non-conforming.
Consider, for example, an application that uses:
popen("command", "r")
to start command, which is part of the same application. The parent writes a prompt to its standard output (presumably the terminal) and then reads from the popen(3) ed stream. The child reads the response from the user, does some transformation on the response (pathname expansion, perhaps) and writes the result to its standard output. The parent process reads the result from the pipe, does something with it, and prints another prompt. The cycle repeats. Assuming that both processes do appropriate buffer flushing, this would be expected to work.
To conform to POSIX.1-2017, pclose () must
use waitpid(3), or some similar function, instead of
wait(3).
The code sample below illustrates how the
pclose () function might be implemented on a system
conforming to POSIX.1-2017.
int pclose(FILE *stream)
{
int stat;
pid_t pid;
pid = <pid for process created for stream by popen()>
(void) fclose(stream);
while (waitpid(pid, &stat, 0) == -1) {
if (errno != EINTR){
stat = -1;
break;
}
}
return(stat);
}
FUTURE DIRECTIONS
None.
SEE ALSO
XBD <stdio.h>
CHANGE HISTORY
First released in Issue 1. Derived from Issue 1 of the SVID.
Issue 7
POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0246 [632] is applied.