PCLOSE(P) POSIX Programmer's Manual PCLOSE(P)NAMEpclose - close a pipe stream to or from a process
SYNOPSIS
#include <stdio.h>
int pclose(FILE *stream);
DESCRIPTION
The pclose() function shall close a stream that was opened by popen(),
wait for the command to terminate, and return the termination status of
the process that was running the command language interpreter. How‐
ever, 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()
* waitpid() 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
IEEE Std 1003.1-2001 that could do one of the above
In any case, pclose() shall not return before the child process created
by popen() has terminated.
If the command language interpreter cannot be executed, the child ter‐
mination status returned by pclose() shall be as if the command lan‐
guage 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() for
the associated stream.
If the argument stream to pclose() is not a pointer to a stream created
by popen(), the result of pclose() 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.
The following sections are informative.
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(), histori‐
cal implementations of pclose() return -1 without setting errno. To
avoid requiring pclose() to set errno in this case,
IEEE Std 1003.1-2001 makes the behavior unspecified. An application
should not use pclose() to close any stream that was not created by
popen().
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 IEEE Std 1003.1-2001, such implementations are not conform‐
ing. 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()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 IEEE Std 1003.1-2001, pclose() must use waitpid(), or
some similar function, instead of wait().
The code sample below illustrates how the pclose() function might be
implemented on a system conforming to IEEE Std 1003.1-2001.
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 ALSOfork() , popen() , waitpid() , the Base Definitions volume of
IEEE Std 1003.1-2001, <stdio.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 PCLOSE(P)
