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PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of
this interface may differ (consult the corresponding Linux manual page for details of
Linux behavior), or the interface may not be implemented on Linux.
NAME
pclose — 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. 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()
* 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 POSIX.1‐2017 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 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() 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.
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.
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(), historical implementations
of pclose() return -1 without setting errno. To avoid requiring pclose() to set errno in
this case, POSIX.1‐2008 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 POSIX.1‐2008, 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()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.1‐2008, 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 POSIX.1‐2008.
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
fork(), popen(), wait()
The Base Definitions volume of POSIX.1‐2017, <stdio.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1-2017, Standard for Information Technology -- Portable Operating System Interface
(POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .
Any typographical or formatting errors that appear in this page are most likely to have
been introduced during the conversion of the source files to man page format. To report
such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .