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pclose.3p (6028B)

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.TH PCLOSE "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH 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.
.\"
.SH NAME
pclose
\(em close a pipe stream to or from a process
.SH SYNOPSIS
.LP
.nf
#include <stdio.h>
.P
int pclose(FILE *\fIstream\fP);
.fi
.SH DESCRIPTION
The
\fIpclose\fR()
function shall close a stream that was opened by
\fIpopen\fR(),
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
\fIpclose\fR(),
then
\fIpclose\fR()
shall return \-1 with
.IR errno
set to
.BR [ECHILD] 
to report this situation. This can happen if the application calls one
of the following functions:
.IP " *" 4
\fIwait\fR()
.IP " *" 4
\fIwaitpid\fR()
with a
.IR pid
argument less than or equal to 0 or equal to the process ID of the
command line interpreter
.IP " *" 4
Any other function not defined in this volume of POSIX.1\(hy2017 that could do one of the above
.P
In any case,
\fIpclose\fR()
shall not return before the child process created by
\fIpopen\fR()
has terminated.
.P
If the command language interpreter cannot be executed, the child
termination status returned by
\fIpclose\fR()
shall be as if the command language interpreter terminated using
.IR exit (127)
or
.IR _exit (127).
.P
The
\fIpclose\fR()
function shall not affect the termination status of any child of the
calling process other than the one created by
\fIpopen\fR()
for the associated stream.
.P
If the argument
.IR stream
to
\fIpclose\fR()
is not a pointer to a stream created by
\fIpopen\fR(),
the result of
\fIpclose\fR()
is undefined.
.P
If a thread is canceled during execution of
\fIpclose\fR(),
the behavior is undefined.
.SH "RETURN VALUE"
Upon successful return,
\fIpclose\fR()
shall return the termination status of the command language
interpreter. Otherwise,
\fIpclose\fR()
shall return \-1 and set
.IR errno
to indicate the error.
.SH ERRORS
The
\fIpclose\fR()
function shall fail if:
.TP
.BR ECHILD
The status of the child process could not be obtained, as described
above.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
None.
.SH "APPLICATION USAGE"
None.
.SH RATIONALE
There is a requirement that
\fIpclose\fR()
not return before the child process terminates. This is intended to
disallow implementations that return
.BR [EINTR] 
if a signal is received while waiting. If
\fIpclose\fR()
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.
.P
If the stream pointed to by
.IR stream
was not created by
\fIpopen\fR(),
historical implementations of
\fIpclose\fR()
return \-1 without setting
.IR errno .
To avoid requiring
\fIpclose\fR()
to set
.IR errno
in this case, POSIX.1\(hy2008 makes the behavior unspecified. An application
should not use
\fIpclose\fR()
to close any stream that was not created by
\fIpopen\fR().
.P
Some historical implementations of
\fIpclose\fR()
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
\fIpclose\fR()
function in POSIX.1\(hy2008, such implementations are not conforming. Also, some
historical implementations return
.BR [EINTR] 
if a signal is received, even though the child process has not
terminated. Such implementations are also considered non-conforming.
.P
Consider, for example, an application that uses:
.sp
.RS 4
.nf
popen("command", "r")
.fi
.P
.RE
.P
to start
.IR 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
\fIpopen\fR()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.
.P
To conform to POSIX.1\(hy2008,
\fIpclose\fR()
must use
\fIwaitpid\fR(),
or some similar function, instead of
\fIwait\fR().
.P
The code sample below illustrates how the
\fIpclose\fR()
function might be implemented on a system conforming to POSIX.1\(hy2008.
.sp
.RS 4
.nf
int pclose(FILE *stream)
{
    int stat;
    pid_t pid;
.P
    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);
}
.fi
.P
.RE
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIfork\fR\^(\|)",
.IR "\fIpopen\fR\^(\|)",
.IR "\fIwait\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<stdio.h>\fP"
.\"
.SH 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 .
.PP
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 .