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popen.3p (7214B)

'\" et
.TH POPEN "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
popen
\(em initiate pipe streams to or from a process
.SH SYNOPSIS
.LP
.nf
#include <stdio.h>
.P
FILE *popen(const char *\fIcommand\fP, const char *\fImode\fP);
.fi
.SH DESCRIPTION
The
\fIpopen\fR()
function shall execute the command specified by the string
.IR command .
It shall create a pipe between the calling program and the executed
command, and shall return a pointer to a stream that can be used to
either read from or write to the pipe.
.P
The environment of the executed command shall be as if a child process
were created within the
\fIpopen\fR()
call using the
\fIfork\fR()
function, and the child invoked the
.IR sh
utility using the call:
.sp
.RS 4
.nf
execl(\fIshell path\fP, "sh", "-c", \fIcommand\fP, (char *)0);
.fi
.P
.RE
.P
where
.IR "shell path"
is an unspecified pathname for the
.IR sh
utility.
.P
The
\fIpopen\fR()
function shall ensure that any streams from previous
\fIpopen\fR()
calls that remain open in the parent process are closed in the new
child process.
.P
The
.IR mode
argument to
\fIpopen\fR()
is a string that specifies I/O mode:
.IP " 1." 4
If
.IR mode
is
.IR r ,
when the child process is started, its file descriptor STDOUT_FILENO
shall be the writable end of the pipe, and the file descriptor
\fIfileno\fR(\fIstream\fR) in the calling process, where
.IR stream
is the stream pointer returned by
\fIpopen\fR(),
shall be the readable end of the pipe.
.IP " 2." 4
If
.IR mode
is
.IR w ,
when the child process is started its file descriptor STDIN_FILENO
shall be the readable end of the pipe, and the file descriptor
\fIfileno\fR(\fIstream\fR) in the calling process, where
.IR stream
is the stream pointer returned by
\fIpopen\fR(),
shall be the writable end of the pipe.
.IP " 3." 4
If
.IR mode
is any other value, the result is unspecified.
.P
After
\fIpopen\fR(),
both the parent and the child process shall be capable of executing
independently before either terminates.
.P
Pipe streams are byte-oriented.
.SH "RETURN VALUE"
Upon successful completion,
\fIpopen\fR()
shall return a pointer to an open stream that can be used to read
or write to the pipe. Otherwise, it shall return a null pointer and
may set
.IR errno
to indicate the error.
.SH ERRORS
The
\fIpopen\fR()
function shall fail if:
.TP
.BR EMFILE
{STREAM_MAX}
streams are currently open in the calling process.
.P
The
\fIpopen\fR()
function may fail if:
.TP
.BR EMFILE
{FOPEN_MAX}
streams are currently open in the calling process.
.TP
.BR EINVAL
The
.IR mode
argument is invalid.
.P
The
\fIpopen\fR()
function may also set
.IR errno
values as described by
.IR "\fIfork\fR\^(\|)"
or
.IR "\fIpipe\fR\^(\|)".
.LP
.IR "The following sections are informative."
.SH EXAMPLES
.SS "Using popen(\|) to Obtain a List of Files from the ls Utility"
.P
The following example demonstrates the use of
\fIpopen\fR()
and
\fIpclose\fR()
to execute the command
.IR ls *
in order to obtain a list of files in the current directory:
.sp
.RS 4
.nf
#include <stdio.h>
\&...
.P
FILE *fp;
int status;
char path[PATH_MAX];
.P
fp = popen("ls *", "r");
if (fp == NULL)
    /* Handle error */;
.P
while (fgets(path, PATH_MAX, fp) != NULL)
    printf("%s", path);
.P
status = pclose(fp);
if (status == -1) {
    /* Error reported by pclose() */
    ...
} else {
    /* Use macros described under wait() to inspect `status\(aq in order
       to determine success/failure of command executed by popen() */
    ...
}
.fi
.P
.RE
.SH "APPLICATION USAGE"
Since open files are shared, a mode
.IR r
command can be used as an input filter and a mode
.IR w
command as an output filter.
.P
Buffered reading before opening an input filter may leave the standard
input of that filter mispositioned. Similar problems with an output
filter may be prevented by careful buffer flushing; for example, with
.IR "\fIfflush\fR\^(\|)".
.P
A stream opened by
\fIpopen\fR()
should be closed by
\fIpclose\fR().
.P
The behavior of
\fIpopen\fR()
is specified for values of
.IR mode
of
.IR r
and
.IR w .
Other modes such as
.IR rb
and
.IR wb
might be supported by specific implementations, but these would not be
portable features. Note that historical implementations of
\fIpopen\fR()
only check to see if the first character of
.IR mode
is
.IR r .
Thus, a
.IR mode
of
.IR "robert the robot"
would be treated as
.IR mode
.IR r ,
and a
.IR mode
of
.IR "anything else"
would be treated as
.IR mode
.IR w .
.P
If the application calls
\fIwaitpid\fR()
or
\fIwaitid\fR()
with a
.IR pid
argument greater than 0, and it still has a stream that was called with
\fIpopen\fR()
open, it must ensure that
.IR pid
does not refer to the process started by
\fIpopen\fR().
.P
To determine whether or not the environment specified in the Shell and Utilities volume of POSIX.1\(hy2017 is
present, use the function call:
.sp
.RS 4
.nf
sysconf(_SC_2_VERSION)
.fi
.P
.RE
.P
(See
.IR "\fIsysconf\fR\^(\|)").
.SH RATIONALE
The
\fIpopen\fR()
function should not be used by programs that have set user (or group)
ID privileges. The
\fIfork\fR()
and
.IR exec
family of functions (except
\fIexeclp\fR()
and
\fIexecvp\fR()),
should be used instead. This prevents any unforeseen manipulation of
the environment of the user that could cause execution of commands not
anticipated by the calling program.
.P
If the original and
\fIpopen\fR()ed
processes both intend to read or write or read and write a common file,
and either will be using FILE-type C functions (\c
\fIfread\fR(),
\fIfwrite\fR(),
and so on), the rules for sharing file handles must be observed (see
.IR "Section 2.5.1" ", " "Interaction of File Descriptors and Standard I/O Streams").
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "Section 2.5" ", " "Standard I/O Streams",
.IR "\fIfork\fR\^(\|)",
.IR "\fIpclose\fR\^(\|)",
.IR "\fIpipe\fR\^(\|)",
.IR "\fIsysconf\fR\^(\|)",
.IR "\fIsystem\fR\^(\|)",
.IR "\fIwait\fR\^(\|)",
.IR "\fIwaitid\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<stdio.h>\fP"
.P
The Shell and Utilities volume of POSIX.1\(hy2017,
.IR "\fIsh\fR\^"
.\"
.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 .