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.TH POPEN "3P" 2013 "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

\fB

execl(\fIshell path\fP, "sh", "-c", \fIcommand\fP, (char *)0);

.fi \fR

.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

\fB

#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 == \(mi1) {

    /* Error reported by pclose() */

    ...

} else {

    /* Use macros described under wait() to inspect `status' in order

       to determine success/failure of command executed by popen() */

    ...

}

.fi \fR

.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\(hy2008 is

present, use the function call:

.sp

.RS 4

.nf

\fB

sysconf(_SC_2_VERSION)

.fi \fR

.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\(hy2008,

.IR "\fB<stdio.h>\fP"

.P

The Shell and Utilities volume of POSIX.1\(hy2008,

.IR "\fIsh\fR\^"

.SH COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form

from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology

-- Portable Operating System Interface (POSIX), The Open Group Base

Specifications Issue 7, Copyright (C) 2013 by the Institute of

Electrical and Electronics Engineers, Inc and The Open Group.

(This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/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 .