PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

POPEN(3P)                 POSIX Programmer's Manual                POPEN(3P)

PROLOG         top

       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         top

       popen — initiate pipe streams to or from a process

SYNOPSIS         top

       #include <stdio.h>

       FILE *popen(const char *command, const char *mode);

DESCRIPTION         top

       The popen() function shall execute the command specified by the
       string 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.

       The environment of the executed command shall be as if a child
       process were created within the popen() call using the fork()
       function, and the child invoked the sh utility using the call:

           execl(shell path, "sh", "-c", command, (char *)0);

       where shell path is an unspecified pathname for the sh utility.

       The popen() function shall ensure that any streams from previous
       popen() calls that remain open in the parent process are closed in
       the new child process.

       The mode argument to popen() is a string that specifies I/O mode:

        1. If mode is r, when the child process is started, its file
           descriptor STDOUT_FILENO shall be the writable end of the pipe,
           and the file descriptor fileno(stream) in the calling process,
           where stream is the stream pointer returned by popen(), shall be
           the readable end of the pipe.

        2. If mode is w, when the child process is started its file
           descriptor STDIN_FILENO shall be the readable end of the pipe,
           and the file descriptor fileno(stream) in the calling process,
           where stream is the stream pointer returned by popen(), shall be
           the writable end of the pipe.

        3. If mode is any other value, the result is unspecified.

       After popen(), both the parent and the child process shall be capable
       of executing independently before either terminates.

       Pipe streams are byte-oriented.

RETURN VALUE         top

       Upon successful completion, popen() 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 errno to indicate the error.

ERRORS         top

       The popen() function shall fail if:

       EMFILE {STREAM_MAX} streams are currently open in the calling
              process.

       The popen() function may fail if:

       EMFILE {FOPEN_MAX} streams are currently open in the calling process.

       EINVAL The mode argument is invalid.

       The popen() function may also set errno values as described by
       fork(3p) or pipe(3p).

       The following sections are informative.

EXAMPLES         top

   Using popen() to Obtain a List of Files from the ls Utility
       The following example demonstrates the use of popen() and pclose() to
       execute the command ls* in order to obtain a list of files in the
       current directory:

           #include <stdio.h>
           ...

           FILE *fp;
           int status;
           char path[PATH_MAX];

           fp = popen("ls *", "r");
           if (fp == NULL)
               /* Handle error */;

           while (fgets(path, PATH_MAX, fp) != NULL)
               printf("%s", path);

           status = pclose(fp);
           if (status == −1) {
               /* Error reported by pclose() */
               ...
           } else {
               /* Use macros described under wait() to inspect `status' in order
                  to determine success/failure of command executed by popen() */
               ...
           }

APPLICATION USAGE         top

       Since open files are shared, a mode r command can be used as an input
       filter and a mode w command as an output filter.

       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 fflush(3p).

       A stream opened by popen() should be closed by pclose().

       The behavior of popen() is specified for values of mode of r and w.
       Other modes such as rb and wb might be supported by specific
       implementations, but these would not be portable features. Note that
       historical implementations of popen() only check to see if the first
       character of mode is r.  Thus, a mode of robert the robot would be
       treated as mode r, and a mode of anything else would be treated as
       mode w.

       If the application calls waitpid() or waitid() with a pid argument
       greater than 0, and it still has a stream that was called with
       popen() open, it must ensure that pid does not refer to the process
       started by popen().

       To determine whether or not the environment specified in the Shell
       and Utilities volume of POSIX.1‐2008 is present, use the function
       call:

           sysconf(_SC_2_VERSION)

       (See sysconf(3p)).

RATIONALE         top

       The popen() function should not be used by programs that have set
       user (or group) ID privileges. The fork() and exec family of
       functions (except execlp() and execvp()), 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.

       If the original and popen()ed processes both intend to read or write
       or read and write a common file, and either will be using FILE-type C
       functions (fread(), fwrite(), and so on), the rules for sharing file
       handles must be observed (see Section 2.5.1, Interaction of File
       Descriptors and Standard I/O Streams).

FUTURE DIRECTIONS         top

       None.

SEE ALSO         top

       Section 2.5, Standard I/O Streams, fork(3p), pclose(3p), pipe(3p),
       sysconf(3p), system(3p), wait(3p), waitid(3p)

       The Base Definitions volume of POSIX.1‐2008, stdio.h(0p)

       The Shell and Utilities volume of POSIX.1‐2008, sh(1p)

COPYRIGHT         top

       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 .

IEEE/The Open Group                 2013                           POPEN(3P)