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



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       popen -- initiate pipe streams to or from a process

SYNOPSIS
       #include <stdio.h>

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

DESCRIPTION
       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
       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
       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()
       or pipe().

       The following sections are informative.

EXAMPLES
   Using popen() to Obtain aa 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
       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().

       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.1-2017 is present, use the function call:


           sysconf(_SC_2_VERSION)

       (See sysconf()).

RATIONALE
       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
       None.

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

       The Base Definitions volume of POSIX.1-2017, <stdio.h>

       The Shell and Utilities volume of POSIX.1-2017, 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 .

       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                  2017                             POPEN(3)
