popen(3p) — Linux manual page

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.1‐2017 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.1‐2017, stdio.h(0p)

       The Shell and Utilities volume of POSIX.1‐2017, sh(1p)

COPYRIGHT         top

       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(3P)

Pages that refer to this page: stdio.h(0p)awk(1p)pclose(3p)stdin(3p)