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.

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 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

       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 .