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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.1‐2008 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.1‐2008, <stdio.h>

       The Shell and Utilities volume of POSIX.1‐2008, 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 .