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POPEN(3S)                                                            POPEN(3S)

       popen, pclose - open or close a pipe (for I/O) from or to a process

       #include <&lt;stdio.h>&gt;

       FILE *popen(command, type)
       char *command, *type;

       FILE *stream;

       The  arguments  to popen() are pointers to null-terminated strings con-
       taining, respectively, a shell command line and an I/O mode,  either  r
       for reading or w for writing.  popen() creates a pipe between the call-
       ing process and the command to be executed.  The value  returned  is  a
       stream  pointer  such  that  one can write to the standard input of the
       command, if the I/O mode is w, by writing to the file stream;  and  one
       can read from the standard output of the command, if the I/O mode is r,
       by reading from the file stream.

       A stream opened by popen() should be closed by  pclose(),  which  waits
       for  the associated process to terminate and returns the exit status of
       the command.

       Because open files are shared, a type r command may be used as an input
       filter,  reading  its standard input (which is also the standard output
       of the process doing the popen()) and providing filtered input  on  the
       stream, and a type w command may be used as an output filter, reading a
       stream of output written to the stream process doing  the  popen()  and
       further  filtering  it  and writing it to its standard output (which is
       also the standard input of the process doing the popen()).

       popen() always calls sh(1), never csh(1).

       csh(1), sh(1), pipe(2V), wait(2V), fclose(3V), fopen(3V), system(3)

       popen() returns a NULL pointer if the pipe or process  cannot  be  cre-
       ated, or if it cannot allocate as much memory as it needs.

       pclose()  returns  -1 if stream is not associated with a `popened' com-

       If the original and `popened' processes concurrently read  or  write  a
       common  file,  neither  should  use buffered I/O, because the buffering
       gets all mixed up.  Similar problems with an output filter may be fore-
       stalled  by  careful  buffer flushing, for instance, with fflush(); see

                                6 October 1987                       POPEN(3S)