popen(3S)                                               popen(3S)



     NAME
          popen, pclose - initiate pipe to/from a process

     SYNOPSIS
          #include <stdio.h>

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

          int pclose(stream)
          FILE *stream;

     DESCRIPTION
          The arguments to popen are pointers to null-terminated
          strings; one string contains a shell command line and the
          other contains an I/O mode.  The mode may be either ``r''
          for reading or ``w'' for writing.  popen creates a pipe
          between the calling program and the command to be executed.
          The value returned is a stream pointer.  If the I/O mode is
          w, one can write to the standard input of the command by
          writing to the file stream; if the I/O mode is r, one can
          read from the standard output of the command, 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 and a type ``w'' as an output
          filter.

     RETURN VALUE
          popen returns a NULL pointer if files or processes cannot be
          created.

          pclose returns -1 if stream is not associated with a command
          opened by popen.

     SEE ALSO
          pipe(2), wait(2), fclose(3S), fopen(3S), system(3S).

     BUGS
          If the original processes and processes opened by popen con-
          currently read or write a common file, neither should use
          buffered I/O, because the buffering gets all mixed up.
          Problems with an output filter may be forestalled by careful
          buffer flushing, for example, by using fflush (see
          fclose(3S)).

          If an illegal type is passed, popen will fork and exec the
          command line passed to it before it discovers that the type



     April, 1990                                                     1

     popen(3S)                                               popen(3S)



          was illegal.  This will result in a NULL pointer being re-
          turned and a broken pipe (with the command executing in the
          background).




















































     2                                                     April, 1990