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POPEN(3)		   Linux Programmers Manual		     POPEN(3)

       popen, pclose - process I/O


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

       int pclose(FILE *stream);

       The  popen()  function opens a process by creating a pipe, forking, and
       invoking the shell.  Since a pipe is by definition unidirectional,  the
       type  argument  may  specify  only  reading  or	writing, not both; the
       resulting stream is correspondingly read-only or write-only.

       The command argument is a pointer to a null-terminated string  contain
       ing  a shell command line.  This command is passed to /bin/sh using the
       -c flag; interpretation, if any, is performed by the shell.   The  type
       argument  is a pointer to a null-terminated string which must be either
       "r" for reading or "w" for writing.

       The return value from popen() is a normal standard I/O  stream  in  all
       respects  save  that  it  must  be  closed  with  pclose()  rather than
       fclose(3).  Writing to such a stream writes to the  standard  input  of
       the  command;  the commands standard output is the same as that of the
       process that called popen(), unless this  is  altered  by  the  command
       itself.	 Conversely,  reading  from  a "popened" stream reads the com
       mands standard output, and the commands standard input is the same as
       that of the process that called popen().

       Note that output popen() streams are fully buffered by default.

       The pclose() function waits for the associated process to terminate and
       returns the exit status of the command as returned by wait4(2).

       The popen() function returns NULL if the fork(2) or pipe(2) calls fail,
       or if it cannot allocate memory.

       The  pclose() function returns -1 if wait4(2) returns an error, or some
       other error is detected.

       The popen() function does not set errno if memory allocation fails.  If
       the  underlying	fork(2)  or pipe(2) fails, errno is set appropriately.
       If the type argument is invalid, and this condition is detected,  errno
       is set to EINVAL.

       If pclose() cannot obtain the child status, errno is set to ECHILD.


       Since  the  standard  input  of a command opened for reading shares its
       seek offset with the process that called popen(), if the original  pro
       cess  has done a buffered read, the commands input position may not be
       as expected.  Similarly, the output from a command opened  for  writing
       may  become intermingled with that of the original process.  The latter
       can be avoided by calling fflush(3) before popen().

       Failure to execute the shell  is  indistinguishable  from  the  shells
       failure	to  execute command, or an immediate exit of the command.  The
       only hint is an exit status of 127.

       sh(1), fork(2),	pipe(2),  wait4(2),  fclose(3),  fflush(3),  fopen(3),
       stdio(3), system(3)

       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

GNU				  1998-05-07			      POPEN(3)

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