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RedHat 9 (Linux i386) - man page for pclose (redhat section 3)

POPEN(3)			    Linux Programmer's Manual				 POPEN(3)

       popen, pclose - process I/O

       #include <stdio.h>

       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 containing a shell command
       line.  This command is passed to /bin/sh using the -c flag;  interpretation,  if  any,  is
       performed  by the shell.  The mode 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().  Writing to such a stream writes to the
       standard input of the command; the command's 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 command's standard output, and  the	command's
       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.

       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 returns an error, or some other error is detected.

       The  popen  function  does  not	set  errno if memory allocation fails.	If the underlying
       fork() or pipe() fails, errno is set appropriately.  If the mode 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 process has done a buffered  read,	the  com-
       mand's 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 shell's failure to execute com-
       mand, or an immediate exit of the command.  The only hint is an exit status of 127.

       A popen() and a pclose() function appeared in Version 7 AT&T UNIX.

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

BSD MANPAGE				    1998-05-07					 POPEN(3)

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