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Unix Version 7 - man page for popen (v7 section 3S)

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POPEN(3S)										POPEN(3S)

       popen, pclose - initiate I/O to/from a process

       #include <stdio.h>

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

       FILE *stream;

       The  arguments  to popen are pointers to null-terminated strings containing respectively a
       shell command line and an I/O mode, either "r" for reading or "w" for writing.  It creates
       a  pipe between the calling process and the command to be executed.  The value returned is
       a stream pointer that can be used (as appropriate) to write to the standard input  of  the
       command or read from its standard output.

       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.

       pipe(2), fopen(3), fclose(3), system(3), wait(2)

       Popen  returns a null pointer if files or processes cannot be created, or the Shell cannot
       be accessed.

       Pclose returns -1 if stream is not associated with a `popened' command.

       Buffered reading before opening an input filter may leave the standard input of that  fil-
       ter  mispositioned.   Similar problems with an output filter may be forestalled by careful
       buffer flushing, e.g. with fflush, see fclose(3).

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