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popen(3)							      popen(3)

NAME
       popen - Initiates a pipe to a process

SYNOPSIS
       #include <stdio.h>

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

LIBRARY
       Standard C Library (libc)

STANDARDS
       Interfaces  documented on this reference page conform to industry stan‐
       dards as follows:

       popen():	 XPG4, XPG4-UNIX

       Refer to the standards(5) reference page	 for  more  information	 about
       industry standards and associated tags.

PARAMETERS
       Points  to  a  null-terminated  string containing a shell command line.
       Points to a null-terminated string containing an I/O mode.

DESCRIPTION
       The popen() function creates a pipe between the calling program	and  a
       shell  command to be executed. It returns a pointer to a FILE structure
       for the stream.

       If the type parameter is the value r, the calling program can read from
       the  standard  output  of the command by reading from the returned file
       stream. If the type parameter is the value w, the calling  program  can
       write  to  the standard input of the command by writing to the returned
       file stream.

       Because open files are shared, a type r command can be used as an input
       filter and a type w command as an output filter.

NOTES
       Programs	 using	the popen() function to invoke an output filter should
       beware of possible deadlock caused by output data remaining in the pro‐
       gram's  buffer.	This can be avoided by either using the setbuf() func‐
       tion to ensure that the output stream is unbuffered, or	by  using  the
       fflush()	 function  to  ensure that all buffered data is flushed before
       calling the pclose() function.

				       Caution

       If the original processes and the  process  started  with  the  popen()
       function	 concurrently  read or write a common file, neither should use
       buffered I/O. If they do, the results are unpredictable.

RETURN VALUES
       Upon successful completion, the popen() function returns a  pointer  to
       the  FILE  structure  for  the  opened stream. In case of error because
       files or processes could not be created, the popen() function returns a
       null pointer.

SEE ALSO
       Functions:  exec(2),  fork(2), fclose(3), fopen(3), pclose(3), pipe(2),
       setbuf(3)

       Standards: standards(5)

								      popen(3)
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