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       popen, pclose - pipe stream to or from a process


       #include <stdio.h>

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

       int pclose(FILE *stream);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       popen(), pclose(): _POSIX_C_SOURCE >= 2 || _XOPEN_SOURCE || _BSD_SOURCE
       || _SVID_SOURCE


       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 type argument is a pointer to a null-terminated string  which  must
       contain  either  the  letter  'r'  for  reading  or  the letter 'w' for
       writing.  Since glibc 2.9, this argument can additionally  include  the
       letter  'e', which causes the close-on-exec flag (FD_CLOEXEC) to be set
       on the underlying file descriptor; see the description of the O_CLOEXEC
       flag in open(2) for reasons why this may be useful.

       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 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(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.



       The 'e' value for type is a Linux extension.


       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 command’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 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)


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