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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 ||


       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  the  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 block 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.  In the event of an error, these functions set
       errno to indicate the cause of the error.


       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.


       For  an  explanation  of  the  terms  used   in   this   section,   see

       │InterfaceAttributeValue   │
       │popen(), pclose() │ Thread safety │ MT-Safe │


       POSIX.1-2001, POSIX.1-2008.

       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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