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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():


       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.   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.



       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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