NAME

       popen, pclose - pipe stream to or from a process

SYNOPSIS

       #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

DESCRIPTION

       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).

RETURN VALUE

       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.

ERRORS

       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.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────────────┬───────────────┬─────────┐
       │Interface         │ Attribute     │ Value   │
       ├──────────────────┼───────────────┼─────────┤
       │popen(), pclose() │ Thread safety │ MT-Safe │
       └──────────────────┴───────────────┴─────────┘

CONFORMING TO

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

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

BUGS

       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.

SEE ALSO

       sh(1), fork(2), pipe(2), wait4(2), fclose(3), fflush(3), fopen(3), stdio(3), system(3)

COLOPHON

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