# popen

## NAME



           popen, pclose - pipe stream to or from a process
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## 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
                   || /* glibc <= 2.19: */ _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



           **popen**(): on success, returns a pointer to an open stream that can
           be used to read or write to the pipe; if the fork(2) or pipe(2)
           calls fail, or if the function cannot allocate memory, NULL is
           returned.
    
           **pclose**(): on success, returns the exit status of the command; if
           wait4(2) returns an error, or some other error is detected, -1 is
           returned.
    
           On failure, both functions set _errno_ to indicate 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
           to indicate the error.  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 │
           └──────────────────────────────────────┴───────────────┴─────────┘
    


## VERSIONS



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


## STANDARDS



           POSIX.1-2008.
    


## CAVEATS



           Carefully read Caveats in system(3).
    


## 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 the command, or an immediate exit of the
           command.  The only hint is an exit status of 127.
    


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