popen()

Updated: October 28, 2024

Execute a command, creating a pipe to it

Synopsis:

#include <stdio.h>

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

Arguments:

command
The command that you want to execute.
mode
The I/O mode for the pipe, which must be "r" or "w"; see below.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The popen() function executes the command specified by command and creates a pipe between the calling process and the executed command.

Depending on the mode argument, you can use the returned stream pointer to read from or write to the pipe.

The executed command has the same environment as its parents. The command is started as follows:

spawnlp (P_NOWAIT, shell_command, shell_command, 
         "-c", command, (char* )NULL );

where shell_command is the command specified by the SHELL environment variable (if it exists), or the sh utility.

The mode argument to popen() is a string that specifies an I/O mode for the pipe:

Note: Use pclose() to close a stream that you used popen() to open.

Returns:

A non-NULL stream pointer on successful completion. If popen() is unable to create either the pipe or the subprocess, it returns a NULL stream pointer and sets errno.

Errors:

EINVAL
The mode argument is invalid.
ENOSYS
There's no pipe manager running.

The popen() function may also set errno values as described by the pipe() and spawnl() functions.

Examples:

/*
 * upper:  executes a given program, converting all input
 *      to upper case.
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <unistd.h>
#include <limits.h>

char   buffer[_POSIX_ARG_MAX];

int main( int argc, char** argv )
  {
    int  i;
    int  c;
    FILE* f;

    for( i = 1; i < argc; i++ ) {
      strcat( buffer, argv[i] );
      strcat( buffer, " " );
    }
    if( ( f = popen( buffer, "w" ) ) == NULL ) {
      perror( "popen" );
      return EXIT_FAILURE;
    }
    while( ( c = getchar() ) != EOF ) {
      if( islower( c ) )
        c = toupper( c );
      putc( c, f );
    }
    pclose( f );
    return EXIT_SUCCESS;
  }

Classification:

POSIX 1003.1

Safety:  
Cancellation point Yes
Interrupt handler No
Signal handler No
Thread Yes