get command output in pipe, C for Linux

Question

I need to run a Linux CLI command and get its stdout output from C.

I can use pipe() to create a pipe, then fork/exec, redirecting child's stdout descriptor into the pipe before calling exec(), and reading from the pipe in parent. Plus I'll need to wait on the child.

Is there a simple call to do fork + redirect + exec + wait, like system() does fork + exec + wait, only system() doesn't do the redirect.

There's popen(), which does fork + redirect + exec, but doesn't do wait, so I can't get exit status.

Answer 1

Use popen() and pclose().

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popen() does not actually wait, of course, but reads on the pipe will block until there is data available.

pclose() waits, but calling it prematurely could cut off some output from the forked process. You'll want to determine from the stream when the child is done...

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Possibly already discussed at http://stackoverflow.com/questions/43116/how-can-i-run-an-external-program-from-c-and-parse-its-output

Answer 2

Is this it?

NAME
       popen, pclose - process I/O

SYNOPSIS
       #include <stdio.h>  

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

       int pclose(FILE *stream);

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 be 
either ‘r’ for reading or ‘w’ for writing.

       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().  
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().

Answer 3

Here is what I use:

   /* simply invoke a app, pipe output*/
    pipe = popen(buf, "r" );
    if (pipe == NULL ) {
        printf("invoking %s failed: %s\n", buf, strerror(errno));
        return 1;
    }

    waitfor(10);

    while(!feof(pipe) ) {
        if( fgets( buf, 128, pipe ) != NULL ) {
            printf("%s\n", buf );
        }
    }

    /* Close pipe */
    rc = pclose(pipe);

Answer 4

GLib has a nice function for this -- g_spawn_sync(): http://library.gnome.org/devel/glib/stable/glib-Spawning-Processes.html#g-spawn-sync

For example, to run a command and get its exit status and output:

const char *argv[] = { "your_command", NULL };
char *output = NULL; // will contain command output
GError *error = NULL;
int exit_status = 0;
if (!g_spawn_sync(NULL, argv, NULL, 0, NULL, NULL, 
                  &output, NULL, &exit_status, &error))
{
  // handle error here
}