how to capture result from system() in C/C++

Question

Possible Duplicate:
How can I run an external program from C and parse its output?

Hi,

Could someone please tell us how to capture a result when executing system() function ?

Actually I wrote a c++ program that displays the machine's IP address, called "ipdisp" and I want when a sever program executes this ipdisp program, the server captes the display IP address. So, is this possible? if yes, how?

thanks for your replies

Answer 1

Yes, you can do this but you can't use system(), you'll have to use popen() instead. Something like:

FILE *f = popen("ipdisp", "r");
while (!feof(f)) {
    // ... read lines from f using regular stdio functions
}
pclose(f);

Answer 2

Greg is not entirely correct. You can use system, but it's a really bad idea. You can use system by writing the output of the command to a temporary file and then reading the file...but popen() is a much better approach. For example:

#include <stdlib.h>
#include <stdio.h>
void
die( char *msg ) {
    perror( msg );
    exit( EXIT_FAILURE );
}

int
main( void )
{
    size_t len;
    FILE *f;
    int c;
    char *buf;
    char *cmd = "echo foo";
    char *path = "/tmp/output"; /* Should really use mkstemp() */

    len = (size_t) snprintf( buf, 0,  "%s > %s", cmd, path ) + 1;
    buf = malloc( len );
    if( buf == NULL ) die( "malloc");
    snprintf( buf, len, "%s > %s", cmd, path );
    if( system( buf )) die( buf );
    f = fopen( path, "r" );
    if( f == NULL ) die( path );
    printf( "output of command: %s\n", buf );
    while(( c = getc( f )) != EOF )
        fputc( c, stdout );
    return EXIT_SUCCESS;
}

There are lots of problems with this approach...(portability of the syntax for redirection, leaving the file on the filesystem, security issues with other processes reading the temporary file, etc, etc.)