Tuesday, April 27, 2010

Reading from the commandline in C

What's the best way to read the output from a commandline command in C? If you use the int system(char*) function you receive back a return code, not the output. Not much use. To receive the output you could call freopen on STDOUT, then read from the file you redirected it to. But when you close the file (which you must), STDOUT is left in a broken state. There has to be a better way, and there is. You use popen, instead of system. Then, the output of the command is supplied via the FILE handle returned by popen. Here's an example that finds out the memory usage of the firefox process under Linux:

#include <stdio.h>
#include <string.h>
static char sysCmd[256];
 * Issue a system command to get memory usage
 * @return the percent value times 100
int get_memory()
    int res = 0;
    float percent;
    sysCmd[0] = 0;
    strcat( sysCmd, "ps aux | awk '{if ($11 ~ /" );
    strcat( sysCmd, "firefox" );
    strcat( sysCmd, "/&& !/awk/) {print $4}}'" );
    FILE *CONSOLE = popen( sysCmd, "r" );
    if ( CONSOLE != NULL )
        res = fscanf( CONSOLE, "%f", &percent );
        if ( res <=0 )
            syslog(LOG_ALERT,"Failed to parse result of %s\n",sysCmd);
        pclose( CONSOLE );
        res = (int)(percent * 100.0f);
        syslog(LOG_ALERT,"Failed to open pipe for %s\n",sysCmd);
    return res;

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