C: Linux command executed by popen() function not showing results

Question

I have the code below that I refer the thread on here to use the popen function

int main(int argc,char *argv[]){    
    FILE* file = popen("ntpdate", "r");
    char buffer[100];
    fscanf(file, "%100s", buffer);
    pclose(file);
    printf("buffer is :%s\n", buffer);
    return 0;
}

It outputs:

21 Apr 03:03:03 ntpdate[4393]: no server can be used, exiting
buffer is:

why printf does not output anything? If I use ls as a command, then printf outputs the ls output. what am I doing wrong ntpdate executing?

If I execute the code below (referring the webpage)

#define COMMAND_LEN 8
#define DATA_SIZE 512

int main(int argc,char *argv[]){


    FILE *pf;
       char command[COMMAND_LEN];
       char data[DATA_SIZE];

       // Execute a process listing
       sprintf(command, "ntpdate");

       // Setup our pipe for reading and execute our command.
       pf = popen(command,"r");

       if(!pf){
         fprintf(stderr, "Could not open pipe for output.\n");
         return;
       }

       // Grab data from process execution
       fgets(data, DATA_SIZE , pf);

       // Print grabbed data to the screen.
       fprintf(stdout, "-%s-\n",data);

       if (pclose(pf) != 0)
           fprintf(stderr," Error: Failed to close command stream \n");

       return 0;
}

I get

21 Apr 03:15:45 ntpdate[5334]: no servers can be used, exiting
-�2}�����"|�4#|�-
 Error: Failed to close command stream 

what are wrongs on the codes above?

Answer 1

Since the output is going to stderr you need to redirect stderr like so:

FILE* file = popen("ntpdate 2>&1", "r");

this will redirect stderr to stdout and so you will see output from both. Second issue fscanf will stop at the first space so you can replace with fgets:

fgets(buffer, 100, file);