C: Linux command executed by popen() function not showing results
c, linux, popen
Solution
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);
Problem
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?