Error message "segmentation fault (core dumped)"

Question

I'm trying to run the following C program:

/*
 * Illustrates system call fork
 */

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int
main (
  int argc,
  char *argv[]
  )
{
  int tC, tF;
  pid_t pid;

  tC = atoi (argv[1]);
  tF = atoi (argv[2]);

  fprintf (stdout, "Main  :                  ");
  fprintf (stdout, "PID=%d; My parent PID=%d\n",
    getpid(), getppid());

  pid = fork();
  if (pid == 0){
    // Child
    sleep (tC);

    fprintf(stdout, "Child : PIDreturned=%d    ", pid);
    fprintf (stdout, "PID=%d; My parent PID=%d\n",
      getpid(), getppid());
  } else {
    // Father
    sleep (tF);

    fprintf(stdout, "Father: PIDreturned=%d ", pid);
    fprintf (stdout, "PID=%d; My parent PID=%d\n",
      getpid(), getppid());
  }

  return 0;
}

I'm able to compile the code, but when I try to run the executable file I get a "segmentation fault (core dump)" error message.

Can anyone tell me what is causing such issue and how to fix it?

You are not checking if the user supplied any arguments. If fewer than 2 arguments are supplied to the program, it'll do `atoi` on `argv` out of bounds. — Ted Lyngmo, Jan 24 '23 at 16:11
How do you run your program? Are you sure you pass two arguments? Always check `argc` *first* before using anything other than `argv[0]` (which technically could be `NULL` as well). Other than that, create a debug build (add the `-g` flag when building) and use a [*debugger*](https://stackoverflow.com/questions/25385173/what-is-a-debugger-and-how-can-it-help-me-diagnose-problems) to catch the crash and locate when and where in your code it happens. — Some programmer dude, Jan 24 '23 at 16:14
[cannot reproduce](https://godbolt.org/z/71Kdz47zn) when supplying valid command line arguments. — yano, Jan 24 '23 at 16:17
thx!! @Someprogrammerdude the issue was exactly that! I wasn't passing the right no. of arguments when running the executable. Also, thx for the tip, I'll make sure to check argc in the code! — Marco Sperindeo, Jan 24 '23 at 19:02

score 5 · Accepted Answer · answered Jan 24 '23 at 16:15

You are not checking if the user supplied any arguments. If fewer than 2 arguments are supplied to the program, it'll do atoi on argv out of bounds (which has undefined behavior and the program may crash as a result).

I suggest that you check it directly at start of main. Example:

int main(int argc, char* argv[]) {
    if(argc != 3) {
        fprintf(stderr, "USAGE: %s child-sleeptime parent-sleeptime\n", argv[0]);
        return 1;
    }

    // ... the rest of `main`

thank you, that was exactly the issue, I wasn't passing the right no. of arguments when running the executable! — Marco Sperindeo, Jan 24 '23 at 19:04

Error message "segmentation fault (core dumped)"

1 Answers1