Question

how do I intercept calls made from other process which I have called from my process. (say – I call make and I would like to intercept and modify call to gcc from make).

Answer 1

Here is a small example with ptrace:

#include <unistd.h>
#include <sys/ptrace.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <stdio.h>
#include <sys/user.h>
#include <sys/prctl.h>

const char *sys_call_name(long num);

int main()
{
  pid_t pid = fork();
  struct user_regs_struct regs;
  if (!pid) { 
    /* child */
    while (1) { printf("C\n"); sleep(1); }
  }
  else { /* parent */
    int status = 0;
    ptrace(PTRACE_ATTACH, pid, NULL, 0); 
    ptrace(PTRACE_SETOPTIONS, pid, NULL, PTRACE_SYSCALL) ;
    while (1) {
      printf("waiting\n");
      pid = wait(&status);
      /* child gone */
      //if (WIFEXITED(status)) { break; }
      ptrace(PTRACE_GETREGS, pid, 0, &regs);
      /* regs.orig_eax is the system call number */
      printf("A system call: %d : %s\n", regs.orig_eax, sys_call_name(regs.orig_eax));
      /* let child continue */
      ptrace(PTRACE_SYSCALL, pid, NULL, 0); 
    }   
  }
  return 0;
}


const char *sys_call_name(long num) {
  switch(num) {
    case   4: return "write";
    case 162: return "nanosleep";
    case 165: return "getresuid";
    case 174: return "rt_sigaction";
    case 175: return "rt_sigprocmask";
    default:  return "unknown";
  }
}