Question

I need to pass two addresses (32 bit) to a process spawned by the execl command from C as follows. In the first program:

  char buffer[18];
  sprintf (&buffer[0],"%x\0 %x\0",lock, count);
  arg_list[0]=&(buffer[0]);
  arg_list[1]=&(buffer[10]);
  execl ("/somedirectory/second_process", arg_list[0], arg_list[1], NULL);

Then in my second program I try and parse the addresses as follows:

  if (argc != 2) {
      printf ("PROCESS 2: Invalid number of arguments. Terminating %i\n", argc);
      return -1;
  }

 if ( !(sscanf (argv[1],"%x",&lock)) || !(sscanf (argv[2],"%x",&count)) ) {
      printf ("PROCESS 2: Problem with parameters passed in");
      return -1;

But my program keeps on giving an error saying that the parameters passed in are invalid. When I try and print out the recieved arguments, my program hangs.

 printf ("The arguments passsed in are %s %s", argv[1], argv[2]);

FIXED:

I was making the execl call incorrectly. It should have been:

  execl ("/somedirectory/second_process", "second_process" arg_list[0], arg_list[1], NULL);

Also in process 2:

  if (argc != 3) {
      printf ("PROCESS 2: Invalid number of arguments. Terminating %i\n", argc);
      return -1;
  }

As the first argument passed in is supposed to be that of the process name as Chris below suggested.

Thanks!

Answer 1

Arrays are ALWAYS 0 based in C, so your two arguments will be argv[0] and argv[1], and argv[2] will be a null pointer, so the sscanf will fail, crash, or hang.

Now normally you should always pass the program name as the first (argv[0]) argument. So you want:

execl ("/somedirectory/second_process", "second_process", arg_list[0], arg_list[1], NULL);

that is, you should actually pass THREE arguments (so argc == 3 in the second program).