NASM Coding: Basics - Examples

3. _exit(num);

   This is a C syscall for terminating the program's execution. The argument

   num

   contains a number representing the exit status (also called exit code):

   0

   means "Success" (no issues happened during the execution), while any non-zero number represents some error. In particular, every non-zero exit code stands for a different type of error (examples:

   1

   is a generic error code;

   11

   is for segmentation fault; and

   13

   is for permission denied errors - e.g., trying to open a file that you don't own.)

   The

   _exit

   syscall takes only $1$ argument, so it uses only the register

   rdi

   to put the argument in, and

   rax

   for the syscall number. Calling

   _exit

   is pretty similar to calling

   return

   inside the

   main()

   function in C.

   mov       rax, 60                 ; syscall code 60 is for 'exit'
   mov       rdi, 0                  ; exit code 0 is for 'Success'
   syscall                           ; invoke operating system to exit the program