CODEWITHSUNDEEP
assembly
Riolku
Riolku

Reputation: 712

What is the difference between calling "ret" vs calling the sys_exit number assembly gcc

In gcc assembly, the main function can either return or exit, where both work. Here I have two programs, where one exits with a syscall int $0x80, and the other simply calls ret. What is the difference? 

.data
hello: .string "Hello, World!"
.globl main
main:
  push %rbx
  movq $hello, %rdi
  call puts
  pop %rbx

  ret

and

.data
hello: .string "Hello, World!"
.globl main
main:
  push %rbx
  movq $hello, %rdi
  call puts
  pop %rbx

  movq $1, %rax
  movq $0, %rbx
  int $0x80

I am aware that ret pops the instruction pointer off the stack, but what does that really do in this case?

Upvotes: 1

Views: 1166

Answers (1)

fuz
fuz

Reputation: 93172

The code that calls main looks like this:

int status = main(argc, argv, envp);
exit(status);

if main returns, exit(status) is executed. exit is a C library function which flushes all stdio streams, invokes atexit() handlers and finally calls _exit(status), which is the C wrapper for the SYS_exit system call. If you use the C runtime (e.g. by having your program start at main or by using any libc functions), I strongly recommend you to never call SYS_exit directly so the C runtime has a chance to correctly deinitialize the program. The best idea is usually to call exit() or to return from main unless you know exactly what you are doing.

Upvotes: 5

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