Understanding gcc -S output
assembly, gcc, x86-64
Solution
Those `.cfisomething` directives result in generation of additional data by the compiler. This data helps traverse the call stack when an instruction causes an exception, so the exception handler (if any) can be found and correctly executed. The call stack information is useful for debugging. This data most probably goes into a separate section of the executable. It's not inserted between the instructions of your code.
`.LFsomething:` are just regular labels that are probably referenced by that extra exception-related data.
`leave` and `ret` are CPU instructions.
`leave` is equivalent to:
movq %rbp, %rsp
popq %rbp
and it undoes the effect of these two instructions
pushq %rbp
movq %rsp, %rbp
and instructions that allocate space on the stack by subtracting something from `rsp`.
`ret` returns from the function. It pops the return address from the stack and jumps to that address. If it was `__libc_start_main()` that called `main()`, then it returns there.
Problem
I did gcc -S on the very complex program below on x86_64: ``` int main() { int x = 3; x = 5; return 0; } ``` And what I got was: ``` .file "main.c" .text .globl main .type main, @function main: .LFB0: .cfi_startproc pushq %rbp .cfi_def_cfa_offset 16 .cfi_offset 6, -16 movq %rsp, %rbp .cfi_def_cfa_register 6 movl $3, -4(%rbp) movl $5, -4(%rbp) movl $0, %eax leave .cfi_def_cfa 7, 8 ret .cfi_endproc .LFE0: .size main, .-main .ident "GCC: (GNU) 4.4.7 20120313 (Red Hat 4.4.7-3)" .section .note.GNU-stack,"",@progbits ``` I was wondering if someone could help me understand the output or refer me to some link explaining. Specifically, What does `cfi ,LFB0,LFE0 , leave` mean? All I could find regarding these is this post but couldn't fully understand what it was for. Also, what does `ret` do in this case? I'm guessing it's returning to `__libc_start_main()` which in turn would call `do_exit()` , is that correct?