C Hello World Disassembled

C program:

#include <stdio.h>
int main(void) {
    puts("Hello World!\n");
    return 0;
}

Compile without assembling - the preferred stack boundary is set to a single word (2² bytes) to avoid confusing boilerplate stack alignment code:

gcc -S -mpreferred-stack-boundary=2 hello.c

Result:

        .file   "hello.c"
        .section        .rodata   # constant data section (read-only data)
.LC0:                             # constant string declaration
        .string "Hello World!\n"
        .text                     # instruction section
.globl main                       # keep 'main' symbol for linking
        .type   main, @function   # 'main' symbol is a function

main:                             # the next instruction is the start of
                                  # 'main'
        pushl   %ebp              # save caller base pointer
        movl    %esp, %ebp        # callee base pointer = current stack pointer
        subl    $4, %esp          # allocate a single word for string address
        movl    $.LC0, (%esp)     # copy string address to top of stack
                                  # (first and only parameter to puts)
        call    puts              # set return address and jump to puts code
                                  # (which is linked in)
        movl    $0, %eax          # return 0
        leave
        ret                       # return to caller(_start in the C library)
        .size   main, .-main      # sizeof(main) is from 'main' to here
        .section        .note.GNU-stack,"",@progbits
        .ident  "GCC: (GNU) 3.4.6 (Gentoo 3.4.6-r2, ssp-3.4.6-1.0, pie-8.7.10)"