Finding locations in machine code (gcc/objdump -d)

Question

If you have a particular line of C code in mind to examine in the machine output, how would you locate it in objdump output. Here is an example

if (cond)
   foo;
   bar();

and I want to see if bar was inlined as I'd like. Or would you use some alternative tool instead of objdump?

Answer 1

You can start objdump using the -S option (like "objdump -Sd a.out"). It will display the sourcecode intermixxed with the assembler code, if the source-files the code was compiled from are available.

Alternatively, you can use the following way:

int main(void) {
    int a = 0;
    asm("#");
    return a;
}

becomes

       .file   "a.c"
        .text
.globl main
        .type   main, @function
main:
        leal    4(%esp), %ecx
        andl    $-16, %esp
        pushl   -4(%ecx)
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %ecx
        subl    $16, %esp
        movl    $0, -8(%ebp)
#APP
# 3 "a.c" 1
        #
# 0 "" 2
#NO_APP
        movl    -8(%ebp), %eax
        addl    $16, %esp
        popl    %ecx
        popl    %ebp
        leal    -4(%ecx), %esp
        ret
        .size   main, .-main
        .ident  "GCC: (GNU) 4.3.2"
        .section        .note.GNU-stack,"",@progbits

Answer 2

You debugger should also let you see source code and matching assembly if you compiled with debug symbols. This is gcc option -g and gdb disass command.

Answer 3

If you're compiling with gcc, you can use -S to generate an assembly file directly. This file usually has some useful information in it, including function names and sometimes line numbers for code (depending on the compile options you use).