Understand assembly code in c

Question

I'm reading some C code embedded with a few assembly code. I understand that __asm__ is a statement to run assembly code, but what does __asm__ do in the following code? According to the output (i.e., r = 16), it seems that __asm__ does not effect the variable r. Isn't it?

#include <stdio.h>
static void foo()
{
    static volatile unsigned int r __asm__ ("0x0019");
    r |= 1 << 4;

    printf("foo: %u\n", r);
}

Platform: Apple LLVM version 6.0 (clang-600.0.56) (based on LLVM 3.5svn) on OSX Yosemite

Answer 1

Strictly speaking, your "asm" snippet simply loads a constant (0x0019).

Here's a 32-bit example:

#include <stdio.h>
static void foo()
{
    static volatile unsigned int r __asm__ ("0x0019");
    static volatile unsigned int s __asm__ ("0x1122");
    static volatile unsigned int t = 0x3344;
    printf("foo: %u %u %u\n", r, s, t);
}

gcc -O0 -S x.c

cat x.c
        .file   "x.c"
        .data
        .align 4
        .type   t.1781, @object
        .size   t.1781, 4
t.1781:
        .long   13124  # Note: 13124 decimal == 0x3344 hex
        .local  0x1122
        .comm   0x1122,4,4
        .local  0x0019
        .comm   0x0019,4,4
        .section        .rodata
.LC0:
        .string "foo: %u %u %u\n"
        .text
        .type   foo, @function
foo:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $24, %esp
        movl    t.1781, %eax
        movl    0x1122, %edx
        movl    0x0019, %ecx
        movl    %eax, 12(%esp)
        movl    %edx, 8(%esp)
        movl    %ecx, 4(%esp)
        movl    $.LC0, (%esp)
        call    printf
        leave
        ret

PS: The "asm" syntax is applicable to all gcc-based compilers.

PPS: I absolutely encourage you to experiment with assembly anywhere you please: embedded systems, Ubuntu, Mac OSX - whatever pleases you.

Here is an excellent book. It's about Linux, but it's also very largely applicable to your OSX:

Programming from the Ground Up, Jonathan Bartlett

Also:

https://www.hackerschool.com/blog/7-understanding-c-by-learning-assembly

http://fabiensanglard.net/macosxassembly/

PPS: x86 assembly syntax comes in two variants: "Intel" and "ATT" syntax. Gcc uses ATT. The ATT syntax is also applicable for any other architecture supported by GCC (MIPS, PPC, etc etc). I encourage you to start off with ATT syntax ("gcc/gas"), rather than Intel ("nasm").