C -> ASM

  • "Programming" means "writing C code".
  • On Linux-like (UNIX, *nix, POSIX) systems, the operating system API is primarily exposed to C programs through the system C library.
  • The hardware doesn't run C though - it runs amd64 machine code (on your laptop) or ARM machine code (on your phone) or maybe some other machine code.
  • Machine code is for machines, not humans, so it's hard to read.
  • Machine code is a series of instructions. If you write the instructions down as text, you get assembly language.
  • To run a C program, you need to translate to machine code (or "binary").
  • Conceptually, and historically, you first translate C -> ASM, then ASM -> binary.
  • You can still do this if you explicitly ask for it.

Note: For the first few homeworks you will be writing ASM programs. You should not have a compiler do this for you. Submitting compiler output for an assembly assignment is worth zero points.


// A C program is a collection of functions.
// Here's a minimal program with one function
main(int argc, char* argv[])
  printf("Hello C program\n");
  return 0;

# Direct C => binary
$ gcc -o hello hello.c
$ ./hello

# C => asm
$ gcc -S -o hello.s hello.c
# take a look at hello.s

# asm => binary
$ gcc -o hello hello.s
$ ./hello

Interesting stuff in hello.s:

  • The string is there, but no newline.
  • The main function exists
    • Starts at label "main"
    • Ends at "ret".
    • Declared ".globl"
  • In the main function another function is called - not printf, but puts.
  • The optimizer got to us.

Let's tell it to be less clever:

# C => asm
$ gcc -fno-builtin -S -o hello.s hello.c
# take a look at hello.s
  • Now the string has a newline.
  • And the function called is "printf".

How about with two functions:

// add1.c
add1(long x)
    return x + 1;

main(int _ac, char* _av[])
  // initial _ marks args as not used
    long x = add1(5);
    printf("%ld\n", x);
    return 0;
# C => asm
$ gcc -S -o add1.s add1.c
# take a look at hello.s
  • Two functions: add1, main
    • each starts at label, ends at "ret"
  • In main, the value 5 is moved to "%rdi"
    • That must be where the function's first argument goes.
    • No, that's "%edi"
    • I said "%rdi", wait a second...
  • Then add1 is called
  • In add1, the value from %rdi goes to some places.
  • Eventually, "addq $1, ..." happens to it.
  • Back in main, %rax is moved to %rsi, and printf is called.

This almost makes sense, but it's a bit of a mess. Let's figure it out.