Lecture Notes: 03 The C Language

More on C #

• A C program is a collection of functions
• Types:
  • Integers: char, short, int, long
  • Floats: float, double
  • Do the sizeof program.
• Pointers:
  • Two global ints (global data)
  • Two local ints (stack)
  • Two functions (text)
  • Two strings (ro data)
  • Array literal; array, elements.
• Describe the memory layout.

Quickly show the add1 program and ASM #

Another example:

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

int
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.

Write average program #

• Read # of its with scanf.
• Allocate stack array with dynamic array.
• Read that # of ints in loop
• Function to calculate average (pointer, size).