C Introduction

Announcements

Homework 7 due tonight at 11:59pm on Gradescope
Midterm 2 is Friday
- Similar format to midterm 1
- Schedule with SDAC early if needed
- Review session on Wednesday in class

Structures

struct - Structures in C

Act like Java classes, but no methods and all public fields
Stores fields adjacently in memory (but may have padding)
Compiler determines padding, use sizeof() to get size
Name of the resulting type includes word struct

struct foo {
    long a;
    int b;
    short c;
    char d;
};

struct foo x;
x.b = 123;
x.c = 4;

Structure Literals

struct a {
    int b;
    double c;
};

/* Both of the following initialize b to 0 and c to 1.0 */
struct a x = { 0, 1.0 };
struct a y = { .b = 0, .c = 1.0 };

typedef

typedef - give new names to any type!

Fairly common to see several names for same data type to convey intent
Ex: unsigned long may be size_t when used in sizes
Examples:
typedef int Integer;
Integer x = 4;
typedef double ** dpp;
Used with anonymous structs:
typedef struct { int x; double y; } foo;
foo z = { 42, 17.4 };

Struct Example

typedef struct {
    long x;
    long y;
    long *array;
    long length;
} foo;

Struct Example

long sum1(foo arg) {
    long ans = arg.x;
    for(long i = 0; i < arg.length; i += 1)
        ans += arg.y * arg.array[i];
    return ans;
}

sum1:
    movq    8(%rsp), %rax
    movq    32(%rsp), %r8
    testq   %r8, %r8
    jle     LBB0_3
    movq    16(%rsp), %rdx
    movq    24(%rsp), %rsi
    xorl    %edi, %edi
.LBB0_2:
    movq    (%rsi,%rdi,8), %rcx
    imulq   %rdx, %rcx
    addq    %rcx, %rax
    incq    %rdi
    cmpq    %rdi, %r8
    jne     .LBB0_2
.LBB0_3:
    retq

Struct Example

long sum2(foo *arg) {
    long ans = arg->x;
    for(long i = 0; i < arg->length; i += 1)
        ans += arg->y * arg->array[i];
    return ans;
}

sum2:
    movq    (%rdi), %rax
    movq    24(%rdi), %r8
    testq   %r8, %r8
    jle     .LBB1_3
    movq    8(%rdi), %rdx
    movq    16(%rdi), %rsi
    xorl    %edi, %edi
.LBB1_2:
    movq    (%rsi,%rdi,8), %rcx
    imulq   %rdx, %rcx
    addq    %rcx, %rax
    incq    %rdi
    cmpq    %rdi, %r8
    jne     .LBB1_2
.LBB1_3:
    retq

Calling Functions

The C code

long a = f(23, "yes", 34uL);

compiles to

movl $23, %edi
leaq label_of_yes_string, %rsi
movq $34, %rdx
callq f
# %rax is "long a" here

without respect to how f was defined. It is the calling convention, not the type declaration of f, that controls this.

Calling Functions

But, if the C code has access to the type declaration of f, then it might perform some implicit casting first; for example, if we declared

long f(double a, const char *b, double c);

long a = f(23, "yes", 34uL);

then the call would be interpreted by C as having implicit casts in it:

long a = f((double)23, "yes", (double)34uL);

Calling Functions

and the arguments would be passed in floating-point registers, like so:

movl $23, %eax
cvtsi2sd %eax, %xmm0           # first floating-point argument

leaq label_of_yes_string, %rdi # first integer/pointer argument

movl $34, %eax
cvtsi2sd %eax, %xmm1           # second floating-point argument

callq f
# %rax is "long a" here

Function Declaration

int f(int x);

Declaration of the function
Function header
Function signature
Function prototype

We want this in every file that invokes f()

Function Definition

int f(int x) {
return 2130 * x;
}

Definition of the function

We only want this in one .c file

Do not want 2 definitions
Which one should the linker choose?

Header Files

C header files: .h files

Written in C, so look like C
Only put header information in them
- Function headers
- Macros
- typedefs
- struct definitions
Essentially: information for the type checker that does not produce any actual binary
#include the header files in our .c files

Big Picture

Header files

Things that tell the type checker how to work
Do not generate any actual binary

C files

Function definitions and implementation
Include the header files

Including Headers

#include "myfile.h"

Quotes: look for a file where I’m writing code
Our header files

#include <string.h>

Angle brackets: look in the standard place for includes
Code that came with the compiler
Likely in /usr/include