C Programming

General-purpose, imperative, low-level, memory management, compiled

"C was originally developed at Bell Labs by Dennis Ritchie between 1972 and 1973 to make utilities running on Unix" -Wiki

Control Statements

If-statements
While-loops
Do-whiles
For-loops
Switch-statements
Gotos

if ( 1 && !0 ){
    printf("Non-zero is true\n");
} else {
    printf("!0 is 1\n");
}

while ("Is this non-zero?") {
    printf("Who thinks yes?\n");
}

int i; /* C99 */
for (; i < 10; i++) {
    print("What is wrong with this?\n");
}

/* What might this snipped be used for? */
switch(ast_node->type) {
    case INT:
        return atoi(ast_node->data);
    case PLUS:
        int a = process(ast_node->left);
        int b = process(ast_node->right);
        return a + b
    default:
        printf("There was an error\n");
        exit(1); /* What does exit 1 mean? */
}

C:
B:
    if (a) {
        goto A;
    }
A: 
    if (b) {
        goto B;
    }
    a = ~a;
    b = ~b;
    goto C;

Pointers & Dynamic Memory

What are pointers used for in C?

referencing memory on the stack or heap
referencing arrays
extra return values

What are pointers used for in Java?

Objects, Objects, Objects

//stack memory, how many bytes are used?
int a[10] = {42, 0}; 

//pointer to stack memory
int *pa = a;

//pointer to heap memory
int *b = malloc(sizeof(int)*10); 

//pointer to heap memory pointing to pointers,
// how many bytes are used?
int **c = malloc(sizeof(int*)*3);

c[0] = b;
c[1] = b;
c[2] = a+1;

**(c+2) = 72;

printf("What is c[2][0]? %d\n", c[2][0]);
printf("What is c[2][-1]? %d\n", c[2][-1]);

Referencing memory on the stack or heap

Arrays are contiguous blocks of memory. Static 2D Arrays are contiguous, but not Dynamic 2D Arrays

Pointers & Dynamic Memory

These data structures and how they are represented in memory are important!

int foo(int x, int y, int *z) {
    if (x > 0 && y > 0){
        *z = x*y;
        return SUCCESS;
    }
    return FAILURE;
}

int main() {
    int a = -1;
    if (foo(1, 2, &a) == FAILURE) {
        return 1;
    }

    printf("%d\n", a);
    return a;
}

How do you return multiple items from a function?

Sometimes one return value is not enough, so pointer as arguments can help receive more data from a function.

Pointers & Dynamic Memory

Stack and Heap

What is the Stack?

What is the Heap?

*if you know where to look

Memory used to separate function frames for local memory usage
Starts at a high address and grows down

Dynamic memory that is globally accessible*
Must be allocated & freed manually

C experts now?

/* 1. What is wrong here? */
int *baz(){
    int i = 10;
    return &i;
}

/* 2. Is this valid? */
int *bar() {
    static int i = 5;
    return &i;
}

/* 3. Is this valid? */
int *foo() {
    void *yeet = malloc(sizeof(double)*10);
    return (int *)yeet;
}

/* 4. Is this valid? */
int main() {
    char *yeett = (char *)foo();
    printf(yeett);
    
    /* 5. Missing something? */
    return 0;
}

Why do we care about details like these?

What does the stack look like?

int *foo(int c, int d) {
    char e;
    void *yeet = malloc(sizeof(c)*d);
    /* Stop! */
    return (int *)yeet;
}

int main(int argc, char *argv[]) {
    int a = 5, 3;
    int b = 7;
    char *bar = foo((b,a),b);
    
    return 0;
}

High

Low

argv

argc

ret addr

old base

ret addr

old base

junk

heap addr