C, Memory

An Interesting Stack Example

int *makeArray() {
    int answer[5];
    return answer;
}

void setTo(int *array, int length, int value) {
    for(int i=0; i<length; i+=1) 
        array[i] = value;
}

int main(int argc, const char *argv[]) {
    int *a1 = makeArray();
    setTo(a1, 5, -2);
    return 0;
}

The Heap: Requesting Memory

void *malloc(size_t size);

• Ask for size bytes of memory

• Returns a (void *) pointer to the first byte

• It does not know what we will use the space for!

• Does not erase (or zero) the memory it returns

malloc Example

typedef struct student_s {
    const char *name;
    int credits;
} student;

student *enroll(const char *name, int transfer_credits) {
    student *ans = (student *)malloc(sizeof(student));
    ans->name = name;
    ans->credits = transfer_credits;
    return ans;
}

The Heap: Freeing Memory

Freeing memory: free

void free(void *ptr);

• Accepts a pointer returned by malloc

• Marks that memory as no longer in use, available to use later

• You should free() memory to avoid memory leaks