Interlude: Memory API

Two Kinds of Program Memory

Variables declared inside a function receive space reserved automatically by the compiler on the call stack; that space vanishes the moment the function returns. Data that must outlive the allocating call has to reside on the heap, where the programmer both requests and later returns the storage.

Calling malloc Correctly

Pass malloc the exact byte count, normally obtained by applying sizeof to a type. Cast the returned void pointer to the needed type. Always test for NULL. When copying a string add one extra byte for the terminator.

Freeing Memory and Typical Pitfalls

Every successful malloc must later be matched by free on the identical pointer. Omitting the allocation entirely, requesting one byte too few, or freeing twice commonly produce crashes or security holes. Many newer languages hide this work behind garbage collection.

Pitfalls

• Passing an uninitialized pointer as the destination of strcpy
• Requesting exactly strlen bytes then writing the terminator as well
• Never calling free after a heap block is finished

Minimum C11 · complete program · Download .c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void) {
    int stack_val = 42;
    printf("Allocated on stack: %d\n", stack_val);
    int *heap_val = (int *)malloc(sizeof(int));
    if (heap_val == NULL) {
        printf("Heap allocation failed\n");
        return 1;
    }
    *heap_val = 99;
    printf("Allocated on heap: %d\n", *heap_val);
    const char *original = "OSTEP";
    char *copy = (char *)malloc(strlen(original) + 1);
    if (copy == NULL) {
        free(heap_val);
        printf("String allocation failed\n");
        return 1;
    }
    strcpy(copy, original);
    printf("String copy: %s\n", copy);
    free(copy);
    free(heap_val);
    printf("All heap memory released\n");
    return 0;
}

Compile locally

gcc -std=c11 -Wall -Wextra -Wpedantic -Werror ostep-14-memory-api.c -o example && ./example

Expected result

Allocated on stack: 42
Allocated on heap: 99
String copy: OSTEP
All heap memory released

CHECK YOUR UNDERSTANDING

Close the answer. Explain it.

Why does sizeof applied to a pointer obtained from malloc report only the pointer width?

Check the sources

• OSTEP chapter PDF

Drafts and official chapters change. The version mark is only the example’s minimum.