Interlude: Memory API

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Types of Memory

In C, there are two types of memory: stack memory (automatic memory) where allocations and deallocations are managed by the compiler, and heap memory, where all allocations and deallocations are handled by you.

#include <stdlib.h>
 
int x = 5; // automatic memory
int* y = malloc(sizeof(int)); // heap memory
*y = 5; 

The malloc() Call

Malloc requires an argument, the size of the block of memory you want. This returns a void * pointer, which is returned for you to cast.

The free() Call

Once you malloc memory, you must free it, otherwise this will cause a memory leak.

Common Errors

Forgetting to Allocate Memory

char *src = "hello";
char *dst; // oops! unallocated
strcpy(dst, src); // segfault and die

Not Allocating enough memory

char *src = "hello";
char *dst = (char *) malloc(strlen(src)); // too small!
strcpy(dst, src); // work properly

Forgetting to Free Memory

Freeing Memory Before you are Done with It

Freeing Memory Repeatedly

Calling Free() Incorrectly