Improper use of pointers are frequently a source of bugs that can include security bugs or program crashes, most often due to segmentation faults.

Not checking for allocation failures

Memory allocation is not guaranteed to succeed, and may instead return a NULL pointer. Using the returned value, without checking if the allocation is successful, invokes undefined behavior. This usually leads to a crash, but there is no guarantee that a crash will happen so relying on that can also lead to problems.

For example, unsafe way:

struct SomeStruct *s = malloc(sizeof *s);
s->someValue = 0; /* UNSAFE, because s might be a null pointer */

Safe way:

struct SomeStruct *s = malloc(sizeof *s);
if (s)
{
    s->someValue = 0; /* This is safe, we have checked that s is valid */
}

Using literal numbers instead of sizeof when requesting memory

For a given compiler/machine configuration, types have a known size; however, there isn’t any standard which defines that the size of a given type (other than char) will be the same for all compiler/machine configurations. If the code uses 4 instead of sizeof(int) for memory allocation, it may work on the original machine, but the code isn’t necessarily portable to other machines or compilers. Fixed sizes for types should be replaced by sizeof(that_type) or sizeof(*var_ptr_to_that_type).

Non-portable allocation:

int *intPtr = malloc(4*1000);    /* allocating storage for 1000 int */
long *longPtr = malloc(8*1000);  /* allocating storage for 1000 long */

Portable allocation:

int *intPtr = malloc(sizeof(int)*1000);     /* allocating storage for 1000 int */
long *longPtr = malloc(sizeof(long)*1000);  /* allocating storage for 1000 long */

Or, better still:

int *intPtr = malloc(sizeof(*intPtr)*1000);     /* allocating storage for 1000 int */
long *longPtr = malloc(sizeof(*longPtr)*1000);  /* allocating storage for 1000 long */

Memory leaks

Failure to de-allocate memory using free leads to a buildup of non-reusable memory, which is no longer used by the program; this is called a memory leak. Memory leaks waste memory resources and can lead to allocation failures.

Logical errors

All allocations must follow the same pattern:

1. Allocation using malloc (or calloc)