13.3 Safe Allocation Macros

The standard C library malloc/realloc/calloc/free APIs are prone to a number of common coding errors. The safe-alloc module provides macros that make it easier to avoid many of them. It still uses the standard C allocation functions behind the scenes.

Some of the memory allocation mistakes that are commonly made are

• passing the incorrect number of bytes to malloc, especially when allocating an array,
• fail to check the return value of malloc and realloc for errors,
• forget to fully initialize memory just allocated with malloc,
• duplicate calls to free by forgetting to set the pointer variable to NULL,
• leaking memory in calls to realloc when that call fails.

The safe-alloc module addresses these problems in the following way:

• It defines macros that wrap around the standard C allocation functions. That makes it possible to use the compiler’s knowledge of the size of objects for allocation; it also allows setting pointers passed in as arguments when appropriate.
• It uses return values only for a success/failure error condition flag, and annotates them with GCC’s __warn_unused_result__ attribute.
• It uses calloc instead of malloc.

Macro: int ALLOC (ptr)

Allocate sizeof(*ptr) bytes of memory and store the address of allocated memory in ptr. Fill the newly allocated memory with zeros.

Returns -1 on failure, 0 on success.

Macro: int ALLOC_N (ptr, count)

Allocate an array of count elements, each sizeof(*ptr) bytes long, and store the address of allocated memory in ptr. Fill the newly allocated memory with zeros.

Returns -1 on failure, 0 on success.

Macro: int ALLOC_N_UNINITIALIZED (ptr, count)

Allocate an array of count elements, each sizeof(*ptr) bytes long, and store the address of allocated memory in ptr. The allocated memory is not initialized.

Returns -1 on failure, 0 on success.

Macro: int REALLOC_N (ptr, count)

Reallocate the memory pointed to by ptr to be big enough to hold at least count elements, each sizeof(*ptr) bytes long, and store the address of allocated memory in ptr. If reallocation fails, the ptr variable is not modified.

Returns -1 on failure, 0 on success.

Macro: void FREE (ptr)

Free the memory stored in ptr and set ptr to NULL.