### 2.24 `malloc`, `realloc`, `free`—manage memory

Synopsis

    #include <stdlib.h>
    void *malloc(size_t nbytes);
    void *realloc(void *aptr, size_t nbytes);
    void *reallocf(void *aptr, size_t nbytes);
    void free(void *aptr);

    void *memalign(size_t align, size_t nbytes);

    size_t malloc_usable_size(void *aptr);

    void *_malloc_r(void *reent, size_t nbytes);
    void *_realloc_r(void *reent,
        void *aptr, size_t nbytes);
    void *_reallocf_r(void *reent,
        void *aptr, size_t nbytes);
    void _free_r(void *reent, void *aptr);

    void *_memalign_r(void *reent,
        size_t align, size_t nbytes);

    size_t _malloc_usable_size_r(void *reent, void *aptr);

Description
These functions manage a pool of system memory.

Use `malloc` to request allocation of an object with at least nbytes bytes of storage available. If the space is available, `malloc` returns a pointer to a newly allocated block as its result.

If you already have a block of storage allocated by `malloc`, but you no longer need all the space allocated to it, you can make it smaller by calling `realloc` with both the object pointer and the new desired size as arguments. `realloc` guarantees that the contents of the smaller object match the beginning of the original object.

Similarly, if you need more space for an object, use `realloc` to request the larger size; again, `realloc` guarantees that the beginning of the new, larger object matches the contents of the original object.

When you no longer need an object originally allocated by `malloc` or `realloc` (or the related function `calloc`), return it to the memory storage pool by calling `free` with the address of the object as the argument. You can also use `realloc` for this purpose by calling it with `0` as the nbytes argument.

The `reallocf` function behaves just like `realloc` except if the function is required to allocate new storage and this fails. In this case `reallocf` will free the original object passed in whereas `realloc` will not.

The `memalign` function returns a block of size nbytes aligned to a align boundary. The align argument must be a power of two.

The `malloc_usable_size` function takes a pointer to a block allocated by `malloc`. It returns the amount of space that is available in the block. This may or may not be more than the size requested from `malloc`, due to alignment or minimum size constraints.

The alternate functions `_malloc_r`, `_realloc_r`, `_reallocf_r`, `_free_r`, `_memalign_r`, and `_malloc_usable_size_r` are reentrant versions. The extra argument reent is a pointer to a reentrancy structure.

If you have multiple threads of execution which may call any of these routines, or if any of these routines may be called reentrantly, then you must provide implementations of the `__malloc_lock` and `__malloc_unlock` functions for your system. See the documentation for those functions.

These functions operate by calling the function `_sbrk_r` or `sbrk`, which allocates space. You may need to provide one of these functions for your system. `_sbrk_r` is called with a positive value to allocate more space, and with a negative value to release previously allocated space if it is no longer required. See Stubs.

Returns
`malloc` returns a pointer to the newly allocated space, if successful; otherwise it returns `NULL`. If your application needs to generate empty objects, you may use `malloc(0)` for this purpose.

`realloc` returns a pointer to the new block of memory, or `NULL` if a new block could not be allocated. `NULL` is also the result when you use ‘`realloc(aptr,0)`’ (which has the same effect as ‘`free(aptr)`’). You should always check the result of `realloc`; successful reallocation is not guaranteed even when you request a smaller object.

`free` does not return a result.

`memalign` returns a pointer to the newly allocated space.

`malloc_usable_size` returns the usable size.

Portability
`malloc`, `realloc`, and `free` are specified by the ANSI C standard, but other conforming implementations of `malloc` may behave differently when nbytes is zero.

`memalign` is part of SVR4.

`malloc_usable_size` is not portable.

Supporting OS subroutines required: `sbrk`.