Malloc

Malloc is a standard library function in the C programming language used to allocate a block of memory from the heap. It is declared as void* malloc(size_t size) and returns a pointer to a block of at least size bytes, or NULL on failure. The contents of the allocated block are indeterminate.

The memory is not initialized; the values are undefined. If zero-initialized storage is required, use calloc or allocate then initialize. To resize an existing block, use realloc; it may move the block to a new location.

The allocated memory must be released with free when it is no longer needed to avoid memory leaks. Freeing a pointer more than once, freeing a pointer that was not obtained from malloc, or using memory after it has been freed yields undefined behavior. Passing NULL to free is a no-op.

Implementation details vary: malloc allocates from the process heap using the allocator supplied by the C standard library. Allocators maintain metadata about blocks and may employ strategies to reduce fragmentation. Overhead per allocation and fragmentation can affect performance and memory usage.

The exact behavior, performance characteristics, and alignment guarantees depend on the C library's malloc implementation. Modern allocators are often thread-safe and may use strategies such as per-thread arenas or segregated free lists to improve concurrency and locality.

Common related functions are calloc, realloc, and free. See also memory management in C.