How to allocate and free aligned memory in C

Question

How do you allocate memory that's aligned to a specific boundary in C (e.g., cache line boundary)? I'm looking for malloc/free like implementation that ideally would be as portable as possible --- at least between 32 and 64 bit architectures.

Edit to add: In other words, I'm looking for something that would behave like (the now obsolete?) memalign function, which can be freed using free.

Answer 1

Here is a solution, which encapsulates the call to malloc, allocates a bigger buffer for alignment purpose, and stores the original allocated address just before the aligned buffer for a later call to free.

// cache line #define ALIGN 64  void *aligned_malloc(int size) {     void *mem = malloc(size+ALIGN+sizeof(void*));     void **ptr = (void**)((uintptr_t)(mem+ALIGN+sizeof(void*)) & ~(ALIGN-1));     ptr[-1] = mem;     return ptr; }  void aligned_free(void *ptr) {     free(((void**)ptr)[-1]); } 

Answer 2

Use posix_memalign/free.

int posix_memalign(void **memptr, size_t alignment, size_t size);   void* ptr; int rc = posix_memalign(&ptr, alignment, size); ... free(ptr) 

posix_memalign is a standard replacement for memalign which, as you mention is obsolete.