What is aligned memory allocation?

Question

I also want to know whether glibc malloc() does this.

Answer 1

The malloc() documentation says:

[...] the allocated memory that is suitably aligned for any kind of variable.

Which is true for most everything you do in C/C++. However, as pointed out by others, many special cases exist and require a specific alignment. For example, Intel processors support a 256 bit type: __m256, which is most certainly not taken in account by malloc().

Similarly, if you want to allocate a memory buffer for data that is to be paged (similar to addresses returned by mmap(), etc.) then you need a possibly very large alignment which would waste a lot of memory if malloc() was to return buffers always aligned to such boundaries.

Under Linux or other Unix systems, I suggest you use the posix_memalign() function:

int posix_memalign(void **memptr, size_t alignment, size_t size);

This is the most current function that one wants to use for such needs.