When is it more appropriate to use valloc() as opposed to malloc()?

Question

C (and C++) include a family of dynamic memory allocation functions, most of which are intuitively named and easy to explain to a programmer with a basic understanding of memory. malloc() simply allocates memory, while calloc() allocates some memory and clears it eagerly. There are also realloc() and free(), which are pretty self-explanatory.

The manpage for malloc() also mentions valloc(), which allocates (size) bytes aligned to the page border.

Unfortunately, my background isn't thorough enough in low-level intricacies; what are the implications of allocating and using page border-aligned memory, and when is this appropriate as opposed to regular malloc() or calloc()?

Answer 1

The manpage for valloc contains an important note:

The function valloc() appeared in 3.0BSD. It is documented as being obsolete in 4.3BSD, and as legacy in SUSv2. It does not appear in POSIX.1-2001.

valloc is obsolete and nonstandard - to answer your question, it would never be appropriate to use in new code.

While there are some reasons to want to allocate aligned memory - this question lists a few good ones - it is usually better to let the memory allocator figure out which bit of memory to give you. If you are certain that you need your freshly-allocated memory aligned to something, use aligned_alloc (C11) or posix_memalign (POSIX) instead.

Answer 2

Allocations with page alignment usually are not done for speed - they're because you want to take advantage of some feature of your processor's MMU, which typically works with page granularity.

One example is if you want to use mprotect(2) to change the access rights on that memory. Suppose, for instance, that you want to store some data in a chunk of memory, and then make it read only, so that any buggy part of your program that tries to write there will trigger a segfault. Since mprotect(2) can only change permissions page by page (since this is what the underlying CPU hardware can enforce), the block where you store your data had better be page aligned, and its size had better be a multiple of the page size. Otherwise the area you set read-only might include other, unrelated data that still needs to be written.

Or, perhaps you are going to generate some executable code in memory and then want to execute it later. Memory you allocate by default probably isn't set to allow code execution, so you'll have to use mprotect to give it execute permission. Again, this has to be done with page granularity.

Another example is if you want to allocate memory now, but might want to mmap something on top of it later.

So in general, a need for page-aligned memory would relate to some fairly low-level application, often involving something system-specific. If you needed it, you'd know. (And as mentioned, you should allocate it not with valloc, but using posix_memalign, or perhaps an anonymous mmap.)