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Comments on What is malloc's standard-defined behavior with respect to the amount of memory it allocates?
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What is malloc's standard-defined behavior with respect to the amount of memory it allocates?
I recently told a friend that malloc(n) allocates and returns a pointer to a block of at least N bytes of memory, as opposed to exactly N; that it is allowed to allocate 'extra' memory to meet e.g. alignment requirements.
He asked what the C standard had to say about this behaviour. I wasn't sure, so I looked it up, and...I can't find any explicit statement on the subject.
Did I miss something in the standard, or was I wrong to begin with? What is malloc's standard-defined behaviour with respect to the block of memory it returns?
(this discussion was in the context of a single-byte buffer-overrun bug that only manifested when N was a multiple of 8; it certainly looked like malloc was rounding up to the word size, although obviously trying to use any such slop is still a bug)
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Since accessing the memory allocated by malloc beyond the size given to the call is undefined behaviour (which means that the standard poses no restriction to the behaviour of a program that does this), and since there's no standard way to determine the length of the allocated block, malloc is indeed allowed to allocate more memory, since there is no standard conforming way for a program to determine whether the allocation was larger than requested.
Note that the standard does not need to give explicit permission for the compiler/standard library to do so, since it already has implicitly given permission by declaring access of that memory as undefined behaviour.
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