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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....
#2: Post edited
by
hkotsubo
·
2021-12-21T18:04:23Z (almost 3 years ago)
Removed markdown from title (doesn't work), added tags
What is `malloc`'s standard-defined behavior with respect to the amount of memory it allocates?
- 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)
#1: Initial revision
by
ajv
·
2021-12-21T17:03:01Z (almost 3 years ago)
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)