Re: difference between malloc and calloc?

Simon Biber wrote:

Eric Sosman wrote:

    There's one possible advantage I can imagine for calloc() over
malloc(), and that's the opportunity for a tiny bit of sanity-
checking.  Here are two ways you might try to allocate memory to
hold N items of SomeType:

    SomeType *p = malloc(N * sizeof *p);
    SomeType *q = calloc(N, sizeof *q);

Now, if N is so large that multiplying it by sizeof(SomeType)
exceeds the valid range of size_t, the argument in the first
form will "wrap around" and you'll silently request less memory
than you wanted; if the request succeeds you'll proceed merrily
along and try to store N items in too small a space, with the
usually unhappy and sometimes baffling consequences.  The second
form, however, will fail and return NULL so your program will be
alerted that the space was not available; there'll be no silent
error.  However, this seems to me to be a very small advantage,
so I'll stick with my original suggestion: malloc() almost always,
calloc() almost never.



Does this mean that

void *my_calloc1(size_t a, size_t b)
{
  void *p = malloc(a * b);
  if(p) memset(p, 0, a * b);
  return p;
}

would not be a valid implementation of calloc, because of the possibility of overflow? 

    Yes, I think it would be invalid.  The Standard says (in
7.20.3.1)

	The calloc function allocates space for an array of
	/nmemb/ objects, each of whose size is /size/.  [...]

This is subject to the general condition in 7.20.3

	[...] If the space cannot be allocated, a null pointer
	is returned.  [...]

Thus, if calloc() cannot find sufficient space for /nmemb/
objects of /size/ bytes each, it must return NULL.  There is no
special dispensation for overflow of nmemb * size; there is just
the requirement for a NULL returned value.

If calloc needs to check for overflow in a * b, how should it do so? 

    The response by "pete" seems to cover what's needed.

--
Eric Sosman
esosman@xxxxxxxxxxxxxxxxxxx
.

Relevant Pages

  • Re: difference between malloc and calloc?
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  • Re: difference between malloc and calloc?
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    (comp.lang.c)
  • Re: Finding a bit string in another bitstring
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