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Comments on Is partial allocation of an object Undefined Behavior?

Post

Is partial allocation of an object Undefined Behavior?

+8
−0

Is it valid to partly allocate an object, as long as you only use the allocated part of it?

#include <stdio.h>
#include <stdlib.h>

struct s {
	int i[100];
};

int main(void)
{
	struct s *s;

	s = malloc(50 * sizeof(int));

	s->i[30] = 7;
	printf("%d\n", s->i[30]);

	free(s);
}
alx@debian$ gcc --version | head -n1
gcc (Debian 12.2.0-13) 12.2.0
alx@debian$ clang --version | head -n1
Debian clang version 14.0.6
alx@debian$ clang -Weverything malloc.c -O3
alx@debian$ clang -Weverything malloc.c
alx@debian$ gcc -Wall -Wextra malloc.c -fanalyzer
alx@debian$ gcc -Wall -Wextra malloc.c -fanalyzer -O3
alx@debian$ ./a.out 
7

The compiler don't seem to complain. It seems to work.

I didn't find anything in the standard that makes this code Undefined Behavior. However, I still suspect of it: the last half of the array within the structure is not allocated. Is it well-defined?

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2 comment threads

If `i` was a flexible array member, everything would be fine. (2 comments)
Going the other direction, assigning a struct pointer to a block of memory allocated much larger than... (1 comment)
If `i` was a flexible array member, everything would be fine.
appguru‭ wrote almost 2 years ago

If i was a flexible array member, everything would be fine.

alx‭ wrote about 1 year ago · edited about 1 year ago

Is it? I just came to this question again because of flexible array members.

See https://gustedt.wordpress.com/2011/03/14/flexible-array-member for more context.

The allocation is usually one of these:

malloc(sizeof(s) + sizeof_member(s, fam[0]) * N);
malloc(offsetof(s, fam) + sizeof_member(s, fam[0]) * N)

In some cases, sizeof(s) can be larger than offsetof(s, fam), as the padding can go beyond the FAM (see the link above). In those cases, the allocation with sizeof(s) is wasting a few bytes, and offsetof(s, fam) calculates the exact size that will be used.

But what if you only allocate a small FAM and offsetof(s, fam) + sizeof_member(s, fam[0]) * 1 is smaller than sizeof(s)? It depends on the answer to this question: if allocating less is just fine, then you don't need to worry about; if not, you'll need to resort to

malloc(MAX(sizeof(s), offsetof(s, fam) + sizeof_member(s, fam[0]) * N));

which is really inconvenient.