Is it legal to use memset() function on `const` array?

Question

In the following code, elements of the const array are cleared by the memset function.

#include <stdio.h>
#include <string.h>

int main() {
    const int a[3] = {1, 2, 3};
    memset(a, 0, sizeof(a));
    printf("%d %d %d\n",a[0],a[1],a[2]);
    return 0;
}

Is it legal to use memset on const array?

Your compiler should be giving you a warning that arg1 to memset has a const mismatch. — lockcmpxchg8b, Nov 21 '17 at 05:34
There are already dozens of questions about modifying a const object, we don't need a new one for each possible modification function. Are you going to ask about `strncpy` onto a const char array next? — M.M, Nov 21 '17 at 05:45
@lockcmpxchg8b that is _if_ you read the warnings. lost of people consider that noise :) — Jean-François Fabre, Nov 21 '17 at 05:55
why `const int a[3] = {1, 2, 3};` and not `const int a[] = {1, 2, 3};` ? — Jean-François Fabre, Nov 21 '17 at 05:56
@Jean-FrançoisFabre "lost of people consider that noise" that's their own damn problem and they are responsible for fixing it themselves. — n. m. could be an AI, Nov 21 '17 at 06:03
@rsp you should operate your compiler in standard-conforming mode. There is no sense in running the compiler in non-standard mode and then trying to use passages from the standard to explain the behaviour — M.M, Nov 21 '17 at 06:03
@rsp let me inyroduce you to some compiler flags. @rspthis is `-Wall -Werror`. `-Wall -Werror` this is @rsp. Hope you will be friends. — n. m. could be an AI, Nov 21 '17 at 06:08
this is a self-answered question. It's ballsy to do that in C, but I suppose OP knows that the operation is illegal. — Jean-François Fabre, Nov 21 '17 at 06:15
GCC generates diagnostics in `-pedantic` mode already for the constraint violation as the `const int *` is assigned to non-const `void *`. You need an *explicit cast here*. It is legal to explicitly cast `const int *` to `void *`, but it is not legal then to change anything through that pointer. — Antti Haapala -- Слава Україні, Nov 21 '17 at 06:44

msc · Accepted Answer · 2017-11-21T06:49:09.693

5

No.

Do not attempt to modify the contents of the array declared as const, otherwise result is undefined behavior.

In that example, the elements of the const int a[3]; are filled by the call to memset which is generated warning because the memset function takes a (non-const) pointer to void, the compiler must implicitly cast away const.

C11 6.7.3 Type qualifiers:

Footnote 132:

The implementation may place a const object that is not volatile in a read-only region of storage. Moreover, the implementation need not allocate storage for such an object if its address is never used.

edited Nov 21 '17 at 06:49

answered Nov 21 '17 at 05:33

msc

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5

The quoted passages are irrelevant as the code contains a constraint violation – M.M Nov 21 '17 at 05:41

Is it legal to use memset() function on `const` array?

1 Answers1