Thread: Strange warnings

Hello. I am trying to implement a recursive merge sort, and I've just barely begun. I am getting some warnings:

merge_sort.c:15: warning: ISO C90 forbids variable-size array 'left'
merge_sort.c:15: warning: ISO C90 forbids mixed declarations and code

Code:

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

static void
merge_sort(int *a, int n)
{
    int mid;
    int *y;

    if (n > 1)
        mid = n / 2;

    y = a + mid;

    int left[mid];
    memcpy(left, a, mid);
}

int
main(int argc, char *argv[])
{
    int uo[] = {2, 65, 23, 65, 734, 23, 45, 23, 67};
    int n = sizeof uo / sizeof uo[0];

    merge_sort(uo, n);

    return 0;
}

I am using gcc 4.2.1 and I am compiling with -Wall and -pedantic.

Any ideas?

You have to initialize mid with some value.
You can't just say int mid; than an array of size mid.
Especially if n is 1 or less.

Your compiler is telling you that Variable Length Arrays are not allowed. So the following is incorrect.

Code:

merge_sort(int *a, int n)
{
    int mid;
...
    int left[mid];

Array sizes must be compile time constants.

Also you must declare all your variables before you can start using them. So the following is incorrect.

Code:

    y = a + mid;
 
    int left[mid];

Jim

GCC by default implements the C89/C90 standard (i.e. the one from 1989/1990), which did not allow VLAs. C99 and C11 do allow VLAs, however. All you have to do is tell GCC that you want to use that standard, like so:
-std=c99
There is preliminary support for C11 (and a flag for it), but it's still in the works, and you aren't using C11-specific features, so C99 is fine for your case.

Note, there is also a bug that can cause a serious problem.

mid is not initialized. If n is <= 1, you never set mid, and thus it has some arbitrary garbage value. That can cause problems by making y point to some strange place, problems with your VLA once you enable C99 (imagine mid is negative), and can also cause a seg fault in the memcpy, should mid be a very large number. Make sure it always has a good value before using it.

With the version of the compiler the OP is using I wouldn't recommend trying to use VLA, they are listed as broken on the gcc C status page. With that version 4.2.1 I'd stick with C90.

I would really recommend trying to update your compiler to a more recent version.

Jim

Originally Posted by jimblumberg

With the version of the compiler the OP is using I wouldn't recommend trying to use VLA, they are listed as broken on the gcc C status page. With that version 4.2.1 I'd stick with C90.

Good to know. They still broken in 4.4, but fixed in 4.6.

Originally Posted by jimblumberg

I would really recommend trying to update your compiler to a more recent version.

Yes, I agree.