Question

I have a few reasons to define a type for a fixed length array such as this:

typedef float fixed_array_t[NX][NY];

I then want to pass references to fixed_array_t instances around to other functions. I’m getting a compiler warning from both GCC and CLANG though I’m seeing correct behavior.

What is this compiler warning telling me and how should my code be modified to avoid the warning? Bonus, why do I have to #define the array size? Compile-time constants apparently don’t work. error: variably modified ‘fixed_array_t’ at file scope

Here is a small demonstration code:

#include <stdio.h>
#define NX 2  // also, why does const int NX = 2; not work?
#define NY 3
typedef float fixed_array_t[NX][NY];

void array_printer( const fixed_array_t arr )
{
    int i, j;
    for (i = 0; i < NX; i++ )
        for( j=0; j < NY; j++ )
            printf("Element [%d,%d]=%f\n", i,j, arr[i][j] );
}

int main( int argc, char ** argv )
{
    fixed_array_t testArray = { {1,2,3}, {4,5,6} };
    array_printer( testArray );
}

GCC warning:

warning: passing argument 1 of ‘array_printer’ from incompatible pointer type

CLANG warning (actually compiling equivalent code in OpenCL):

warning: incompatible pointer types passing 'fixed_array_t' (aka 'real [2][3]'), expected 'real const (*)[3]'

Yet program operation is fine:

Element [0,0]=1.000000
Element [0,1]=2.000000
Element [0,2]=3.000000
Element [1,0]=4.000000
Element [1,1]=5.000000
Element [1,2]=6.000000

Answer 1

If you just want the problem solved, you can use a struct to encapsulate your array:

$ cat struct.c ; make CFLAGS=-Wall -Wextra struct ; ./struct
#include <stdio.h>

#define NX 2
#define NY 3

typedef struct fixed {
    float arr[NX][NY];
} fixed_t;

void array_printer( const fixed_t f)
{
    int i, j;
    for (i = 0; i < NX; i++ )
        for( j=0; j < NY; j++ )
            printf("Element [%d,%d]=%f\n", i,j, f.arr[i][j] );
}


int main(int argc, char *argv[]) {
    fixed_t f = {.arr={ {1,2,3}, {4,5,6} }};
    array_printer(f);
    return 0;
}

cc -Wall    struct.c   -o struct
Element [0,0]=1.000000
Element [0,1]=2.000000
Element [0,2]=3.000000
Element [1,0]=4.000000
Element [1,1]=5.000000
Element [1,2]=6.000000

No warnings, no errors, and only slightly more annoying to use (f.arr[][] rather than arr[][] in the array_printer, for example).

If you amend the struct fixed to include the NX and NY dimensions, you could even have multiple different-sized objects in your program. (Though you’d slightly lose the benefits of compile-time-known bounds, I’m not sure how much that really buys you.)