GCC: array type has incomplete element type

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25

I have declared a struct, and I try to pass an array of those structs (as well as a double array of doubles, and an integer) into a function. I get an "array type has incomplete element type" message from gcc when I compile it. What have I got wrong in how I pass the struct into the function?

typedef struct graph_node {
  int X;
  int Y;
  int active;
} g_node;

void print_graph(g_node graph_node[], double weight[][], int nodes);

I have also tried struct g_node graph_node[], but I get the same thing.

2012-04-04 00:15
by Joshua Soileau


30

It's the array that's causing trouble in:

void print_graph(g_node graph_node[], double weight[][], int nodes);

The second and subsequent dimensions must be given:

void print_graph(g_node graph_node[], double weight[][32], int nodes);

Or you can just give a pointer to pointer:

void print_graph(g_node graph_node[], double **weight, int nodes);

However, although they look similar, those are very different internally.

If you're using C99, you can use variably-qualified arrays. Quoting an example from the C99 standard (section §6.7.5.2 Array Declarators):

void fvla(int m, int C[m][m]); // valid: VLA with prototype scope

void fvla(int m, int C[m][m])  // valid: adjusted to auto pointer to VLA
{
    typedef int VLA[m][m];     // valid: block scope typedef VLA
    struct tag {
        int (*y)[n];           // invalid: y not ordinary identifier
        int z[n];              // invalid: z not ordinary identifier
    };
    int D[m];                  // valid: auto VLA
    static int E[m];           // invalid: static block scope VLA
    extern int F[m];           // invalid: F has linkage and is VLA
    int (*s)[m];               // valid: auto pointer to VLA
    extern int (*r)[m];        // invalid: r has linkage and points to VLA
    static int (*q)[m] = &B;   // valid: q is a static block pointer to VLA
}

Question in comments

[...] In my main(), the variable I am trying to pass into the function is a double array[][], so how would I pass that into the function? Passing array[0][0] into it gives me incompatible argument type, as does &array and &array[0][0].

In your main(), the variable should be:

double array[10][20];

or something faintly similar; maybe

double array[][20] = { { 1.0, 0.0, ... }, ... };

You should be able to pass that with code like this:

typedef struct graph_node
{
    int X;
    int Y;
    int active;
} g_node;

void print_graph(g_node graph_node[], double weight[][20], int nodes);

int main(void)
{
    g_node g[10];
    double array[10][20];
    int n = 10;

    print_graph(g, array, n);
    return 0;
}

That compiles (to object code) cleanly with GCC 4.2 (i686-apple-darwin11-llvm-gcc-4.2 (GCC) 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.9.00)) and also with GCC 4.7.0 on Mac OS X 10.7.3 using the command line:

/usr/bin/gcc -O3 -g -std=c99 -Wall -Wextra -c zzz.c
2012-04-04 00:35
by Jonathan Leffler
This is exactly what it was, thanks - Joshua Soileau 2012-04-04 00:38
This is exactly what I needed, it compiles just fine with this. I have another question, in my main(), the variable I am trying to pass into the function is a 'double array[][]', so how would I pass that into the function? Passing array[0][0] into it gives me incompatible argument type, as does &array and &array[0][0] - Joshua Soileau 2012-04-04 00:51
+1 Great answer. It might also be worth mentioning that the function argument double weight[][10] is the same as double (*weight)[10], since array arguments in function declarations are treated as pointers to the first element (which is why you don't need to provide the size of the first dimension) C89 §6.7.1, probably in a similar location in C99 - Timothy Jones 2012-04-04 01:26


6

The compiler needs to know the size of the second dimension in your two dimensional array. For example:

void print_graph(g_node graph_node[], double weight[][5], int nodes);
2012-04-04 00:36
by Ed S.