Warnings of Pointer to array of matrices In ANSI C GCC C90

Question

I'm having strange warnings in Linux. In Visual Studio on Windows, the code compiles and works well but I need to compile it with GCC c90 I get these warnings:

I've initialized the matrix like this:

typedef float mat[4][4];

Now i want to create an array of matrices:

mat MAT_A = { 0 };
mat MAT_B = { 0 };
mat MAT_C = { 0 };
mat *matrices[3] = {MAT_A, MAT_B, MAT_C};

I've declared the function:

void get_input(mat** matrices);

and use it:

get_input(&matrices);

The code is working well but I need to compile it with gcc c90. And I get this warnings:

warning: initialization from incompatible pointer type [-Wincompatible-pointer-types]  
mat *(*matrices)[3] = {MAT_A, MAT_B, MAT_C};                     
                       ^~~~~

and

warning: passing argument 1 of ‘get_input’ from incompatible pointer type [-Wincompatible-pointer-types]
    flag = get_input(&matrices);
                     ^

Answer 1

Due to this typedef declaration

typedef float mat[4][4];

arrays declared like

mat MAT_A = { 0 };

are implicitly converted to its first element in this declaration

mat *matrices[3] = {MAT_A, MAT_B, MAT_C};

of the type float( * )[4]. But the variable matrices is declared as having the element type float ( * )[4][4]. These types are not compatible.

That is in fact you have the following declaration

float ( *matrices[3] )[4][4] = {MAT_A, MAT_B, MAT_C};

You should declare the array eother like

float ( *matrices[3] )[4] = {MAT_A, MAT_B, MAT_C};

In this case the function should be declared like

void get_input( float ( ** matrices)[4] );

and called like

get_input( matrices );

or like

mat *matrices[3] = { &MAT_A, &MAT_B, &MAT_C };

in the last case the function is called like

get_input( matrices );

Also the function get_input declared like

void() get_input(mat** matrices);

actually is equivalent to

void() get_input( float ( ** matrices )[4][4]);

But in this call of the function

get_input(&matrices);

the argument has the type

float( * ( * ) [3])[4][4]

Answer 2

The mat *matrices[3] is an array of 3 pointers to mat. So one needs to initialize it with pointers to mat.

mat *matrices[3] = {&MAT_A, &MAT_B, &MAT_C};

Moreover, when calling get_input(&matrices);, the &matrices is a pointer to an array of 3 pointers to mat. This is not what get_input() expects what is a pointer to a pointer to mat. Just use:

get_input(matrices);

The expression matrices is transformed to a pointer to its first element which is a pointer to mat. As result, the type of the expression is a pointer to a pointer to mat, exactly what get_input expects.

Now the code compiles without a warning. See https://godbolt.org/z/84qd4rrnx

Answer 3

Change

mat *matrices[3] = {MAT_A, MAT_B, MAT_C};

to

mat *matrices[3] = {&MAT_A, &MAT_B, &MAT_C};