#define COLUMNS as a function argument in C

Question

It is more question than a problem. I make my header file with a function declaration like this:

void fw_iteracja_wsk_rows_a(float (*tab)[COLUMNS], int ROWS);

How can I adjust/change the COLUMNS value depending on the array size in other files?

Currently, no matter what I try, COLUMNS takes the value from the header file, making the function not universal.

To be clear im using VS2022 with my travel to learn C. This is a definition of the function:

void fw_iteracja_wsk_rows_a(float (*tab)[COLUMNS], int ROWS)
{
    float (*p_rows)[COLUMNS];
    float* p_columns;
    p_rows = tab;
    p_columns = *p_rows;
    for (p_rows; p_rows < tab + ROWS; p_rows++)
    {
        for (p_columns; p_columns < *p_rows + COLUMNS; p_columns++)
        {
            printf("%.1f, ", *p_columns);
        }
        printf("\n");
    }
}

Declaration is in file: C_PATMY_2DARRAY.h Also in this file is: #define COLUMNS 12. Definition is in file: C_PATMY_2DARRAY.c

When I #include "C_PATMY_2DARRAY.h in other .c file and try to use this function with array with diffrent columns number. I cant becouse function is getting COLUMNS value form header file. I did try #ifndef #undef but it is not working.

Grateful for the tip.

@babon - i can agree with overcomplicating. But, did it becouse i need this pointer on array: float (*tab)[COLUMNS] for being able to make iteration with pointers.

Maybe someone can tell me how to declare universal functions using a pointer to an array. When I need to specify the number of columns in the declaration and definition.

Hi! This is a really important time to share your code with us so we can understand the issue. (Please be sure to use proper formatting). It is not a silly question, but it is hard to know what the problem is without the code. Thanks ... — greg spears
– greg spears, Commented Apr 25 at 20:32
Looks like you are overcomplicating things. Define the function as void fw_iteracja_wsk_rows_a(float *tab, int COLUMNS, int ROWS). Let the caller pass the number of columns and rows. Inside fw_iteracja_wsk_rows_a manually calculate the offset of each element in the array and print it. — babon
– babon, Commented Apr 26 at 8:59
The duplicate shows how to do what you're trying to do. If not, explain why. — dbush
– dbush, Commented Apr 26 at 15:59
You can pass the number of columns as an argument to the function call, and change the tab parameter to be a pointer to a variably modified array (VMA) like this: void fw_iteracja_wsk_rows_a(int ROWS, int COLUMNS, float (*tab)[COLUMNS]);. However note that support for pointers to VMAs is optional in some versions of the C standard. You may need to rename the COLUMNS parameter if you have a macro of the same name in scope. — Ian Abbott
– Ian Abbott, Commented Apr 28 at 10:36

Lundin · Accepted Answer · 2025-04-28 10:56:08Z

2

The normal way in standard C would be to not use a fixed constant, but to pass the array sizes along as parameters:

void func (int rows, int cols, float tab[rows][cols])
{
  ...
  tab[i][j] = something;
}

And since array parameters get adjusted to a pointer to the first element, that's 100% equivalent to:

void func (int rows, int cols, float (*tab)[cols]);

This works on any decent C compiler released after year 1999.

The next problem is that VS2022 is not a decent C compiler and it is using an older standard even than the one from year 1999. So it won't support modern standard C.

On such very old and outdated compilers, you would have to resort to "mangled 2D arrays", ie represented as a 1D array:

void func (int rows, int cols, float* tab)
{
 ...  tab[i*cols + j] = something;
}

Alternatively investigate how to set VS to use gcc or clang instead of outdated MSVC.

answered Apr 28 at 10:56

Lundin

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1 Comment

Patryk M Apr 29 at 18:23

This was the answer I couldn't find anywhere. gcc is unfortunately not an easy thing to adapt in VS2022. Other way clang I installed the packages, changed the project properties and it's there. But that's where the easy part ends, unfortunately I have to learn it. Right now i will stay with MSVC i need to know him better.

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