Why aren't constants(#define) in C read by the header file?

Question

I am trying to find a way to pass definitions onto a header file for using it when formatting an array. My code is something like this:

#define CONSTANT 10
#include "myHeaderFile.h"    
int main(headerFunction(array)){}

Header file:

int headerFunction(array[][CONSTANT]) { // Multidimensional array
}

It gives me an error that CONSTANT hasn't been set. To be more exact: error: ‘CONSTANT’ undeclared here (not in a function)

All the code:

#include <stdio.h>
#define W 946
#define H 528
#include "myFunctions.h"


// Video resolution




// Allocate a buffer to store one frame
unsigned char frame[H][W][3] = {0};




int main(void)
{
    int x, y, count;
     
    // Open an input pipe from ffmpeg and an output pipe to a second instance of ffmpeg
    FILE *pipein = popen("ffmpeg -i 1.mp4 -f image2pipe -c:v rawvideo -pix_fmt rgb24 - ", "r"); // ffmpeg -i 1.mp4 -f image2pipe -vcodec rawvideo -pix_fmt rgb24 -       
    FILE *pipeout = popen("ffmpeg -y -f rawvideo -vcodec rawvideo -pix_fmt  rgb24 -s 946x528  -i - -f mp4 -q:v 5 -an -vcodec mpeg4 output.mp4", "w");
     
    // Process video frames
    while(1)
    {
        // Read a frame from the input pipe into the buffer
        count = fread(frame, 1, H*W*3, pipein);
          
        // If we didn't get a frame of video, we're probably at the end
        if (count != H*W*3) break;
         
        // Process this frame
        flip(frame,x,y,H,W);

        // Write this frame to the output pipe
        fwrite(frame, 1, H*W*3, pipeout);
    }
     
    // Flush and close input and output pipes
    fflush(pipein);
    pclose(pipein);
    fflush(pipeout);
    pclose(pipeout);
}

Headers File:

int invertRGB(unsigned char frame [H][W][3],int x, int y,int H, int W){
      for (y=0 ; y<H ; ++y) for (x=0 ; x<W ; ++x)
      {
          frame[y][x][0] = 255 - frame[y][x][0]; 
          frame[y][x][1] = 255 - frame[y][x][1]; 
          frame[y][x][2] = 255 - frame[y][x][2];            
      }
      return frame;
}

int flip(unsigned char frame [H][W][3],int x, int y, int H, int W){
      unsigned char frameTemp[H][W][3];
      for (y=0 ; y<H ; ++y) for (x=0 ; x<W ; ++x)
      {   
          frameTemp[(H-1)-y][(W-1)-x][0] = frame[y][x][0];
          frameTemp[(H-1)-y][(W-1)-x][1] = frame[y][x][1]; 
          frameTemp[(H-1)-y][(W-1)-x][2] = frame[y][x][2];
      }
      for (y=0 ; y<H ; ++y) for (x=0 ; x<W ; ++x)
      {
          frame[y][x][0] = frameTemp[y][x][0];
          frame[y][x][1] = frameTemp[y][x][1]; 
          frame[y][x][2] = frameTemp[y][x][2]; 
      }
      return frame;
}

This is my attempt at trying to create a video editor :)

Header files don't read anything. The compiler reads the header file as if it were included in the source. #defines only affect what comes after them. — William Pursell, Aug 17 '20 at 17:16
You might want to [buy a good book about the C programming language](https://stackoverflow.com/questions/562303/the-definitive-c-book-guide-and-list). — user3386109, Aug 17 '20 at 17:26
Already have ... KN Kings C Programming ... A modern approach — DAVID, Aug 17 '20 at 17:27
Header file shouldn't include definitions of functions. Header files is for declareing things. — MikeCAT, Aug 17 '20 at 17:33
Generally you should write only definitions of functions like `int invertRGB(unsigned char frame [H][W][3],int x, int y,int H, int W);` and definitions of functions should be in another source file (`.c` file). This is because function definitions in header file will lead to multiple definition errors when the header file is used from multiple source files to be compiled into one executable file. — MikeCAT, Aug 17 '20 at 17:39
In this case, separating functions into another source file will make it difficult to use the definition of macros `H` and `W`, so it seems it is good to copy-and-pasting the contents of the header to the main source file and removing header inclusion. — MikeCAT, Aug 17 '20 at 17:41
@MikeCAT Problem is I'm going to be adding more functions and it is going to get messy ... — DAVID, Aug 17 '20 at 17:42
@MikeCAT Also how would I pass the macros constant over to the function when defining the array's size? — DAVID, Aug 17 '20 at 18:08
`int main(headerFunction(array)){}` - that is definitely not a valid definition for `main`. Is that what's in your actual code? — John Bode, Aug 17 '20 at 18:22
If you're going to pass height and width values to your functions as arguments, then you do not need to convey the same information via macros, too, and *vice versa*. If you are going to use arguments then be aware that you would be leveraging C's *optional* variable-length array feature, which is not universally implemented. In that case, the parameters for the variable-length dimensions need to appear earlier in the parameter list than the variable-length arrays themselves. — John Bollinger, Aug 17 '20 at 20:06

MikeCAT · Answer 1 · 2020-08-17T17:37:14.443

2

It is because CONSTANT is not defined at the point that the header file is read.

Try this:

#define CONSTANT 10
#include "myHeaderFile.h"
int main(headerFunction(array)){}

In "All the code", you used the same names for constants (used in unsigned char frame [H][W][3]) and arguments (used in int H, int W), so the arguments is also treated as macros and replaced with integers, making the definition invalid. In this case the size is fixed, so you won't need the arguments H and W and they should be removed. Don't forget to remove the extra arguments also from calling points.

edited Aug 17 '20 at 17:37

answered Aug 17 '20 at 17:14

MikeCAT

73,922
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I then get this error: error: expected ‘;’, ‘,’ or ‘)’ before numeric constant – DAVID Aug 17 '20 at 17:15
Another solution is locating (carrying) that `#define CONSTANT 10` line into `myHeaderFile.h` before the `headerFunction`. – ssd Aug 17 '20 at 17:16
1

@DAVID the `array` in `int headerFunction(array[][CONSTANT])` does not have a type. – Weather Vane Aug 17 '20 at 17:16
I didn't set it up properly in the code but that isn't the problem ... – DAVID Aug 17 '20 at 17:17
1

@DAVID it is, because people will says that is what your problem is. Please post exact code. – Weather Vane Aug 17 '20 at 17:18
The third line of the OP's snippet is garbage. I would recommend deleting it from this answer, since it's not relevant to the question being asked. – user3386109 Aug 17 '20 at 17:24
The third line is against the wording of the C standard to declare `main`. – RobertS supports Monica Cellio Aug 17 '20 at 17:25
@RobertSsupportsMonicaCellio Why? – DAVID Aug 17 '20 at 17:26
@RobertSsupportsMonicaCellio it is just "like this"... – MikeCAT Aug 17 '20 at 17:26
@MikeCAT Anyway I get the before mentioned error ... – DAVID Aug 17 '20 at 17:27
@MikeCAT Use foo instead. – RobertS supports Monica Cellio Aug 17 '20 at 17:27
@MikeCAT Where? – DAVID Aug 17 '20 at 17:27
@RobertSsupportsMonicaCellio `int foo(headerFunction(array)){}` is also invalid. `int main(void){headerFunction(array);}` is better. – MikeCAT Aug 17 '20 at 17:28
@DAVID https://stackoverflow.com/a/5020691/12139179 – RobertS supports Monica Cellio Aug 17 '20 at 17:31
@RobertSsupportsMonicaCellio Already read it and doesn't solve anything. I posted all the code. – DAVID Aug 17 '20 at 17:32
@MikeCAT I meant only the misuse of `main`. But yes, the parameter syntax is also invalid. – RobertS supports Monica Cellio Aug 17 '20 at 17:34
@MikeCAT I guess we have here an [XY problem](https://meta.stackexchange.com/questions/66377/what-is-the-xy-problem). – RobertS supports Monica Cellio Aug 17 '20 at 17:35

Why aren't constants(#define) in C read by the header file?

1 Answers1