Unusual usage of .h file in C

Question

During reading article about filtering, I've found some strange using of .h file - use it for filling array of coefficients:

#define N 100 // filter order float h[N] = { #include "f1.h" }; //insert coefficients of filter float x[N]; float y[N];  short my_FIR(short sample_data) {   float result = 0;    for ( int i = N - 2 ; i >= 0 ; i-- )   {     x[i + 1] = x[i];     y[i + 1] = y[i];   }    x[0] = (float)sample_data;    for (int k = 0; k < N; k++)   {     result = result + x[k]*h[k];   }   y[0] = result;    return ((short)result); } 

So, is it normal practice to use float h[N] = { #include "f1.h" }; this way?

Answer 1

Preprocessor directives like #include are just doing some textual substitution (see the documentation of GNU cpp inside GCC). It can occur at any place (outside of comments and string literals).

However, a #include should have its # as the first non-blank character of its line. So you'll code

float h[N] = {   #include "f1.h" }; 

^{The original question did not have #include on its own line, so had wrong code.}

It is not normal practice, but it is permitted practice. In that case, I would suggest using some other extension than .h e.g. use #include "f1.def" or #include "f1.data" ...

Ask your compiler to show you the preprocessed form. With GCC compile with gcc -C -E -Wall yoursource.c > yoursource.i and look with an editor or a pager into the generated yoursource.i

I actually prefer to have such data in its own source file. So I would instead suggest to generate a self-contained h-data.c file using e.g. some tool like GNU awk (so file h-data.c would start with const float h[345] = { and end with };...) And if it is a constant data, better declare it const float h[] (so it could sit in read-only segment like .rodata on Linux). Also, if the embedded data is big, the compiler might take time to (uselessly) optimize it (then you could compile your h-data.c quickly without optimizations).

Answer 2

As already explained in previous answers, it is not normal practice but it's a valid one.

Here is an alternative solution:

File f1.h:

#ifndef F1_H #define F1_H  #define F1_ARRAY                   \ {                                  \      0, 1, 2, 3, 4, 5, 6, 7, 8, 9, \     10,11,12,13,14,15,16,17,18,19, \     20,21,22,23,24,25,26,27,28,29, \     30,31,32,33,34,35,36,37,38,39, \     40,41,42,43,44,45,46,47,48,49, \     50,51,52,53,54,55,56,57,58,59, \     60,61,62,63,64,65,66,67,68,69, \     70,71,72,73,74,75,76,77,78,79, \     80,81,82,83,84,85,86,87,88,89, \     90,91,92,93,94,95,96,97,98,99  \ }  // Values above used as an example  #endif 

File f1.c:

#include "f1.h"  float h[] = F1_ARRAY;  #define N (sizeof(h)/sizeof(*h))  ...