How to use #define to define a variable value from a text file?

Question

Somewhere in my code I use int array[L]. I specify the array length: #define L 64. Everything is ok. But what if I want to specify this value 64 in a (external) text file, not in the code? How can I read a text file into #define?

I wouldn't recommend defining macro values outside the source code. If you absolutely have to, you'd have to open the file read-only and parse the text file to get your value. — David Corbin, Jul 22 '16 at 18:14
Your code would not compile since L would not pre-process to an integer litteral. By the way, a header file is an external text file. If you want dynamically sized array, this won't work. — clarasoft-it, Jul 22 '16 at 18:19
most compilers will let you specify a Macro definition on the command line. Usually it's -D. So, in your Makefile, you'll want to read the external file, and parse out the value for L. Than specify that value as the parameter to the -D compiler flag. You will soon become a Makefile guru. Check this out: http://stackoverflow.com/questions/6767413/create-a-variable-in-a-makefile-by-reading-contents-of-another-file — bruceg, Jul 22 '16 at 18:20

score 2 · Accepted Answer · answered Jul 22 '16 at 18:18

But what if I want to specify this value 64 in a (external) text file, not in the code?

There are two methods to set the value of a macro that I can think of.

Define it in code. A text file won't do for this purpose.
Define it as command line option in the compiler (-DL=64).

If you use make, you can define the option in the appropriate Makefile.

If you use an IDE, you'll need to figure out how to do that in the various settings that IDEs provide.

score 0 · Answer 2 · answered Jul 22 '16 at 18:22

Read the text file, and then malloc the array:

int *array; 
int L; 
int main(void)
{ 
    L = readLengthFromTextFile();
    array = malloc( L );
    if ( array == NULL ) {
        printf( "array allocation failed\n" );
        exit(1);
    }

    // use the array as if declared as:    int array[L];

    free( array );
}

How to use #define to define a variable value from a text file?

2 Answers2