Question

This is my first post, so if I’m being too vague or giving information that everyone would intuitively assume, please let me know.

I’m very new to writing in C and am just trying to get a better understanding of preprocessing. I’m writing a simple program that can take in arguments either directly from the console using gcc -Wall -std=c99 -DSEED=argument, where my argument should be a an integer, or if the -D is not defined the user will input it.

The SEED value is simply used in srand(). I’m very confused why my code will not compile if I put in an -DSEED=a as my argument while if I put -DSEED=1 it will compile. I’m getting a “âaâ undeclared (first use in this function)” error and really don’t understand the difference between the two. I thought the #define matched up the variable type with the input, so if I put in an “a” #SEED would be a char and if I put in a “1” #SEED would be an int.

If the SEED is not defined I’m using a #ifndef SEED command and this works well.
I think I’m supposed to “stringify” the input SEED and then can check if it is an integer or not. After reading some articles online I’m trying to use:

#ifndef SEED
    //code
#else
    #define TO_STRING( input ) #input
    char c;
    c = TO_STRING( SEED )
    //Then I was going to use c to figure out if it was an int.
#endif

This is not working and anyone able to point out any misconceptions that you think that I may have would be greatly appreciated.

EDIT – So I did figure out why I was receiving the error message when trying the -DSEED=a, because it was reading it as a variable.

Answer 1

To stringify a #define you need to use a two-step approach:

#define _STRINGIFY(s) #s
#define STRINGIFY(s) _STRINGIFY(s)

...

#define SEED 123

...

const char * pszSeed = STRINGIFY(SEED); /* 'pszSeed' would point to "123" form here on. */

If you only want to use one character simply access it via *pszSeed or pszSeed[0].