Stringify C preprocess

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].