Printing out a Variable from Compiler options

Question

I am trying to compile a very simple C program:

# program test.c
#include <stdio.h>

int main() 
{
    printf("HELLO WORLD \"%s\"\n\n", FOO);
}

and compiled with

gcc -o prog -D FOO=bar test.c

Basically what I am looking to have the following output:

HELLO WORLD "bar"

When I attempt to compile the above, I get the error

<command line>:1:13: note: expanded from here
#define FOO bar

I'm even going so far as to do the following (and yes, I know this is not good):

#indef FOO
    define ZZZ sprintf("%s", FOO)
#endif

score 6 · Answer 1 · answered Aug 18 '22 at 21:56

6

#define FOO bar makes no sense in your example; what is the bare word bar?

If you want to replace it with the string "bar", then you need to produce

#define FOO "bar"

You can achieve this by adding quotes to your CLI flag, which typically should be escaped:

gcc test.c -D FOO=\"bar\" -o prog
./prog
HELLO WORLD "bar"

answered Aug 18 '22 at 21:56

user229044

Ugh, it's embarrassing when the answer is THIS simple. Thanks for your help! – KingFish Aug 18 '22 at 21:58
bar != "bar". So it is not something OP wants – 0___________ Aug 18 '22 at 22:01
@Paul but you asked something completely different. – 0___________ Aug 18 '22 at 22:03
Example when your method will be useless https://godbolt.org/z/5Pb58qr68. Usually definitions passed to the compiler have some meaning. And `bar` cannot be replaced by `"bar"` – 0___________ Aug 18 '22 at 22:05

0___________ · Answer 2 · 2022-08-18T22:10:18.800

4

You need to stringify it.

#define STR(x) XSTR(x)
#define XSTR(x) #x


#include <stdio.h>

int main() 
{
    printf("HELLO WORLD \"%s\"\n\n", STR(FOO));
}

output:

HELLO WORLD "bar"

Two macros have to be used as we need to expand the macro first and then stringify it.

edited Aug 18 '22 at 22:10

answered Aug 18 '22 at 22:01

0___________

Not your downvote, but if you believe this is the correct solution, with the other answer being incorrect, why not vote to close this topic as a [duplicate](https://stackoverflow.com/q/240353/2505965)? – Oka Aug 18 '22 at 22:32

2 Answers2