Evaluate preprocessor directives in C

Question

I have a question about C preprocessor. I have define the macros:

#define num2str(x)      str(x)

#define str(x) #x

and I pass a number to a string constant:

const   char    myString[]     = num2str(NUMBER);

When I put for example #define NUMBER 10 works fine. But I want to put num2str(NUMBER + 10), and the result is "10 + 10" instead of "20". Is there any way to evaluate the expression NUMBER+10 and obtain "20"?

Is there *any* way.. Use something else then C preprocessor. Like m4. — KamilCuk
– KamilCuk, Commented Jul 16, 2021 at 11:59
While the C preprocessor does have the capability to evaluate certain simple constant expressions, those results are only usable in preprocessor expressions such as #if expressions. I don't know of a way to actually have the evaluated results placed into the preprocessor output. — Tom Karzes
– Tom Karzes, Commented Jul 16, 2021 at 12:05

Eric Postpischil · Accepted Answer · 2021-07-16 11:47:38Z

1

In C, macro replacement does not do arithmetic.

There are some kludges that can implement limited arithmetic. Determining whether such a kludge would serve your purpose requires further information about your problem.

answered Jul 16, 2021 at 11:47

Eric Postpischil

233k15 gold badges199 silver badges379 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

John Bode · Accepted Answer · 2021-07-16 13:08:52Z

1

Macros are text substitutions¹, nothing more. Macro arguments are not evaluated, they are simply expanded in place.

^{Strictly speaking they’re token substitutions, but the distinction is pretty minor in this case.}

answered Jul 16, 2021 at 13:08

John Bode

125k19 gold badges130 silver badges211 bronze badges

Collectives™ on Stack Overflow

Evaluate preprocessor directives in C

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related