CODEWITHSUNDEEP
cmacros
senya247
senya247

Reputation: 23

Stringize function return value?

Suppose I have a random function func(), how would i stringize its return value? I tried doing this-

#define STR1(x) #x
#define STR2(x) STR1(x)

int func(void){
    return 1;
}

int main(void)
{
   puts((STR2(func()));
}

But this literally prints func(), I read about double stringization somewhere, and this would've worked if I had used STR2 on a macro, since it would've expanded the macro first, and then stringized that, but why does this not compute the result of func before stringizing it?

Upvotes: 0

Views: 112

Answers (2)

Eric Postpischil
Eric Postpischil

Reputation: 223747

Stringification is a preprocessing operation in macro replacement, and it only operates on source code tokens. It cannot be used on run-time values.

To convert a value to a string, you can use snprintf. However, as you are directly outputting the string, you can simply use printf("%d", func());.

Supposing you want the string for more than just puts, you can convert the value to a string by including <stdio.h> and <stdlib.h> and using:

    // Store the value so we do not call func multiple times.
    int t = func();

    // Ask snprintf how much space we need.  Add one for the terminating null character.
    size_t n = snprintf(NULL, 0, "%d", t) + 1;

    // Ask for memory and check that we got it.
    void *p = malloc(n);
    if (!p)
    {
        fprintf(stderr, "Error, unable to allocate memory.\n");
        exit(EXIT_FAILURE);
    }

    // Convert the value to a string.
    snprintf(p, n, "%d", t);

    // Use the string as desired.
    puts(p);

    // Release the memory.
    free(p);

Upvotes: 1

Tasos Papastylianou
Tasos Papastylianou

Reputation: 22255

The preprocessor will see this (lines added for convenience):

1.  #define STR1(x) #x
2.  #define STR2(x) STR1(x)

3.  int func(void){
4.      return 1;
5.  }

6.  int main(void)
7.  {
8.     puts((STR2(func()));
9.  }

It will then process the first preprocessing directive on line one. This will result in the following pre-processedcode:

2.  #define STR2(x) #x

3.  int func(void){
4.      return 1;
5.  }

6.  int main(void)
7.  {
8.     puts((STR2(func()));
9.  }

The preprocessor directive of line 2 is now ready to be processed. This will result in the following preprocessed code:

3.  int func(void){
4.      return 1;
5.  }

6.  int main(void)
7.  {
8.     puts(( "func()" );
9.  }

This code is now fully pre-processed, and ready to be passed to the compiler. (which, incidentally, will cause a compiler error because of the unbalanced parenthesis)

Upvotes: 0

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