CODEWITHSUNDEEP
cmacrosc-preprocessor
jonathan
jonathan

Reputation: 5

Macros print output

#include <stdio.h>

#define N 100

void f(void);


int main(void)
{
f();
#ifdef N
#undef N
#endif

return 0;
}

void f(void){
#if defined(N)
  printf("N is %d\n",N);
#else
  printf("N is undefined\n");
#endif // defined
}

Why does this output print N is undefined shouldn't it print N is 100 because of the function call on f before it reaches the undef that removes the value 100?

Upvotes: 0

Views: 67

Answers (1)

luser droog
luser droog

Reputation: 19504

Preprocessor directives and macros are processed at a very early stage of compilation, they have no existence at runtime.

Running your code just through the preprocessor (cpp -P -- warning: remove the #include first) shows the actual C code that is being compiled.

void f(void);
int main(void)
{
f();
return 0;
}
void f(void){
  printf("N is undefined\n");
}

As to why this expansion is chosen rather than the alternate message, consider these lines in your source.

#ifdef N
#undef N
#endif

Regardless of whether it's defined or not initially, it will not be defined after these lines are (pre-)processed.

Upvotes: 1

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