C Preprocessor include directive

Question

When I include another source(I.e stdio.h) the preprocessor is smart enough to include only the functions that I am using in my code?

Example: Assuming this small program, would be ease to include only what I am using, and what the printf functions uses, including them recursively, but what about bigger programs?

#include <stdio.h> 

int main(void) {
   printf("Hello World\n"); 
   return 0;
}

Answer 1

No. On the contrary:

#include performs textual replacement: it opens the file and copies all¹ of its contents into your main C file. In the process it executes all preprocessor instructions in the included file. Amongst other things, this means that it will recursively include all files that are #included in the header.

#include does not know and does not care which part of the included file you end up using.

---

¹ As mentioned, preprocessor instructions are executed in the included file. This can modify what gets included. For example, assume the following header file header.h:

#ifndef HEADER_H
#define HEADER_H

#ifdef NDEBUG
#  define LOG(...) ((void) 0)
#else
#  define LOG(...) log_message(__FILE__, __LINE__, __VA_ARGS__)

inline void log_message(const char* filename, int line, ...) {
    // Logging code omitted for brevity.
}
#endif

// other stuff

#endif

Now, if your main.c file looks as follows:

#define NDEBUG
#include "header.h"

int main(void) {
    // …
    LOG("hello");
}

… then, after preprocessing, your main.c file would looks something like this (I’m omitting some irrelevant stuff):

# 1 "main.c"

# 1 "./header.h" 1




# 13 "./header.h"

// other stuff


# 3 "main.c" 2

int main(void) {
    // …
    ((void) 0);
}

… in other words, only the part of header.h that corresponds to #ifdef NDEBUG was included, not the part in the #else clause. If we had included header.h without defining NDEBUG, then the included header code would have contained the definition of log_message.