Library Calls: Difference between revisions

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#include <stdio.h>

int answer = 42;

printf("The answer is %d!\n", answer);

 

We all know this construct from our userspace programming experiences. We never really thought about how it worked, at least for some time. You ''#include'', you get to use "the system".

Let us have a look at the above code fragment, again. You could just as well write:

 

int printf(const char * format, ...);

char * msg = "World";

printf("Hello %s!\n", msg);

 

The header ''<stdio.h>'', in this case, does serve no other purpose than to tell the compiler that there is a function ''printf()'' that takes a parameter list consisting of at least a ''const char'' pointer, and returns ''int''. Nothing else is necessary, and mere convenience so you don't have to write your own declarations or ''#include'' each function individually.

So you provide a lightweight <tt>printf()</tt> for kernel space. You don't even have to bother to call it <tt>kprintf()</tt> or something as your kernel binary will never be '''linked''' with userspace code. Using some preprocessor ''#ifdef'' magic, you can even use the very same ''<stdio.h>'' header file as for userspace code, reducing redundancy and potential error sources: The preprocessor symbol <tt>__STDC_HOSTED__</tt> (with two leading and trailing underscores that the Wiki stubbornly interprets as boldface markup) is undefined when you set ''-ffreestanding''...

 

#ifdef __STDC_HOSTED__

// Userspace declarations

// Kernelspace declarations

#endif

 

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