How do compilers know where to find #include <stdio.h>?

Question

I am wondering how compilers on Mac OS X, Windows and Linux know where to find the C header files.

Specifically I am wondering how it knows where to find the #include with the <> brackets.

#include "/Users/Brock/Desktop/Myfile.h"    // absolute reference
#include <stdio.h>                         // system relative reference?

I assume there is a text file on the system that it consults. How does it know where to look for the headers? Is it possible to modify this file, if so where does this file reside on the operating system?

Answer 1

When the compiler is built, it knows about a few standard locations to look for header file. Some of them are independent of where the compiler is installed (such as /usr/include, /usr/local/include, etc.) and some of the are based on where the compiler is installed (which for gcc, is controlled by the --prefix option when running configure).

Locations like /usr/include are well known and 'knowledge' of that location is built into gcc. Locations like /usr/local/include is not considered completely standard and can be set when gcc is built with the --with-local-prefix option of configure.

That said, you can add new directories for where to search for include files using the compiler -I command line option. When trying to include a file, it will look in the directories specified with the -I flag before the directories I talked about in the first paragraph.