What are the GCC default include directories?

When I compile a very simple source file with gcc I don't have to specify the path to standard include files such as stdio or stdlib.

How does GCC know how to find these files?

Does it have the /usr/include path hardwired inside, or it will get the paths from other OS components?

Solution

In order to figure out the default paths used by gcc/g++, as well as their priorities, you need to examine the output of the following commands:

echo | gcc -xc -E -v -

echo | gcc -xc++ -E -v -

The credit goes to Qt Creator team.

Here's a breakdown of the flags:

-x selects the language, C or C++ respectively
-E makes gcc to run the preprocessor only, so no compilation takes place
-v prints all the commands run, which is the key to dumping the standard paths
- is the "input file" to preprocess, as a convention - stands for stdin (or stdout, depending on the context);

echo | feeds an empty string to gcc so effectively we preprocess an empty file generated on the fly