https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92720
--- Comment #6 from joseph at codesourcery dot com <joseph at codesourcery dot com> --- For the documentation of implementation-defined preprocessor behavior (as required to be documented by a C implementation for anything the C standard says is implementation-defined; in this case, where C11 6.10.2#2 and #3 refer to "How the places are specified or the header identified is implementation-defined." and "searched for in an implementation-defined manner"), see cpp.texi. It says: @item Source file inclusion. For a discussion on how the preprocessor locates header files, @ref{Include Operation}. In turn, that section "Include Operation" has more details. It doesn't mention includes with an absolute path, but I think that's because the general concept of resolving a path relative to a directory, and how that handles absolute paths, is one for the underlying operating system, not for any particular application running on that operating system.