isystem

isystem is a compiler option used by several C/C++ compilers, most notably GCC and Clang, to designate a directory as a system include directory. When a path is added with -isystem, headers found there are treated as system headers by the preprocessor.

The practical effect of marking a directory as system includes is that the compiler treats those headers differently from user headers. A common consequence is reduced diagnostic noise: warnings generated within headers located in system include directories are often suppressed or treated differently from warnings in files you write. This behavior makes it easier to integrate third-party libraries without being overwhelmed by warnings originating from their headers. In addition to -isystem, compilers still use -I to add include paths, but those paths are typically treated as user headers, with different warning and diagnostic handling.

Usage examples include:

- gcc -isystem /path/to/include your_source.c

- clang++ -isystem /path/to/include your_source.cpp

Notes:

- The exact diagnostic and search-order behavior can vary between compilers and versions, so it is advisable to consult the specific compiler documentation for details on how -isystem interacts with other options such as -I, -W, and -Wp.

- -isystem is commonly used when integrating libraries or vendor-supplied headers to minimize warnings and to influence how those headers are perceived by the compiler.

In summary, isystem provides a way to mark certain include directories as system headers, primarily to reduce warnings and noise from external code while maintaining control over header search paths.