What is the environment variable for GCC/G++ to look for .h files during compilation: LIBRARY_PATH, C_PATH, C_INCLUDE_PATH or CPLUS_PATH?

Question

Is there an environment variable for GCC/G++ to look for .h files during compilation?

I google my question, there are people say LIBRARY_PATH, C_PATH, C_INCLUDE_PATH, CP

Answer 1

Just look at the actual gcc documentation. It's all explained there.

To summarize:

• LIBRARY_PATH is for the linker, not for header files (used when looking for libraries requested by a -l option)
• CPATH specifies directories to look for header files in (like the -I option)
• C_INCLUDE_PATH and CPLUS_INCLUDE_PATH are like CPATH, but for C/C++ respectively.

Answer 2

Also, if you're not sure which paths are being checked on your system you can use

cpp -v

Which will tell you which paths are being checked for .h files, the output includes sections:

#include "..." search starts here:
#include <...> search starts here:

Answer 3

From: http://gcc.gnu.org/onlinedocs/cpp/Environment-Variables.html

CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH

Each variable's value is a list of directories separated by a special character, much like PATH, in which to look for header files. The special character, PATH_SEPARATOR, is target-dependent and determined at GCC build time. For Microsoft Windows-based targets it is a semicolon, and for almost all other targets it is a colon.

CPATH specifies a list of directories to be searched as if specified with -I, but after any paths given with -I options on the command line. This environment variable is used regardless of which language is being preprocessed.

The remaining environment variables apply only when preprocessing the particular language indicated. Each specifies a list of directories to be searched as if specified with -isystem, but after any paths given with -isystem options on the command line.

In all these variables, an empty element instructs the compiler to search its current working directory. Empty elements can appear at the beginning or end of a path. For instance, if the value of CPATH is :/special/include, that has the same effect as '-I. -I/special/include'.

I think that most setups avoid using the environment variables and instead pass the include directories in the command line using the -I option. there will usually be a makefile variable or an IDE setting to control what gets passed to -I.