"g++ -I..." and CPATH Environment Variable

Linux Apps Tutorials - Herong's Tutorial Examples

∟"g++ -I..." and CPATH Environment Variable

This section provides a tutorial example on how to use 'g++ -I...' option and/or CPATH environment variable to specify paths of include header files for GCC compiler.

If you are compiling a C/C++ program that uses custom header files, you need to specify the path name where the header files are located.

1. Create a simple header file, ./include/myheader.h:

herong$ vi ./include/myheader.h

#define Name "herong"

2. Create a C++ program file that uses the header file:

herong$ vi HeaderTest.cpp

#include "iostream"
#include "string"
#include "myheader.h"
int main(void) {
  std::string who (Name);
  std::cout <<  "Hello " <<  who <<  "!" <<  std::endl;
  return 0;
}

3. Compile it without specifying the include path:

herong$ g++ HeaderTest.cpp
HeaderTest.cpp:3:10: fatal error: myheader.h: No such file or directory
 #include "myheader.h"
          ^~~~~~~~~~~~
compilation terminated.

4. Compile it with "-I ..." option to specify include path:

herong$ g++ -I ./include -v HeaderTest.cpp

...
#include "..." search starts here:
#include <...>  search starts here:
 ./include
 /usr/include/c++/8
 /usr/include/c++/8/x86_64-redhat-linux
 /usr/include/c++/8/backward
 /usr/lib/gcc/x86_64-redhat-linux/8/include
 /usr/local/include
 /usr/include
End of search list.
...

herong$ ./a.out
Hello herong!

5. Compile it with "CPATH=..." environment variable to specify include path:

herong$ export CPATH=./include

herong$ g++ -v HeaderTest.cpp

...
#include "..." search starts here:
#include <...>  search starts here:
 ./include
 /usr/include/c++/8
 /usr/include/c++/8/x86_64-redhat-linux
 /usr/include/c++/8/backward
 /usr/lib/gcc/x86_64-redhat-linux/8/include
 /usr/local/include
 /usr/include
End of search list.
...

herong$ ./a.out
Hello herong!