`"apue.h"`is not a global environment PATH , but program can retrieve it

Question

I am learning from APUE3e

#include "apue.h"
#include <dirent.h>

int main(int argc, char *argv[])
{
    DIR *dp;
    struct dirent *dirp;
  ....

In the header, import a user-definded module "apue.h". the program run correctly after compiled.

The apue.h is placed in /Library/Developer/CommandLineTools/usr/include,

What confuse me is that /Library/Developer/CommandLineTools/usr/include is not in the $PATH but could be retrieved by the program globally.

$ echo $PATH | grep include
#does not return anything.

How could the program get it done?

Answer 1

$PATH is where the shell looks for executables, not header files. This answer shows how you can get the compiler to show you where it's looking for header files.