Development System - Header Files
For programming in C and other languages, we need header files to provide definitions of constants and declarations for system and library function calls. For C, these are almost always located in /usr/include and subdirectories thereof. You can normally find header files that depend on the particular incarnation of Linux that you are running in /usr/include/sys and /usr/include/linux.
Other programming systems will also have include files that are stored in directories that get searched automatically by the appropriate compiler. Examples include /usr/include/X11 for the X Window System and /usr/include/g++ for GNU C++.
We can use include files in subdirectories or nonstandard places by specifying the -I flag to the C compiler.
For example,
$ gcc -I/usr/openwin/include fred.c
will direct the compiler to look in the directory /usr/openwin/include, as well as the standard places, for header files included in the fred.c program. Refer to the manual page for the C compiler (man gcc) for more details.
It’s often convenient to use the grep command to search header files for particular definitions and function prototypes. Suppose we need to know the name of the #defines used for returning the exit status from a program. Simply change to the /usr/include directory and grep for a probable part of the name like this:
$ grep EXIT_ *.h
…
stdlib.h:#define EXIT_FAILURE 1 /* Failing exit status. */
stdlib.h:#define EXIT_SUCCESS 0 /* Successful exit status. */
…
$
Here grep searches all the files in the directory with a name ending in .h for the string EXIT_. In this example, it has found (among others) the definition we need in the file stdlib.h