The #include preprocessor directive is used to paste code of given file into current file. It is used include system-defined and user-defined header files. If included file is not found, compiler renders error.
By the use of #include directive, we provide information to the preprocessor where to look for the header files. There are two variants to use #include directive.
The #include <filename> tells the compiler to look for the directory where system header files are held. In UNIX, it is \usr\include directory.
The #include "filename" tells the compiler to look in the current directory from where program is running.
#include directive example
Let's see a simple example of #include directive. In this program, we are including stdio.h file because printf() function is defined in this file.
Note 1: In #include directive, comments are not recognized. So in case of #include <a//b>, a//b is treated as filename.
Note 2: In #include directive, backslash is considered as normal text not escape sequence. So in case of #include <a\nb>, a\nb is treated as filename.
Note 3: You can use only comment after filename otherwise it will give error.