The preprocessor directive #include is an instruction to read in the entire contents of another file at that point. This is generally used to read in header files for library functions. Header files contain details of functions and types used within the library. They must be included before the program can make use of the library functions.
Library header file names are enclosed in angle brackets, < >. These tell the preprocessor to look for the header file in the standard location for library definitions. This is /usr/include for most UNIX systems.
Another use for #include for the programmer is where multi-file programs are being written. Certain information is required at the beginning of each program file. This can be put into a file called globals.h and included in each program file. Local header file names are usually enclosed by double quotes, " ". It is conventional to give header files a name which ends in .h to distinguish them from other types of file.
Our globals.h file would be included by the following line.
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