The Runtime Library :
One of the reasons for C being such a small language is that it defers many operations to a runtime library. The runtime library is a collection of object files. Each file contains the machine instructions for a function that performs one of a wide variety of services. The functions are divided into groups, such as I/0, memory management, mathematical operations, string manipulation, etc. For each group, there is a source file, called the header file, that contains information that we need to use these functions. By conventions, the names for header files end with a. h extension. For example, the standard group of I/O functions has an associated header file called stdio.h.
To include the header file in a program, we should insert the following statement in our source file
# include < filename >
e.g.
# include < stdio.h >
Usually, this would be one of the first lines in our source file. We shall discuss the # include directive and other preprocessor commands in more detail later on. The complete process of compiling and running a C program is as shown the following figure
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