Define Multi File Program?

While writing large programs we must divide programs up into modules. These would be separate source files and the main() function would be in one file main.c say the other files will contain functions. This is especially true of programs that make use of lengthy functions, where each function may take up a separate file. Or, if there are many small related functions within a program, it may be desirable to place a few functions within every of several files. The individual files will be compiled alone and then linked together to form one executable object program. This facilitates the debugging and editing of the program, ever since each file can be maintained at a manageable size.

Where a function is spread over numerous files every file will contain one or more functions. One file will include main while the others will hold functions which are called by others. These other files are able to be treated as a library of functions. Programmers generally start designing a program by dividing the problem into simply managed sections. Each one of these sections might be implemented as one or more functions and all functions from each section will typically live in a single file. Where the file holds the definition of functions which return values there is a further limitation on calling these functions from another file. Except functions in another file are told about the function definitions and they will be unable to compile them correctly. The most excellent solution to this problem is to write a header file for each of the C files. This will contain the same name as the C file, but ending in .h. The header file holds definitions of all the functions used in the C file.

when a function in another file calls a function from our C file it is able to define the function by making a #include of the appropriate .h file.

In the above program two functions are stored in showresult.h and sum_func.h and main() function is saved in mainfile.c. Throughout preprocessing phase# include and # include copy the whole content of the corresponding header files into the mainfile.c.