The main Function and Program Execution

Every C program has a primary (main) function that must be named main. If your code adheres to the Unicode programming model, you can use the wide-character version of main, wmain. The main function serves as the starting point for program execution. It usually controls program execution by directing the calls to other functions in the program. A program usually stops executing at the end of main, although it can terminate at other points in the program for a variety of reasons. At times, perhaps when a certain error is detected, you may want to force the termination of a program. To do so, use the exit function. See the Run-Time Library Reference for information on and an example using the exit function.

Functions within the source program perform one or more specific tasks. The main function can call these functions to perform their respective tasks. When main calls another function, it passes execution control to the function, so that execution begins at the first statement in the function. A function returns control to main when a return statement is executed or when the end of the function is reached.

You can declare any function, including main, to have parameters. The term “parameter” or “formal parameter” refers to the identifier that receives a value passed to a function. See Parameters for information on passing arguments to parameters. When one function calls another, the called function receives values for its parameters from the calling function. These values are called “arguments.” You can declare formal parameters to main so that it can receive arguments from the command line using this format:

main( int argc, char *argv[ ], char *envp[ ] )

When you want to pass information to the main function, the parameters are traditionally named argc and argv, although the C compiler does not require these names. The types for argc and argv are defined by the C language. Traditionally, if a third parameter is passed to main, that parameter is named envp. Examples later in this chapter show how to use these three parameters to access command-line arguments. The following sections explain these parameters.

See Using wmain for a description of the wide-character version of main.