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When a program begins running, the system calls the function main, which marks the entry point of the program. By default, main has the storage class extern. Every program must have one function named main, and the following constraints apply:

No other function in the program can be called main.

main cannot be defined as inline or static.

main cannot be called from within a program.

 The address of main cannot be taken.

 The main function cannot be overloaded.

The function main can be defined with or without parameters, using any of the following forms:

int main (void)

int main ( )

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

int main (int argc, char ** argv)

Although any name can be given to these parameters, they are usually referred to as argc and argv. The first parameter, argc (argument count) is an integer that indicates how many arguments were entered on the command line when the program was started. The second parameter, argv (argument vector), is an array of pointers to arrays of character objects. The array objects are null-terminated strings, representing the arguments that were entered on the command line when the program was started.

The first element of the array, argv[0], is a pointer to the character array that contains the program name or invocation name of the program that is being run from the command line. argv[1] indicates the first argument passed to the program, argv[2] the second argument, and so on.