The main() function

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 main function cannot be declared with the constexpr specifier.

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 following example program backward prints the arguments entered on a command line such that the last argument is printed first:

#include <stdio.h>
int main(int argc, char *argv[])
{
  while (--argc > 0)
    printf("%s ", argv[argc]);
  printf("\n"); 
}

Invoking this program from a command line:

   backward string1 string2

gives the following output:

   string2 string1