Why does MPI_Init accept pointers to argc and argv?

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

    if (argc < 2) {
        fprintf(stderr, "Need a command line argument.\n");
        exit(1);
    }

    int arg = atoi(argv[1]);
    // now you have an integer
    // do whatever with it...
}

main( int argc, char** argv )
           |            |
           |            |
           |            +----pointer to supplied arguments
           +--no. of arguments given at command line (including executable name)

display_image image1.jpg

argc will be 2
argv[0] points to display_image
argv[1] points to image1

if(argc !=2 )
   ^^ Checks whether no. of supplied argument is not exactly two

# define _2a(list,a1,a2)                 list a1;a2;

main _2a((argc,argv), int argc, char * argv[])
{
...
}

main (argc,argv) int argc; char * argv[];
{
...
}

main(argc, argv)
int argc;
char *argv[];
{
    ...
}

#define _2a(list,a1,a2) (a1, a2)

main (int argc, char * argv[])
{
...
}

#ifdef __STDC__
#define _2a(list,a1,a2) (a1, a2) 
#else
#define _2a(list,a1,a2) list a1;a2;
#endif