If you are passing in pointers as arguments to a function, but do not wish for the value contained in the pointer to be modified, then it is sometimes helpful to make the argument declaration a constant. This makes the compiler present a warning or an error if the value is accidentally modified within your function. This technique is especially useful when one of the arguments to your function is a string. In this case, you will probably be passing in a char* argument for convenience. However, passing in a string in this manner allows the function to modify the argument, which may not be desirable. Using a const char* argument can prevent this. Example:
int string_key(const char *key, const char *id_string)
{
/* within this function it is now impossible to accidentally modify
* the character strings pointed to by key or id_string
*/
return(0);
}