How to get the real and total length of char * (char array)?

Question

For a char [], I can easily get its length by:

char a[] = \"aaaaa\";
int length = sizeof(a)/sizeof(char); // length=6

However,

Answer 1

You can find the length of a char* string like this:

char* mystring = "Hello World";
int length = sprintf(mystring, "%s", mystring);

sprintf() prints mystring onto itself, and returns the number of characters printed.