In C.

For my lab I'm suppose to open a file and calculate the number of characters in the file. I don't really know how to do that. This is my best guess, but it does work. How do I calculate number of characters in a file?

#include int main(). char ch; int len; char filename[] ="test.txt"; FILE* file = fopen(filename, "r"); while(ch !=EOF) len++; printf("%d", len); return 0