get file length

Whats the most crossplatform way to get file length in C?

>>5
Not portable. Files opened in text mode need not return byte offsets from ftell() (the values might be "seek descriptors", suitable for passing to fseek() but useless otherwise).

>>8
Not portable. Files opened in binary mode need not support SEEK_END.

size_t n = 0; while (fgetc(f) != EOF) ++n;