SHA256_FdChunk
#ifndef SHA256_FdChunk
char *SHA256_FdChunk(int, char *, off_t, off_t);
SHA256_FdChunk(fd2, digest1, sb1.st_size, sb2.st_size - sb1.st_size);
SHA256_FdChunk(fd2, digest2, sb1.st_size, sb2.st_size - sb1.st_size);