The canonical way to determine the size of a file in bytes, using Common Lisp, is to open the file with an element type of (unsigned-byte 8) and then calculate the length of the stream. This is less than ideal. In most cases it is better to get the size of the file from its metadata, using a system call.

This library exports a single function, file-size-in-octets. It returns the size of a file in bytes, using system calls when possible.