FILE is $_ from #fopen.
The variable _ is set to the number of items successfully read.
If an error occurs, or the end-of-file is reached, the return
value is a short item count (or zero).
fread does not distinguish between end-of-file and error
length is limited to 2048.
See help on C function: size_t fread( void *ptr, size_t size, size_t nmemb, FILE *stream);