valid_read - checks if a certain person has read permission on a file
int valid_read( string file, mixed user, string func );
Every time a user tries to read a file, thie driver calls valid_read in the master object to check if the read should be allowed. The argu‐ ments are the filename, the name of the person making the read, and the calling function name. If valid_read returns non-zero, the read is allowed.