23.1.3.1 The EOF-ERROR-P argument

Eof-error-p in input function calls controls what happens if input is from a file (or any other input source that has a definite end) and the end of the file is reached. If eof-error-p is true (the default), an error of type end-of-file is signaled at end of file. If it is false, then no error is signaled, and instead the function returns eof-value.

Functions such as read that read the representation of an object rather than a single character always signals an error, regardless of eof-error-p, if the file ends in the middle of an object representation. For example, if a file does not contain enough right parentheses to balance the left parentheses in it, read signals an error. If a file ends in a symbol or a number immediately followed by end-of-file, read reads the symbol or number successfully and when called again will act according to eof-error-p. Similarly, the function read-line successfully reads the last line of a file even if that line is terminated by end-of-file rather than the newline character. Ignorable text, such as lines containing only whitespace[2] or comments, are not considered to begin an object; if read begins to read an expression but sees only such ignorable text, it does not consider the file to end in the middle of an object. Thus an eof-error-p argument controls what happens when the file ends between objects.