The C Programming Language: EOF

EOF is a macro, defined in <stdio.h>, which expands to a negative integral constant expression that is returned by several functions to indicate end-of-file, that is, no more input from a stream.

Functions in <ctype.h> accept it as an argument value, and some functions declared in <stdio.h> also use it as an error return value.

Many implementations choose the value -1 for EOF, but don't count on it.