Raw data (bytes) and signed/unsigned variables

Question

I've been told that whenever you work with bytes, you should declare your variables as unsigned chars. In Windows' data types, BYTE is declared as an unsigned char.

My questions:

Why?

Unsigned is a representation of integers from 0 to 255 and signed 128 to -127.

If that's the case, then how is EOF in binaries (-1) caught?

EOF is declared in stdio.h as a -1 #define macro.

Answer 1

When you read chars from a stream, the return type of functions like std::getc is int, and not char. The constant EOF is of type int, and not char or unsigned char.

Even in the C++ I/O API, the I/O streams like std::ifstream deal with types char_type (that is the type of characters in the stream), and int_type that is a type that can hold all values of char_type, plus EOF.

Answer 2

EOF is a status information, and is distinct from the data. However some functions have habit of using single return type for both. Example:

/* Return next data byte (0 - 255) or EOF (-1) if there was an error */
int readByte(...);

Point is that you need to have larger type than plain byte to be able to do this.