Is char signed or unsigned by default?

The book is wrong. The standard does not specify if plain char is signed or unsigned.

In fact, the standard defines three distinct types: char, signed char, and unsigned char. If you #include <limits.h> and then look at CHAR_MIN, you can find out if plain char is signed or unsigned (if CHAR_MIN is less than 0 or equal to 0), but even then, the three types are distinct as far as the standard is concerned.

Do note that char is special in this way. If you declare a variable as int it is 100% equivalent to declaring it as signed int. This is always true for all compilers and architectures.

As Alok points out, the standard leaves that up to the implementation.

For gcc, the default is signed, but you can modify that with -funsigned-char. note: for gcc in Android NDK, the default is unsigned. You can also explicitly ask for signed characters with -fsigned-char.

On MSVC, the default is signed but you can modify that with /J.

C99 N1256 draft 6.2.5/15 "Types" has this to say about the signed-ness of type char:

The implementation shall define char to have the same range, representation, and behavior as either signed char or unsigned char.

and in a footnote:

CHAR_MIN, defined in <limits.h>, will have one of the values 0 or SCHAR_MIN, and this can be used to distinguish the two options. Irrespective of the choice made, char is a separate type from the other two and is not compatible with either.