Multi-character constant warnings

According to the standard (§6.4.4.4/10)

The value of an integer character constant containing more than one character (e.g., 'ab'), [...] is implementation-defined.

long x = '\xde\xad\xbe\xef'; // yes, single quotes

This is valid ISO 9899:2011 C. It compiles without warning under gcc with -Wall, and a “multi-character character constant” warning with -pedantic.

From Wikipedia:

Multi-character constants (e.g. 'xy') are valid, although rarely useful — they let one store several characters in an integer (e.g. 4 ASCII characters can fit in a 32-bit integer, 8 in a 64-bit one). Since the order in which the characters are packed into one int is not specified, portable use of multi-character constants is difficult.

For portability sake, don't use multi-character constants with integral types.

This warning is useful for programmers that would mistakenly write 'test' where they should have written "test".

This happen much more often than programmers that do actually want multi-char int constants.