Surprisingly (to me at least), this is valid ISO 9899:1999 C. It compiles without warning under gcc with -Wall, and a “multi-character character constant” warning with -pedantic.

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.

A bit of experimentation shows that gcc stores the leftmost character in the literal as the most-significant byte in the integer. But is this a reliable behavior in other common compilers? It’s a cool little piece of esoteric C syntax, but I’m guessing nobody interested in portable code should be using it when an integer constant works just as well.