ANSI 220.127.116.11 The value of an integer character constant that contains more than one character or a wide character constant that contains more than one multibyte character
The regular character constant, 'ab' has the integer value (int)0x6162. When there is more than one byte, previously read bytes are shifted left by the value of CHAR_BIT and the next byte is compared using the bitwise-OR operator with the low CHAR_BIT bits. The number of bytes in the multibyte character constant cannot exceed sizeof(int), which is 4 for 32-bit target code.
The multibyte character constant is read as above and this is converted to a wide-character constant using the mbtowc run-time function. If the result is not a valid wide-character constant, an error is issued. In any event, the number of bytes examined by the mbtowc function is limited to the value of MB_CUR_MAX.