Editing FourCC (section)

=== Compiler support ===

FourCC is written in big endian relative to the underlying ASCII character sequence, so that it appears in the correct byte order when read as a string. Many C compilers, including GCC, define a [[C syntax#Character constants|multi-character literal]] behavior of right-aligning to the least significant byte, so that {{code|'1234'}} becomes <code>0x3'''1'''3'''2'''3'''3'''3'''4'''</code> in ASCII.<ref>{{cite web |title=The C Preprocessor: Implementation-defined behavior |url=https://gcc.gnu.org/onlinedocs/cpp/Implementation-defined-behavior.html |website=gcc.gnu.org}}</ref> This is the conventional way of writing FourCC codes used by Mac OS programmers for OSType. ([[Classic Mac OS]] was exclusively big-endian.)

On little-endian machines, a byte-swap on the value is required to make the result correct. Taking the avc1 example from above: although the literal {{tt|'avc1'}} already converts to the integer value {{tt|0x61766331}}, a little-endian machine would have reversed the byte order and stored the value as {{tt|31 63 76 61}}. To yield the correct byte sequence {{tt|61 76 63 31}}, the pre-swapped value {{tt|0x31637661}} is used.