The keyword __attribute__ allows you to specify special attributes of struct and union types when you define such types. This is mostly encountered when designing code with particular care on memory footprint optimisation. I have encountered this several times in some Apple headers while digging for documentation for my kext projects, and I thought it would be useful to explain it a bit. For example (from AppleUSBCDCCommon.h):
typedef struct
{
UInt8 bmRequestType;
UInt8 bNotification;
UInt16 wValue;
UInt16 wIndex;
UInt16 wLength;
} __attribute__((packed)) Notification;
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