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I'm using the Keil uVision3. How do I pack structs?
Do I use __packed or is there a pragma?
is this correct? it doesn't compile...
typedef struct { UINT8 key_diff; UINT8 key_state; } __packed SCmd_KeyPress;
The most common - and probably the only valid reason - for packing structures is to save space when doing embedded work. It is a trade-off between data size and code size, and often also a trade-off in speed.
If you need to work with hardware, or write data to a file or build packets for transfer over a network, you should instead byte-pack your data yourself, similar to:
pos = packet.data; pos = insert_u8(pos,my_8bit_value); pos = insert_u16(pos,my_16bit_value); ... assert(pos - packet.data == expected_packet_size);
The reason for this is that when permanently storing information, or accessing hardware or communicating, you have more things to take into account. A packed structure will just save you the extra space between individual members, but may still have the incorrect byte order.