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struct with members' sizes

I'm using C51 v3.20, and when I compiled the following structure, the compiler allocates 6 bytes for it.

typedef struct t_channel_info {
	int target_temp:9;
	unsigned char control_mode:2;
	unsigned char status:2;
	unsigned char warming:1;
	int current_temp:9;
	unsigned char warm_proportion;
} t_channel_info;

Should not it allocate only (9+2+2+1+9+8)/8=4 bytes?

But, when I partition a 9-bit integer to 8 and 1 bits chars, the size reduces to 5 bytes.

Is there an explanation?

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0 Scott deWolski over 23 years ago

I don't use C51 but this might help. Bit fields usually are assigned as an int which would give you the first 4 members in one int, the 5th member in the 2nd int and the last in it's own byte (2 + 2 + 1 = 5). If padding is added it could become 6 bytes.

typedef struct t_channel_info {
	int target_temp:9;               // byte 1&2
	unsigned char control_mode:2;    // byte 2
	unsigned char status:2;          // byte 2
	unsigned char warming:1;         // byte 2
	int current_temp:9;              // byte 3&4
	unsigned char warm_proportion;   // byte 5
} t_channel_info;

It is possible C51 can put bit fields in bytes which might give you;

typedef struct t_channel_info {
	int target_temp:9;               // byte 1&2
	unsigned char control_mode:2;    // byte 3
	unsigned char status:2;          // byte 3
	unsigned char warming:1;         // byte 3
	int current_temp:9;              // byte 4&5
	unsigned char warm_proportion;   // byte 6
} t_channel_info;

Access each of the members in some test code and look at the assembly language to figure what is happening.
Best luck
Scott

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0 Scott deWolski over 23 years ago

typedef struct t_channel_info {
	int target_temp:9;               // byte 1&2
	unsigned char control_mode:2;    // byte 2
	unsigned char status:2;          // byte 2
	unsigned char warming:1;         // byte 2
	int current_temp:9;              // byte 3&4
	unsigned char warm_proportion;   // byte 5
} t_channel_info;

It is possible C51 can put bit fields in bytes which might give you;

typedef struct t_channel_info {
	int target_temp:9;               // byte 1&2
	unsigned char control_mode:2;    // byte 3
	unsigned char status:2;          // byte 3
	unsigned char warming:1;         // byte 3
	int current_temp:9;              // byte 4&5
	unsigned char warm_proportion;   // byte 6
} t_channel_info;

Access each of the members in some test code and look at the assembly language to figure what is happening.
Best luck
Scott

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