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struct with members' sizes

Bahri Okuroglu

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?

  • 0

    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




  • 0

    The best you can do with a simple struct is 5 bytes:

    typedef struct t_channel_info
{
    //int at 0-1
             int target_temp:9;
	unsigned int control_mode:2;
	unsigned int status:2;
	unsigned int warming:1;

    //int at 2-3
	         int current_temp:9;

    //char at 4
	unsigned char warm_proportion:8;
} t_channel_info;

t_channel_info . . . . . . . . . . . .  * TAG *  -----  STRUCT   -----  5
  target_temp. . . . . . . . . . . . .  MEMBER   -----  FIELD    0000H  9.0
  control_mode . . . . . . . . . . . .  MEMBER   -----  FIELD    0000H  2.9
  status . . . . . . . . . . . . . . .  MEMBER   -----  FIELD    0000H  2.11
  warming. . . . . . . . . . . . . . .  MEMBER   -----  FIELD    0000H  1.13
  current_temp . . . . . . . . . . . .  MEMBER   -----  FIELD    0002H  9.0
  warm_proportion. . . . . . . . . . .  MEMBER   -----  FIELD    0004H  8.0

    With a union you can get 4 bytes:
    (The structures are broken out to get a better cross reference.)

    struct PartA
{
    //int at 0-1
             int target_temp:9;
	unsigned int control_mode:2;
	unsigned int status:2;
	unsigned int warming:1;
} a;

struct PartB
{
    //char at 0
             char dummy1;

    //int at 1-2
	         int dummy2:7;
	         int current_temp:9;

    //char at 3
	unsigned char warm_proportion;
};

typedef union
{
	struct PartA a;
	struct PartB b;
} t_channel_info2;

t_channel_info2. . . . . . . . . . . .  TYPEDEF  -----  UNION    -----  4
  a. . . . . . . . . . . . . . . . . .  MEMBER   -----  STRUCT   0000H  2
  b. . . . . . . . . . . . . . . . . .  MEMBER   -----  STRUCT   0000H  4

PartA. . . . . . . . . . . . . . . . .  * TAG *  -----  STRUCT   -----  2
  target_temp. . . . . . . . . . . . .  MEMBER   -----  FIELD    0000H  9.0
  control_mode . . . . . . . . . . . .  MEMBER   -----  FIELD    0000H  2.9
  status . . . . . . . . . . . . . . .  MEMBER   -----  FIELD    0000H  2.11
  warming. . . . . . . . . . . . . . .  MEMBER   -----  FIELD    0000H  1.13

PartB. . . . . . . . . . . . . . . . .  * TAG *  -----  STRUCT   -----  4
  dummy1 . . . . . . . . . . . . . . .  MEMBER   -----  CHAR     0000H  1
  dummy2 . . . . . . . . . . . . . . .  MEMBER   -----  FIELD    0001H  7.0
  current_temp . . . . . . . . . . . .  MEMBER   -----  FIELD    0001H  9.7
  warm_proportion. . . . . . . . . . .  MEMBER   -----  U_CHAR   0003H  1

  • 0 in reply to Jon Young

    Thanks for your kind replies.

    Based on your comments and links, I modified the struct as follows:

    typedef struct tt_channel_info {
	unsigned int warm_proportion:7;
	int target_temp:9;
	unsigned int control_mode:2;
	unsigned int status:2;
	unsigned int warming:1;
	int current_temp:9;
} tt_channel_info;

tt_channel_info. . . . . . . . . . . .  TYPEDEF  -----  STRUCT   -----  4
  warm_proportion. . . . . . . . . . .  MEMBER   -----  FIELD    0000H  7.0
  target_temp. . . . . . . . . . . . .  MEMBER   DATA   FIELD    0000H  9.7
  control_mode . . . . . . . . . . . .  MEMBER   DATA   FIELD    0002H  2.0
  status . . . . . . . . . . . . . . .  MEMBER   DATA   FIELD    0002H  2.2
  warming. . . . . . . . . . . . . . .  MEMBER   DATA   FIELD    0002H  1.4
  current_temp . . . . . . . . . . . .  MEMBER   DATA   FIELD    0002H  9.5

    Yes, I changed the size of warm_proportion to 7 bits. It is OK to have values between 0-127 for this field.


  • 0 in reply to Bahri Okuroglu

    Since you have control over the layout, I suggest the following modification to reduce code size and increase speed.

    typedef struct tt_channel_info {
	int target_temp:9;  //keep the fields > 8 bits at int offset 0
	unsigned int warm_proportion:7;

	int current_temp:9; //keep the fields > 8 bits at int offset 0
	unsigned int control_mode:2;
	unsigned int status:2;
	unsigned int warming:1;
} tt_channel_info;

  • 0

    for information please mail me at
    smansouria@yahoo.com
    with complete description
    to get your answer