联合和类型转换中的位填充结构

Question

I have a unsigned 64-bit word and a bit padded structure which are both given below.The structure is inside a union which contains several(11 to be exact) similar but slightly structures.

uint64_t final_data_word;

#pragma pack(1)
typedef struct control_block_format_1_s
{
    uint8_t block_type_field:8;
    uint8_t control_word_0:7;
    uint8_t control_word_1:7;
    uint8_t control_word_2:7;
    uint8_t control_word_3:7;
    uint8_t control_word_4:7;
    uint8_t control_word_5:7;
    uint8_t control_word_6:7;
    uint8_t control_word_7:7;
}control_block_format_1_t;

typedef union
{
    control_block_format_1_t *cb_1;
    control_block_format_2_t *cb_2;
    control_block_format_3_t *cb_3;
    control_block_format_4_t *cb_4;
    control_block_format_5_t *cb_5;
    control_block_format_6_t *cb_6;
    control_block_format_7_t *cb_7;
    control_block_format_8_t *cb_8;
    control_block_format_9_t *cb_9;
    control_block_format_10_t *cb_10;
    control_block_format_11_t *cb_11;
}block_payload_union_t;
#pragma pack()

I want to interpret the 64 bits in the 64-bit word as fields in the structure.so I am doing the below operation

block_payload_union_t *block_pload =(block_payload_union_t*)malloc(sizeof(block_payload_union_t*));
block_pload->cb_1 = (control_block_format_1_t*)(&final_data_word);

but I am not getting the expected values for the last field in my structure.Can anyone see any problem with what I am doing?Any suggestions or comments are appreciated.

@Jonathan I have added the following comments to my code.
printf("sizeof(union) = %zu\\n", sizeof(block_payload_union_t));

printf("sizeof(cb1) = %zu\\n", sizeof(control_block_format_1_t));

printf("FDW = 0x%.16lx\\n", final_data_word);

//printf("*bp->llp = 0x%.16lx\\n", *block_pload->llp);

printf("bp->cb1->block_type_fld = 0x%.2X\\n", block_pload->cb_1->block_type_field);

printf("bp->cb1->control_word_0 = 0x%.2X\\n", block_pload->cb_1->control_word_0);

printf("bp->cb1->control_word_1 = 0x%.2X\\n", block_pload->cb_1->control_word_1);

printf("bp->cb1->control_word_2 = 0x%.2X\\n", block_pload->cb_1->control_word_2);

printf("bp->cb1->control_word_3 = 0x%.2X\\n", block_pload->cb_1->control_word_3);

printf("bp->cb1->control_word_4 = 0x%.2X\\n", block_pload->cb_1->control_word_4);

printf("bp->cb1->control_word_5 = 0x%.2X\\n", block_pload->cb_1->control_word_5);

printf("bp->cb1->control_word_6 = 0x%.2X\\n", block_pload->cb_1->control_word_6);

printf("bp->cb1->control_word_7 = 0x%.2X\\n", block_pload->cb_1->control_word_7);

The output I got without #pragma pack() was as follows

final data word 0x1e00000000000000

sizeof(union) = 8

sizeof(cb1) = 9

FDW = 0x1e00000000000000

bp->cb1->block_type_fld = 0x00

bp->cb1->control_word_0 = 0x00

bp->cb1->control_word_1 = 0x00

bp->cb1->control_word_2 = 0x00

bp->cb1->control_word_3 = 0x00

bp->cb1->control_word_4 = 0x00

bp->cb1->control_word_5 = 0x00

bp->cb1->control_word_6 = 0x1E

bp->cb1->control_word_7 = 0x78

The output with #pragma pack() was as follows

final data word 0x1e00000000000000

sizeof(union) = 8

sizeof(cb1) = 8

FDW = 0x1e00000000000000

bp->cb1->block_type_fld = 0x00

bp->cb1->control_word_0 = 0x00

bp->cb1->control_word_1 = 0x00

bp->cb1->control_word_2 = 0x00

bp->cb1->control_word_3 = 0x00

bp->cb1->control_word_4 = 0x00

bp->cb1->control_word_5 = 0x00

bp->cb1->control_word_6 = 0x00

bp->cb1->control_word_7 = 0x0F

which is similar to the output you got on Jonathan's machine.

Answer 1

You should be using sizeof(block_payload_union_t) instead of sizeof(block_payload_union_t *) in the call to malloc() . However, on a 64-bit machine, it probably gives you the same size (8), so you get away with it, for all it is wrong.

It is slightly odd that your block_payload_union_t contains pointers to your field layouts instead of holding the actual values.

You have not shown us the declaration of final_data_word . Have you checked the size of your union versus the size you expect?

---

On a Mac running Lion (10.7.2), I get this output from the program which follows:

Output

With #pragma pack(1) :

sizeof(union) = 8
sizeof(cb1) = 8
FDW = 0xFEDCBA9876543210
*bp->llp = 0xFEDCBA9876543210
bp->cb1->block_type_fld = 0x10
bp->cb1->control_word_0 = 0x32
bp->cb1->control_word_1 = 0x28
bp->cb1->control_word_2 = 0x59
bp->cb1->control_word_3 = 0x43
bp->cb1->control_word_4 = 0x29
bp->cb1->control_word_5 = 0x17
bp->cb1->control_word_6 = 0x37
bp->cb1->control_word_7 = 0x7F

Without #pragma pack(1) :

sizeof(union) = 8
sizeof(cb1) = 9
FDW = 0xFEDCBA9876543210
*bp->llp = 0xFEDCBA9876543210
bp->cb1->block_type_fld = 0x10
bp->cb1->control_word_0 = 0x32
bp->cb1->control_word_1 = 0x54
bp->cb1->control_word_2 = 0x76
bp->cb1->control_word_3 = 0x18
bp->cb1->control_word_4 = 0x3A
bp->cb1->control_word_5 = 0x5C
bp->cb1->control_word_6 = 0x7E
bp->cb1->control_word_7 = 0x10

What are you getting?

Program

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

#pragma pack(1)
typedef struct control_block_format_1_s
{
    uint8_t block_type_field:8;
    uint8_t control_word_0:7;
    uint8_t control_word_1:7;
    uint8_t control_word_2:7;
    uint8_t control_word_3:7;
    uint8_t control_word_4:7;
    uint8_t control_word_5:7;
    uint8_t control_word_6:7;
    uint8_t control_word_7:7;
} control_block_format_1_t;

typedef union
{
    long long *llp;
    control_block_format_1_t *cb_1;
    //control_block_format_2_t *cb_2;
    //control_block_format_3_t *cb_3;
    //control_block_format_4_t *cb_4;
    //control_block_format_5_t *cb_5;
    //control_block_format_6_t *cb_6;
    //control_block_format_7_t *cb_7;
    //control_block_format_8_t *cb_8;
    //control_block_format_9_t *cb_9;
    //control_block_format_10_t *cb_10;
    //control_block_format_11_t *cb_11;
} block_payload_union_t;
#pragma pack()

int main(void)
{
    long long final_data_word = 0xFEDCBA9876543210;
    block_payload_union_t *block_pload =(block_payload_union_t*)malloc(sizeof(block_payload_union_t));
    block_pload->cb_1 = (control_block_format_1_t*)(&final_data_word);
    printf("sizeof(union) = %zu\n", sizeof(block_payload_union_t));
    printf("sizeof(cb1) = %zu\n", sizeof(control_block_format_1_t));
    printf("FDW = 0x%.16llX\n", final_data_word);
    printf("*bp->llp = 0x%.16llX\n", *block_pload->llp);
    printf("bp->cb1->block_type_fld = 0x%.2X\n", block_pload->cb_1->block_type_field);
    printf("bp->cb1->control_word_0 = 0x%.2X\n", block_pload->cb_1->control_word_0);
    printf("bp->cb1->control_word_1 = 0x%.2X\n", block_pload->cb_1->control_word_1);
    printf("bp->cb1->control_word_2 = 0x%.2X\n", block_pload->cb_1->control_word_2);
    printf("bp->cb1->control_word_3 = 0x%.2X\n", block_pload->cb_1->control_word_3);
    printf("bp->cb1->control_word_4 = 0x%.2X\n", block_pload->cb_1->control_word_4);
    printf("bp->cb1->control_word_5 = 0x%.2X\n", block_pload->cb_1->control_word_5);
    printf("bp->cb1->control_word_6 = 0x%.2X\n", block_pload->cb_1->control_word_6);
    printf("bp->cb1->control_word_7 = 0x%.2X\n", block_pload->cb_1->control_word_7);
    return(0);
}

Answer 2

What's the point in having pointers as members in your union? You could achieve the same result via casts.

If what you want is to access the data (and not the pointers) through the eleven different structures, then in the union you don't want to use pointers but the structs directly:

typedef union
{
    control_block_format_1_t cb_1;
    control_block_format_2_t cb_2;
    control_block_format_3_t cb_3;
    control_block_format_4_t cb_4;
    control_block_format_5_t cb_5;
    control_block_format_6_t cb_6;
    control_block_format_7_t cb_7;
    control_block_format_8_t cb_8;
    control_block_format_9_t cb_9;
    control_block_format_10_t cb_10;
    control_block_format_11_t cb_11;
} block_payload_union_t;

Now the size of your block_payload_union_t struct is just 64-bits, and you can work with this data using any of the eleven versions.

Answer 3

#pragma pack(1) directive would cause that member to be packed in the structure on a 1-byte boundary, however #pragma pack directive aligns all bit fields in a structure/union on 1-bit boundaries. This is why the last field is kicked off. Check the sizeof of control_block_format_1_s whether it is 8 or 9.