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);
}