C structure with bits printing in hexadecimal

I have defined a structure as below

   struct {
        UCHAR   DSatasetMGMT : 1;
        UCHAR   AtriburDeallocate : 1;
        UCHAR   Reserved6 : 6;
        UCHAR   Reserved7 : 7;
        UCHAR   DSatasetMGMTComply : 1;
    }DatasetMGMTCMDSupport;

It is a 2 byte structure represented in bits. How should I print the whole 2 bytes of structure in hexadecimal. I tried

"DatasetMGMTCMDSupport                          :   0x%04X\n"

And

0x%04I64X\n

But not getting expected result.

I am getting 0x3DC18003 with 0x%04X\n while the correct data is 0x8003 "

I am using 64 bit windows system.

I need to know how to print 2 byte structure in hexadecimal.

Try using 0x%04hx\n. This tells printf to print out only the two bytes. You can read more about it here: https://en.wikipedia.org/wiki/Printf_format_string#Length_field

In contrast, the I64 in 0x%04I64X\n tells printf to print out a 64 bit integer, which is 8 bytes, and 0x%04X\n tells it to print out a default-size integer, which might be 4 bytes on your system.

The width 04 specifies a minimum width. Since the value needs more digits, they are printed.

From a C Standard point of view, you cannot rely on a particular layout of bit fields. Hence, any solution will at best have implementation-defined behaviour.

That being said, your expected output can be obtained. The structure fits in 2 bytes and if you print sizeof(DatasetMGMTCMDSupport) it should give the result 2.

The byte representation of DatasetMGMTCMDSupport can be printed and that is what you were attempting, but since your system has integer size 4, two additional bytes are included. To fix this, the following can be done:

#include <stdint.h>
#include <string.h>
#include <stdio.h>
...
    uint16_t a;
    memcpy(&a, &DatasetMGMTCMDSupport, sizeof(a));
    printf("0x%04X", (unsigned)a);

This copies the 2 bytes of DatasetMGMTCMDSupport into a 2-byte integer variable and prints the hexadecimal representation of those 2 bytes only. If you are on a little-endian system, you should see 0x8003.

A more general approach would be to directly print the bytes of DatasetMGMTCMDSupport:

    for(unsigned i = 0; i < sizeof(DatasetMGMTCMDSupport); i++)
    {
        printf("%02X", (unsigned)((unsigned char *)&DatasetMGMTCMDSupport)[i]);
    }

This will most likely print 0380 (notice the byte order: first byte printed first).

To reverse the byte order is straightforward:

    for(unsigned i = 0; i < sizeof(DatasetMGMTCMDSupport); i++)
    {
        printf("%02X", (unsigned)((unsigned char *)&DatasetMGMTCMDSupport)[sizeof(DatasetMGMTCMDSupport)-1-i]);
    }

which should give 8003.