What's the difference between the following structures?

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struct pack
{
 unsigned int a:9;
} p;

And

struct pack
{
 unsigned int a;
} p;

Please explain the difference.
Thanks in advance.

Posted 30-Mar-11 8:18am

UL UL ALBAB

Updated 30-Mar-11 8:35am

Sergey Alexandrovich Kryukov

v3

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3 solutions

Solution 2

Found you an answer: [^]

Posted 30-Mar-11 8:27am

twohowlingdogs

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Sergey Alexandrovich Kryukov 30-Mar-11 14:31pm

I provided more detailed answer before. Why would you add the Answer without adding any additional value?
--SA

twohowlingdogs 30-Mar-11 14:36pm

I saw this question before you posted so I did not see your answer. All the information needed was in the link so I didn't think I needed to elaborate.

Sergey Alexandrovich Kryukov 30-Mar-11 15:24pm

Understood, sure.
--SA

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Sergey Alexandrovich Kryukov · Accepted Answer · 2011-03-30T08:25:00

In first sample a bit specifier is used.
Normally, if makes sense if you have a structure with more than one member with the bit specifier, such as ":9" in your sample.

It allows you to pack together some members which normally would use more bits then specified. The real reason for doing that is not saving of some memory. Rather, it is used to match exact binary layout of the structure required by some hardware, hardware API or binary compatibility with other 3rd-party code.

For detailed explanation, see: http://docs.embarcadero.com/products/rad_studio/delphiAndcpp2009/HelpUpdate2/EN/html/devwin32/bitfields_xml.html[^].

Pay special attention for Note on Compatibility — it is not a trivial issue.

—SA

OriginalGriff · Accepted Answer · 2011-03-30T08:36:00

In the first example, you are explicitly saying how many bits of the integer should the value of "a" take up - bits 0 to 8 inclusive, for a total of 9.
In the second, you are saying that "a" occupies an entire unsigned int (usually 32 bits).

So if you had an instance of each strucure called "b" and you did this:

b.a = 0xffff;
printf("0x%x", (unsigned int) b);

Then the first way would print "0x1ff" and the second "0xffff";

You can use this to combine values into a single integer, or byte which can help with communications, either between processes or external hardware.

What's the difference between the following structures?

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