Data structure alignment: Difference between revisions

Revision as of 14:32, 6 July 2006

Revision as of 15:04, 8 August 2006

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==Typical alignment of C structs on x86==

[[Data structure]] members are stored sequentially in a memory so that in the structure below the member Data1 will always precede Data2 and Data2 will always precede Data3:

struct MyData

{

short Data1;

short Data2;

short Data3;

};

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The type of each member of the structure usually has a default alignment, meaning that it will, unless otherwise requested by the programmer, be aligned on a pre-determined boundary. The following typical alignments are valid for compilers from [[Microsoft]], [[Borland]], and [[GNU]] when compiling for x86:

* A '''char''' (one byte) will be 1-byte aligned.<br/>

* A '''short''' (two bytes) will be 2-byte aligned.<br/>

* An '''int''' (four bytes) will be 4-byte aligned.<br/>

* A '''float''' (four bytes) will be 4-byte aligned.<br/>

* A '''double''' (eight bytes) will be 8-byte aligned on Windows and 4-byte aligned on Linux.

Here is a structure with members of various types, totaling '''8 bytes''' before compilation:

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struct MixedData

{

char Data1;

short Data2;

int Data3;

char Data4;

};

After compilation the data structure will be supplemented with padding bytes to ensure a proper alignment for each of its member:

struct MixedData ( /* after compilation) */

{

char Data1;

char Padding0[1];

short Data2;

int Data3;

char Data4;

char Padding1[3];

};

The compiled size of the structure is now '''12 bytes'''. It is important to note that the last member is padded with the number of bytes required to conform to the largest type of the structure. In this case 3 bytes are added to the last member to pad the structure to the size of a long word.

It’sIt is possible to change the alignment of structures to reduce the memory they require (or to conform to an existing format) by changing the compiler’s alignment (or “packing”) of structure members.

Requesting that the “MixedData”''MixedData'' structure above be aligned to a one byte boundary will have the compiler discard the pre-determined alignment of the members and no padding bytes would be inserted.

While there is no standard way of defining the alignment of structure member, some compilers use ''#pragma'' directives to specify packing inside source files. Here is an example:

#pragma pack( push ) ; /* push current alignment to stack */

#pragma pack( 1 ) ; /* set alignment to 1 byte boundary */

struct MyPackedData

{

byte char Data1;

long Data2;

byte char Data3;

};

#pragma pack( pop ) ; /* restore original alignment from stack */

This structure would have a compiled size of '''6 bytes'''. The above directives are available in compilers from [[Microsoft]], [[Borland]] and many others.