Optimal structure field ordering

[Mark Stevans]

In article <2775 at ttrdc.UUCP> levy at ttrdc.UUCP (Daniel R. Levy) writes:
<In article <163 at navtech.uucp>, mark at navtech.uucp (Mark Stevans) writes:
<# Due to the alignment requirements of various processor architectures, the
<# declared order of fields within a C structure can significantly effect the
<# space required to store the structure.  The easy way to save space on almost
<# all C implementations is to sort your structure fields in order of descending
<# size of the field type.  For arrays, just use the base type.
<# A brief example.  The following program:
<# 	typedef struct { char buf[5]; short s; char c; long l; } Biggie;
<# 	typedef struct { short s; long l; char buf[5]; char c; } Smallie;
<# 	main()
<# 	{
<# 		printf("Biggie is %d bytes long, but Smallie is only %d bytes long.\n",
<# 				sizeof (Biggie), sizeof (Smallie));
<# 	}
<# When compiled and run on a Sun-3/50 produces the output:
<# 	Biggie is 14 bytes long, but Smallie is only 12 bytes long.
<I get better results with:
<typedef struct { long l; short s; char buf[5]; char c; } Smallie;

How goofy can I get?  My example doesn't properly follow my proposed rule!
Just goes to prove the old adage: avoid comments, since they might conflict
with your code.  Thanks for the correction, DRL.  But anyhows, what do y'all
think of the rule itself:

	"The space requirement of any given C structure may be easily
	optimized by reorganizing the structure fields in order of decreasing
	base type size."

Some responder pointed out correctly that there is no guarantee in the C
language that the compiler will preserve your field ordering -- but I wouldn't
worry about that.  I have yet to see a C compiler that did any reordering of
structure fields.  It would make it a challenge to examine structures through
"adb"!  Perhaps some compilers put the elements in reverse order, but that's
about as strange as reality currently gets.

						Mark "Shorty" Stevans