dave@aspect.UUCP (Dave Corcoran) (06/20/91)
printf("Hello World\n");
My Sun cc prints 4 for both structs, prints 3 for the array
-----------------8<--------------------
struct a {char x;char y;char z};
struct b {char x[3]};
char c[3];
main()
{
printf("%d\n",sizeof (struct a));
printf("%d\n",sizeof (struct b));
printf("%d\n",sizeof c);
}
-----------------8<--------------------
Is there any way to cause cc to force sizeof return the actual size of the
structs without rounding up to the next highest sizeof (short)?
Thanx
--
David Corcoran -@@
uunet!aspect!dave ~
Having the right to do something is not the same as being right in doing it.
-- C.K. Chestersonok@goanna.cs.rmit.oz.au (Richard A. O'Keefe) (06/20/91)
In article <10569@aspect.UUCP>, dave@aspect.UUCP (Dave Corcoran) writes: > My Sun cc prints 4 for both structs, prints 3 for the array > struct a {char x;char y;char z}; > struct b {char x[3]}; > char c[3]; Syntax error: that should be struct a {char x; char y; char z;}; ^ and the same for struct b. Omitting the last ; before the } of a struct was a Berkeley extension. Many compilers do not support it, and it is not standard. > Is there any way to cause cc to force sizeof return the actual size of the > structs without rounding up to the next highest sizeof (short)? Four bytes *IS* the actual size of the structs in that implementation of C. Remember, "sizeof (T)" means "if you had an array of N elements each of type (T), the number of bytes in the array would be N times <what>?". If you have an array of (struct a)s, they will be 4 bytes apart, not 3. There isn't anything you can portably do that would profit from trying to outsmart the compiler in this way, and very little that is _safe_ even for a program that is never going to see another compiler or machine. (You could try taking offsetof the last field + sizeof the last field. But of course the last field itself may contain padding.) -- I agree with Jim Giles about many of the deficiencies of present UNIX.
Dave.Harris@f14.n15.z1.fidonet.org (Dave Harris) (06/22/91)
In a message of <Jun 20 21:49>, Dave Corcoran (1:114/15) writes: >My Sun cc prints 4 for both structs, prints 3 for the array >-----------------8<-------------------- >struct a {char x;char y;char z}; >struct b {char x[3]}; >char c[3]; >main() >{ > printf("%d\n",sizeof (struct a)); > printf("%d\n",sizeof (struct b)); > printf("%d\n",sizeof c); >} >-----------------8<-------------------- >Is there any way to cause cc to force sizeof return the actual size of >the >structs without rounding up to the next highest sizeof (short)? >Thanx I assume that you are getting 4 when expecting 3? If so... Normally, 4 IS the size of this struct when it is stored in memory. Why you would want 3 returned escapes me. You use the 3 to malloc() (or any number of other address calculations and), you are just asking for bugs. There might be some options that allow you to specify alignment on a byte boundary as opposed to a word boundary. This will cost you some speed however. Dave Harris -- Uucp: ...{gatech,ames,rutgers}!ncar!asuvax!stjhmc!15!14!Dave.Harris Internet: Dave.Harris@f14.n15.z1.fidonet.org
mouse@thunder.mcrcim.mcgill.edu (der Mouse) (06/22/91)
In article <10569@aspect.UUCP>, dave@aspect.UUCP (Dave Corcoran) writes: > My Sun cc prints 4 for both structs, prints 3 for the array > struct a {char x;char y;char z}; > struct b {char x[3]}; > char c[3]; [code to print out sizeof() the above things omitted -dM] > Is there any way to cause cc to force sizeof return the actual size > of the structs without rounding up to the next highest sizeof(short)? It *is* returning the actual size of the structs. The size of a struct is defined to include any padding the compiler sees fit to include. der Mouse old: mcgill-vision!mouse new: mouse@larry.mcrcim.mcgill.edu