On Fri, Jul 25, 2003 at 11:28:05AM +0200, Eric de Jong wrote: > Thank you, Jan Zizka, Roman Kellner > > I would expect that the allignment has only influence on the address of > variables, not on the total size of a structure. It looks like the compiler > calculates the size of the structure to be a multiple of the largest item in the > structure. > What I want is to allign each variable in the structure to it's own size, but > the total size of the structure to be the actual size. Is this possible? Without using packed and aligned attributes, this is not possible. This can be a severe performance penalty on many architectures and are not supported by other compilers. According to the ISO C90 and C99 standards, the alignment of a structure or union is at least the maximum of the alignments of each of its members. A compiler can make structure and unions have even more alignment than the alignment of the members, for example some ABI's on word based machines will align any structure to a word boundary, even if all of the fields are chars, and on the machine I just did an ABI for, the compiler will align any structure with 2 or more words to double word alignment so the double word memory instructions can be more often used. This wording has been in the C specification ever since I can remember (and I used C on UNIX V6 PDP-11 machines). The reason for this is to make arrays and p++ work correctly. If the size were smaller, p++ would wind up pointing to unalgined memory. Note, also you are not guaranteed that short/int/long are the same size or alignment on different machines either. If you are reading files produced by another computer, you have to worry about endian issues as well. -- Michael Meissner email: gnu@xxxxxxxxxxxxxxxxx http://www.the-meissners.org
