> -----Original Message----- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of > Basile STARYNKEVITCH > Sent: Wednesday, January 24, 2007 10:30 AM > To: gcc@gcc.gnu.org > Subject: variable-sized array fields in structure? > > Hello all, > > It is common to have structures which end with an "undefined" > variable-length array like > > struct foo_st { > struct bar_st* someptr; > int len; > struct biz_st *tab[] /* actual size is len */; > }; > > I'm sorry to be unable to get the exact wording of this construct, > which I am sure is in some recent (C99? maybe) standard, unfortunately > I don't have these standards at hand.
It is discussed in section 6.7.2.1 of the C99 standard, in the Semantics section, paragraph #15 that explicitly allows the last element of a structure to have an array with no bound, called a flexible array. I don't have an online version of C90, but it may have been in there as well. > There is even a length attribute in GTY to help support this > http://gcc.gnu.org/onlinedocs/gccint/GTY-Options.html > > I believe the correct idiom in GCC source is to put an explicit > dimension to 1 (probably because 0 would make some old compilers > unhappy), ie to code instead > > struct foo_st { > struct bar_st* someptr; > int len; > struct biz_st *tab[1] /* 1 is dummy, actual size is len */; > }; Pre-ANSI/ISO compilers did not allow this, and 1 was used quite heavily in the community for things like char name[1];. > Unfortunately, when debugging (or taking sizeof), this makes a little > difference. > > My small suggestion would be > > 1. To explicitly document that such undefined variable-sized array > fields should be declared of dimension VARYING_SIZE (or some other > word), i.e. to code > > struct foo_st { > struct bar_st* someptr; > int len; > struct biz_st *tab[VARYING_SIZE] /* actual size is len */; > }; > > 2. To have a definition of VARYING_SIZE is some of our header files > (config.h, or system.h or others) which is 1 for old compilers and > empty for new ones (including gcc itself), maybe > > #if (defined __STDC_VERSION__ && __STDC_VERSION__ >= 199901L) > #define VARYING_SIZE 1 > #else > #define VARYING_SIZE /*empty*/ > #endif Probably reasonable. > > > Is there some point that I forgot? Probably yes, since my suggestion > is quite obvious but not yet in GCC? > > Thanks for reading. > > -- > Basile STARYNKEVITCH http://starynkevitch.net/Basile/ > email: basile<at>starynkevitch<dot>net mobile: +33 6 8501 2359 > 8, rue de la Faïencerie, 92340 Bourg La Reine, France > *** opinions {are only mines, sont seulement les miennes} *** > -- Michael Meissner AMD, MS 83-29 90 Central Street Boxborough, MA 01719
