Dave Korn wrote: > Andrew Haley wrote: >> Dave Korn wrote: >>> Dave Korn wrote: >>>> Dave Korn wrote: >>>> >>>>> ELF GAS/LD seem happy enough when presented with a ".comm foo,0" >>>>> directive, but PE does rather literally what you asked, and gives you >>>>> no data object, leading to i0 in the above being an undefined >>>>> reference at link time. >>>> After a bit further digging, it turns out that this is because PE uses >>>> the same representation for an external symbol as would represent a >>>> common of size 0, so if you define such a common, it magically becomes >>>> an undefined external symbol instead!
>> I don't see what the big deal is here: just allocate one byte to a >> statically- allocated zero-length array. > > The "big deal" is that this is blatantly and trivially invalid code, and we > silently accept it and generate nonsensical assembler output without the least > hint of any kind of a diagnostic. Of course we have to fix the assembler output. For ant two declarations a and b, &a != &b, even when a is a zero-length array. So, you have to allocate at least one byte. > " 6.7.5.2 Array declarators > Constraints > 1 In addition to optional type qualifiers and the keyword static, the [ and > ] may delimit an expression or *. If they delimit an expression (which > specifies the size of an array), the expression shall have an integer type. If > the expression is a constant expression, it shall have a value greater than > zero. " But zero-length arrays are a gcc extension. There's nothing that limits them to the last member of a struct. zero-length arrays must be rejected with -pedantic, but not otherwise. >> Sure, but in that case it's clearly the user's fault: they're writing >> beyond the bounds of an array. > > But how, as a user, would you attempt to write /within/ the bounds of such > an array? You can't. > Exactly. So why should we let the user create this meaningless > construct in the first place? Because it's a documented gcc extension. Andrew.
