On Jan 13, 2009, at 1:37 PM, Andrew Ross wrote: > On Tue, Jan 13, 2009 at 12:51:03PM -0700, Jerry wrote: >> >> On Jan 9, 2009, at 12:51 AM, Andrew Ross wrote: >> >>> >>> Jerry, >>> >>> While adapting example 12 to use plcol1 and plscmap1l I came across >>> one >>> interesting issue with ada. >>> >>> Several example use something like >>> >>> A_Boolean : Boolean_Array_1D(1 .. 1); >>> >>> A_Boolean(1) := False; >>> >>> plscmap1l(HLS, i, h, l, s, A_Boolean); >>> >>> Note that in these cases the h, l, s arrays are only of length 2. >>> This >>> works for these examples, but when I try for example 12 with colour >>> arrays of length 5 then it doesn't work. I assume this is trying to >>> emulate the C usage of NULL for the array pointer to signal no >>> colours >>> are reversed. Explicitly defining an array of length 5 and willing >>> with >>> False values works as expected. >> >> Andrew, >> >> Thanks for catching this. >> >> I'm not sure what happened to lead to my converting several examples >> to use A_Boolean : Boolean_Array_1D(1 .. 1) in this way, but it now >> seems wrong to me. I currently do not check that the five arrays >> passed to plscmap1l are actually the same length and so passing a >> length-one array to the reverse flag slot when the others are more >> than length 1 probably caused the C code to execute a out-of-bounds >> array access. It was only a matter of luck that this didn't cause >> problems on the other examples. When you tried this with Example 12, >> some of these out-of-bounds accesses probably resulted in non-zero >> numbers, interpreted by C as True, and caused several of the colors >> to be wrong. That's my best guess as to what happened. >> >> I looked into what would be required to emulate the C behavior and >> found that to implement the C expectation exactly in Ada would >> require asking the user to pass a pointer to the reversing array in >> stead of the array itself. This would be very high level of weirdness >> to ask of the user. Alternately, it might be possible to hack up >> something whereby if the user passed a null array (an array of zero >> length) then call a different version of the thin-binding routine. I >> looked into this and it looked like at best a _lot_ of trouble. And >> all this to emulate a frankly dubious C trick. >> >> So I changed all of the examples to use a proper-length reverse flag >> array and added a note to the documentation that this is a variance >> with the C documentation. One could argue I suppose if this is a >> variance with the actual C API. > > Jerry, > > I think that is fine. Some other bindings do the same thing. The NULL > pointer trick in C is really a convenience to the user. Doesn't always > make sense. One other trick that some languages use it to treat the > pointer as NULL if the argument is missing. This depends on whether > the > language allows default values for arguments (e.g. C++). If neither of > these is relevant in Ada, then don't worry. > > Andrew > Fixed with overloaded procs in rev. 9300. Jerry
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