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> Undefined means : the compiler can omit this case and assume it never happens. That's not the whole picture though. The standard actually says that anything

by gh02t 9y ago

> Undefined means : the compiler can omit this case and assume it never happens. That's not the whole picture though. The standard actually says that anything can happen in undefined cases. > However, if any such execution contains an undefined operation, this International Standard places no requirement on the implementation executing that program with that input (not even with regard to operations preceding the first undefined operation). The standard doesn't specify what the resulting program should do at all. It's equally valid to e.g. emit code that fails gracefully instead totally ignoring the undefined branch.

Retra 9y ago
Anything can happen because the compiler will assume it doesn't happen. "Undefined behavior" essentially means, "If you try to do this, you have written a program that is not C, and you should not be handing it to a C compiler expecting it to do anything correctly." If you were to try to write a program that accurately detected whether the given input is a meaningful C program, you'd have to solve the Halting Problem (and more, once you take IO into account.) Thus undefined behavior must exist in the places where your program is syntactically valid but has no definite semantics. (And you can't enumerate all of these places in a Turing complete language.) Not even a C compiler will be able to tell you if it actually has been given C code; all it can say is "I parsed it, and here're some semantics for it."
- dragonwriter 9y ago
  No, undefined behavior is still valid C (if it wasn't, it would be an invalid construct, not a valid conatruct with UB.) It's just C whose actual behavior is unspecified (and, because the standard is outright perverse, makes the entire program's behavior undefined) by the standard and, if you are especially lucky, explicitly defined by the particular C implementation. But maybe not even then.
littlestymaar 9y ago
> > Undefined means : the compiler can omit this case and assume it never happens. > That's not the whole picture though. The standard actually says that anything can happen in undefined cases. Well, that's right the standard says the compiler can do anything they want, including erasing all data from your hard drive or summoning a demon in your nose [1], but in practice I've never seen any compiler attempt such things. What compilers usually do with UB is assuming it will never happen and use this invariant as an optimization hint. [1] : http://catb.org/jargon/html/N/nasal-demons.html http://catb.org/jargon/html/N/nasal-demons.html

by gh02t 9y ago

Retra 9y ago

Anything can happen because the compiler will assume it doesn't happen. "Undefined behavior" essentially means, "If you try to do this, you have written a program that is not C, and you should not be handing it to a C compiler expecting it to do anything correctly." If you were to try to write a program that accurately detected whether the given input is a meaningful C program, you'd have to solve the Halting Problem (and more, once you take IO into account.) Thus undefined behavior must exist in the places where your program is syntactically valid but has no definite semantics. (And you can't enumerate all of these places in a Turing complete language.) Not even a C compiler will be able to tell you if it actually has been given C code; all it can say is "I parsed it, and here're some semantics for it."

dragonwriter 9y ago
No, undefined behavior is still valid C (if it wasn't, it would be an invalid construct, not a valid conatruct with UB.) It's just C whose actual behavior is unspecified (and, because the standard is outright perverse, makes the entire program's behavior undefined) by the standard and, if you are especially lucky, explicitly defined by the particular C implementation. But maybe not even then.

littlestymaar 9y ago

> > Undefined means : the compiler can omit this case and assume it never happens. > That's not the whole picture though. The standard actually says that anything can happen in undefined cases. Well, that's right the standard says the compiler can do anything they want, including erasing all data from your hard drive or summoning a demon in your nose [1], but in practice I've never seen any compiler attempt such things. What compilers usually do with UB is assuming it will never happen and use this invariant as an optimization hint. [1] : http://catb.org/jargon/html/N/nasal-demons.html http://catb.org/jargon/html/N/nasal-demons.html