Undefined behavior in C is a reading error
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Undefined behavior in C is a reading error
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Re: Undefined behavior in C is a reading error
#2Re: Undefined behavior in C is a reading error
#3If C is just a portable assembler then what if the assembly itself has undefined behaviour. :)
Re: Undefined behavior in C is a reading error
#4The author seems to be missing this essential text: "the implementor may augment the language by providing a definition of the officially undefined behavior."
Making a system call is undefined behavior in the C standard, but it's not undefined behavior in clang-on-FreeBSD, because the implementors of clang on FreeBSD have defined what those system calls do.
Ditto for "asm" (UD unless/until you're running on a compiler which defines what that does), all of the tricks which make "malloc" work, and all of his other examples of acceptable uses of code which the C standard does not define.
Re: Undefined behavior in C is a reading error
#5I don't see any utility in inventing a new reading of the standard. Getting everyone to agree on a new interpretation of a sentence can't possibly be easier than getting everyone to agree on a more clearly worded sentence. The actual thing you'd have to convince everyone of (the utility of the new consensus) and the people you'd have to convince (compiler writers, documentation authors) are the same in both cases.
Re: Undefined behavior in C is a reading error
#6The author seems to be missing this essential text: "the implementor may augment the language by providing a definition of the officially undefined behavior." Making a system call is undefined behavior in the C standard, but it's not undefined behavior in clang-on-FreeBSD, because the implementors of clang on FreeBSD have defined what those system calls do. Ditto for "asm" (UD unless/until you're running on a compile…
The thing about UB is that it tends to happen when the C standard refuses to specify when a program segment is erroneous or valid. Some C environments treat memory as a large array of undifferentiated bytes or words, by design. Other C environments have tagged, bounds-checked regions of memory, again by design. (For example, the C compiler for the Lisp machine.) Usually, indirecting through a null pointer or walking off the end of an array are erroneous, but sometimes you want to read from memory location 0, or scan through all of available memory. The C standard allows for both kinds of environments by stating that these behaviors are undefined, allowing the implementation to error out or do something sensible, depending on the environment.
The idea that UB is carte blanche for implementations to do whatever is an unintended consequence of the vague language of the standard. Maybe a future C standard should use "safe" and "unsafe" instead of UB for some of these operations, and clarify that unsafe code will be erroneous in a safe environment and do something sensible but potentially dangerous in an unsafe environment so you must really know what you're doing.
Re: Undefined behavior in C is a reading error
#7The author seems to be missing this essential text: "the implementor may augment the language by providing a definition of the officially undefined behavior." Making a system call is undefined behavior in the C standard, but it's not undefined behavior in clang-on-FreeBSD, because the implementors of clang on FreeBSD have defined what those system calls do. Ditto for "asm" (UD unless/until you're running on a compile…
The C standards have the perfectly fine name "implementation dependent" to describe those things. Undefined behavior is much less constrained than implementation dependent, adn thus more problematic.
Re: Undefined behavior in C is a reading error
#8Duplicate, https://news.ycombinator.com/item?id=27217981
That only got 1 comment.
Re: Undefined behavior in C is a reading error
#9If C is just a portable assembler then what if the assembly itself has undefined behaviour. :)
This exists, but the effect of undefined behavior in CPU architectures is a little bit more forgiving than the interpretation of UB in C to mean "literally the entire program has no meaning". Instead, usually the program will execute correctly up to the invalid instruction, and then something happens, and then the CPU will continue executing from that state. It's actually fairly difficult to build an instruction with undefined behavior that contaminates unrelated parts of the program.
Though it HAS happened: notably, brucedawson explains here [1] that the 360 has an instruction so badly thought out that merely having it in an executable page is enough to make your program otherwise meaningless due to speculative execution.
[1] https://randomascii.wordpress.com/2018/01/07/finding-a-cpu-d...
Re: Undefined behavior in C is a reading error
#10If C is just a portable assembler then what if the assembly itself has undefined behaviour. :)
This exists, but the effect of undefined behavior in CPU architectures is a little bit more forgiving than the interpretation of UB in C to mean "literally the entire program has no meaning". Instead, usually the program will execute correctly up to the invalid instruction, and then something happens, and then the CPU will continue executing from that state. It's actually fairly difficult to build an instruction with…
Wow! Interesting to see hints that meltdown exists years before it was officially published.