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It's certainly not "illegal", it's merely undefined. This may be a pedantic difference but we're talking about an area where being technically correct _really_

It's certainly not "illegal", it's merely undefined. This may be a pedantic difference but we're talking about an area where being technically correct _really_ is the best kind of correct. I bring up VM behavior because, particularly given the timeframe, it's important to consider that there once was a time when VM did not exist, and SEGV meant an actual _segmentation_ fault.

mikeash 14y ago
The code in question is not valid C code. It just happens not to require an error to be raised, but I think "illegal" sums it up just fine. That's not really a specific term in this situation. As for VM and not-VM, I don't really see why it matters. It would be perfectly valid for a C compiler to generate code that does bounds checking on every array access, for example. And there were far weirder ways to do memory hardware than the standard MMU with 4kB pages that we have today, and the legacy of that can be found throughout the C standard. For example, the fact that it's illegal to store many values in a pointer, not dereference but merely store, comes from systems that really would check that stuff and crash your app if you had a bad pointer value in the wrong place, even if you didn't try to load what it pointed to.
- mnarayan01 14y ago
  > The code in question is not valid C code. If it were not valid C code, then a standard-compliant C compiler would be required to reject it. Now don't get me wrong, having code that is dependent dereferencing past the end of an array is not a good idea. Having such code in a popular benchmark is a bad situation to be in, particularly when it interferes with progress in compiler optimization. It is not, however, "illegal" for any standard definition of illegal, and in this context clarity is important as ambiguity can interfere with compiler optimization :) Edit: Assume you have a machine/OS with a page size of 1 byte. Can a compiler generate a program that does not raise an exception in the situation mentioned by the OP? Saying such a program is illegal would surely imply that it could not.
  - mikeash 14y ago
    > If it were not valid C code, then a standard-compliant C compiler would be required to reject it. Nope! Here's a direct quote out of the copy of the standard I have lying about, from the section that discusses when diagnostic output is required: "The intent is that an implementation should identify the nature of, and where possible localize, each violation. Of course, an implementation is free to produce any number of diagnostics as long as a valid program is still correctly translated. It may also successfully translate an invalid program." That last sentence is particularly pertinent. Many (most?) languages have two kinds of programs: valid and invalid. C has three: valid, invalid requiring the compiler to reject it, and invalid but the compiler may still accept it. For whatever reason, people have a very hard time understanding the implications of the third category, or, as in your case, even realizing that it exists.
    - mnarayan01 14y ago
      Is that C89 or C99? I only see that in the latter, though the C89 spec I see may have footnotes removed. From C99: > A conforming implementation shall produce at least one diagnostic message (identified in an implementation-defined manner) if a preprocessing translation unit or translation unit contains a violation of any syntax rule or constraint, even if the behavior is also explicitly specified as undefined or implementation-defined. would seem to imply that a standard compliant compiler would need to produce a message for an invalid program. I will say it is both interesting and surprising (to me) that at least C99 does allow for the successful translation of an invalid C program. Regardless, while informative, I'll probably stop posting on this topic before I lose the ability to ;)
      - mikeash 14y ago
        Your quote is where the footnote comes from. I think what you're missing (and what I missed when I first read that while looking this up) is the "violation of any syntax rule or constraint" part. In other words, a diagnostic is required for syntax errors only. For other stuff, like bad pointer dereferences, division by zero, etc., it's optional.
cygx 14y ago
It's certainly not "illegal", it's merely undefined. Potayto, potahto. Violating language semantics results in undefined behaviour. You can only get less legal if you add syntax errors into the mix. You could make the argument that some undefined behaviour should only be considered nonportable instead of erroneous, but personally I consider code that triggers undefined behaviour as invalid or possibly written in a non-standard C dialect.

mikeash 14y ago

The code in question is not valid C code. It just happens not to require an error to be raised, but I think "illegal" sums it up just fine. That's not really a specific term in this situation. As for VM and not-VM, I don't really see why it matters. It would be perfectly valid for a C compiler to generate code that does bounds checking on every array access, for example. And there were far weirder ways to do memory hardware than the standard MMU with 4kB pages that we have today, and the legacy of that can be found throughout the C standard. For example, the fact that it's illegal to store many values in a pointer, not dereference but merely store, comes from systems that really would check that stuff and crash your app if you had a bad pointer value in the wrong place, even if you didn't try to load what it pointed to.

mnarayan01 14y ago
> The code in question is not valid C code. If it were not valid C code, then a standard-compliant C compiler would be required to reject it. Now don't get me wrong, having code that is dependent dereferencing past the end of an array is not a good idea. Having such code in a popular benchmark is a bad situation to be in, particularly when it interferes with progress in compiler optimization. It is not, however, "illegal" for any standard definition of illegal, and in this context clarity is important as ambiguity can interfere with compiler optimization :) Edit: Assume you have a machine/OS with a page size of 1 byte. Can a compiler generate a program that does not raise an exception in the situation mentioned by the OP? Saying such a program is illegal would surely imply that it could not.
- mikeash 14y ago
  > If it were not valid C code, then a standard-compliant C compiler would be required to reject it. Nope! Here's a direct quote out of the copy of the standard I have lying about, from the section that discusses when diagnostic output is required: "The intent is that an implementation should identify the nature of, and where possible localize, each violation. Of course, an implementation is free to produce any number of diagnostics as long as a valid program is still correctly translated. It may also successfully translate an invalid program." That last sentence is particularly pertinent. Many (most?) languages have two kinds of programs: valid and invalid. C has three: valid, invalid requiring the compiler to reject it, and invalid but the compiler may still accept it. For whatever reason, people have a very hard time understanding the implications of the third category, or, as in your case, even realizing that it exists.
  - mnarayan01 14y ago
    Is that C89 or C99? I only see that in the latter, though the C89 spec I see may have footnotes removed. From C99: > A conforming implementation shall produce at least one diagnostic message (identified in an implementation-defined manner) if a preprocessing translation unit or translation unit contains a violation of any syntax rule or constraint, even if the behavior is also explicitly specified as undefined or implementation-defined. would seem to imply that a standard compliant compiler would need to produce a message for an invalid program. I will say it is both interesting and surprising (to me) that at least C99 does allow for the successful translation of an invalid C program. Regardless, while informative, I'll probably stop posting on this topic before I lose the ability to ;)
    - mikeash 14y ago
      Your quote is where the footnote comes from. I think what you're missing (and what I missed when I first read that while looking this up) is the "violation of any syntax rule or constraint" part. In other words, a diagnostic is required for syntax errors only. For other stuff, like bad pointer dereferences, division by zero, etc., it's optional.

cygx 14y ago

It's certainly not "illegal", it's merely undefined. Potayto, potahto. Violating language semantics results in undefined behaviour. You can only get less legal if you add syntax errors into the mix. You could make the argument that some undefined behaviour should only be considered nonportable instead of erroneous, but personally I consider code that triggers undefined behaviour as invalid or possibly written in a non-standard C dialect.