4 ms·

they are not necessarily illegal operations; embedded systems and kernel development would be two such examples - an area where C is frequently used. i can lis

by foxhill 11y ago

they are not necessarily illegal operations; embedded systems and kernel development would be two such examples - an area where C is frequently used. i can list many reasons why i continue to use C, please, there's no need to suggest or imply that C has outlived its usefulness - it clearly hasn't.

to3m 11y ago
Yes. The standard leaves a lot of things undefined precisely so that they can be legal, or do something meaningful - or, indeed, not - without causing the implementation to become non-standard because of it. This is what "undefined" means: what it say! It doesn't mean "illegal", and it doesn't mean "bad".
- scott_s 11y ago
  I think you are confused "implementation defined" and "undefined". If something is "implementation defined", it is legal for a C program to do it, even though the standard does not define what exactly must happen. Compiler writers are allowed to define a reasonable behavior for their situation. However, "undefined" does mean "illegal". Not only does the standard not say what will happen, the standard says that such behavior is not allowed in well-formed C programs. C compilers, then, tend to optimize based on the assumption that certain behaviors will not happen. This can causes weirdness when they encounter programs that exhibit this behavior - and yes, such programs have a bug. But often, the bug manifests in an even stranger way. Check out the work of John Regehr for more (academic publications: http://www.cs.utah.edu/~regehr/papers/; http://www.cs.utah.edu/~regehr/papers/; blog: http://blog.regehr.org/ http://blog.regehr.org/).
  - to3m 11y ago
    Well-formedness doesn't appear to have a definition in C11. In the C++11 standard, the wording rather implies to me that it is a static property, suggesting that a well-formed program could therefore still induce undefined behaviour at runtime. The C11 standard is however quite clear on what undefined behaviour is: "behaviour...for which this International Standard imposes no requirements". That sounds like a far cry from illegality to me! It sounds more like it is merely... yes... undefined. This means that gcc (et al) are allowed to do what they do. It doesn't mean it's not crap.
    - scott_s 11y ago
      I think you omitted the "erroneous" part of that definition. From http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf: 3.4.1 1 implementation-defined behavior unspecified behavior where each implementation documents how the choice is made 2 EXAMPLE An example of implementation-defined behavior is the propagation of the high-order bit when a signed integer is shifted right. 3.4.3 1 undefined behavior behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements 2 NOTE Possible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message). 3 EXAMPLE An example of undefined behavior is the behavior on integer overflow. And later, 4. Conformance 2 If a ‘‘shall’’ or ‘‘shall not’’ requirement that appears outside of a constraint or runtime constraint is violated, the behavior is undefined. Undefined behavior is otherwise indicated in this International Standard by the words ‘‘undefined behavior’’ or by the omission of any explicit definition of behavior. There is no difference in emphasis among these three; they all describe ‘‘behavior that is undefined’’. ... 5 A *strictly conforming* program shall use only those features of the language and library specified in this International Standard.3) It shall not produce output dependent on any unspecified, undefined, or implementation-defined behavior, and shall not exceed any minimum implementation limit. Substitute "strictly conforming" where I said (colloquially) "well-formed". Footnote 3 basically says that a program can still be strictly conforming if it uses of conditional features has conditional guards. All of that above tells me: "implementation-defined" and "undefined" are different concepts in the C standard, and programs with "undefined" behavior are erroneous - or, colloquially, illegal programs. Oddly, I still agree with your final conclusion: gcc and other compilers are definitely allowed to do what they do; they are following the standard.

by foxhill 11y ago

to3m 11y ago

Yes. The standard leaves a lot of things undefined precisely so that they can be legal, or do something meaningful - or, indeed, not - without causing the implementation to become non-standard because of it. This is what "undefined" means: what it say! It doesn't mean "illegal", and it doesn't mean "bad".

scott_s 11y ago
I think you are confused "implementation defined" and "undefined". If something is "implementation defined", it is legal for a C program to do it, even though the standard does not define what exactly must happen. Compiler writers are allowed to define a reasonable behavior for their situation. However, "undefined" does mean "illegal". Not only does the standard not say what will happen, the standard says that such behavior is not allowed in well-formed C programs. C compilers, then, tend to optimize based on the assumption that certain behaviors will not happen. This can causes weirdness when they encounter programs that exhibit this behavior - and yes, such programs have a bug. But often, the bug manifests in an even stranger way. Check out the work of John Regehr for more (academic publications: http://www.cs.utah.edu/~regehr/papers/; http://www.cs.utah.edu/~regehr/papers/; blog: http://blog.regehr.org/ http://blog.regehr.org/).
- to3m 11y ago
  Well-formedness doesn't appear to have a definition in C11. In the C++11 standard, the wording rather implies to me that it is a static property, suggesting that a well-formed program could therefore still induce undefined behaviour at runtime. The C11 standard is however quite clear on what undefined behaviour is: "behaviour...for which this International Standard imposes no requirements". That sounds like a far cry from illegality to me! It sounds more like it is merely... yes... undefined. This means that gcc (et al) are allowed to do what they do. It doesn't mean it's not crap.
  - scott_s 11y ago
    I think you omitted the "erroneous" part of that definition. From http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf: 3.4.1 1 implementation-defined behavior unspecified behavior where each implementation documents how the choice is made 2 EXAMPLE An example of implementation-defined behavior is the propagation of the high-order bit when a signed integer is shifted right. 3.4.3 1 undefined behavior behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements 2 NOTE Possible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message). 3 EXAMPLE An example of undefined behavior is the behavior on integer overflow. And later, 4. Conformance 2 If a ‘‘shall’’ or ‘‘shall not’’ requirement that appears outside of a constraint or runtime constraint is violated, the behavior is undefined. Undefined behavior is otherwise indicated in this International Standard by the words ‘‘undefined behavior’’ or by the omission of any explicit definition of behavior. There is no difference in emphasis among these three; they all describe ‘‘behavior that is undefined’’. ... 5 A *strictly conforming* program shall use only those features of the language and library specified in this International Standard.3) It shall not produce output dependent on any unspecified, undefined, or implementation-defined behavior, and shall not exceed any minimum implementation limit. Substitute "strictly conforming" where I said (colloquially) "well-formed". Footnote 3 basically says that a program can still be strictly conforming if it uses of conditional features has conditional guards. All of that above tells me: "implementation-defined" and "undefined" are different concepts in the C standard, and programs with "undefined" behavior are erroneous - or, colloquially, illegal programs. Oddly, I still agree with your final conclusion: gcc and other compilers are definitely allowed to do what they do; they are following the standard.