Recent Features
| |||||||||||||
Not AgainNot AgainPosted Apr 23, 2012 9:48 UTC (Mon) by mpr22 (subscriber, #60784)In reply to: Not Again by ekj Parent article: PHP: a fractal of bad design (fuzzy notepad)
With regard to your first sentence: because people want to be able to compile programs which contain functions that perform arithmetic on signed integers, and the results of signed integer arithmetic overflow may be outside the implementation's reasonable ability to control. (Unsigned integer arithmetic, on the other hand, has strictly-defined overflow behaviour summarized as (UINT_MAX + 1) == 0) With regard to your second sentence: given a contradiction, everything is true.
(Log in to post comments)
Not Again Posted Apr 23, 2012 9:57 UTC (Mon) by ekj (guest, #1524) [Link]
Wouldn't arithmethic operations on signed integers that overflow be unspecified rather than undefined ? I was thinking mostly of constructs that are by themselves -always- undefined, not subject to "undefined *if* the sum of these two overflow" which the compiler cannot generally know about beforehand.
What is the rationale for letting "void main(void)" compile and produce a program that you can run (if you dare!) despite the fact that it means, according to the C-spec: "Do nothing, or anything whatsoever."
Not Again Posted Apr 23, 2012 10:40 UTC (Mon) by anselm (subscriber, #2796) [Link] What is the rationale for letting "void main(void)" compile and produce a program that you can run (if you dare!) despite the fact that it means, according to the C-spec: "Do nothing, or anything whatsoever." According to the C standard, the prohibition on prototypes for »main« other than »int main(void)« and »int main (int, char **)« applies only to what the standard calls a »hosted environment«, i.e., an operating system like Linux. The standard makes certain stipulations about how such an environment is supposed to call into a C program, and this is where the restrictions on »main()« come from. The output from a C compiler could, however, be useful in what the standard calls a »freestanding environment«, where – among other differences – the implementation defines how a program is actually started. It could force a different prototype for »main()« or call a differently-named function altogether. (An obvious example of a »freestanding environment« would be the Linux kernel, which runs on the bare machine, without the benefit of an underlying operating system, since of course it is supposed to be the operating system that would make up part of a »hosted environment« for ISO C.) Having said that, it is probably safe to say that 99%+ of programs compiled with, say, GNU C, are intended to be run in the hosted environment, which is why, in the highly recommended »-Wall« mode, gcc emits warnings complaining about non-conforming definitions of »main()« unless the »-ffreestanding« option is specified on the command line. If you're serious you could use the »-Werror=main« option to turn this warning into an error.
Not Again Posted Apr 23, 2012 10:42 UTC (Mon) by mpr22 (subscriber, #60784) [Link] The result of void main(void) is only formally undefined if you're targeting a hosted implementation (which, admittedly, application programmers generally are). If you're using a freestanding implementation, then both the type and the name of your program's entry point are implementation-defined, so main might not be magic and even if it is, it might legitimately have a return type of void. (Note to self: check whether it's defining void main(/*whatever*/) or returning from main having done so, that crosses the undefined-behaviour threshold on hosted implementations.)
|
|||||||||||||