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I answered a question, If you use wrong format specifier then the behaviour of your code is undefined.

In that question, OP used %lu format specifier for sizeof operator.

printf("%lu \n", sizeof(*"327"));

But, I got some comments, %lu not UB for that code.

So, Is it true for sizeof(*"327")?

msc
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  • The discussion in the comments seemed to be about whether `size_t` is `typedef`d to be `unsigned long` or not, so whether it invokes UB or not seems to depend on the compiler? – UnholySheep Oct 13 '17 at 12:16
  • Well, I think the bigger problem is, "Why would you ever do that." There's nothing really "wrong" with the format specifier, though that may not be the specifier that will properly display the value.. That simply means that it will be displayed incorrectly, not that it is necessarily UB. – David Hoelzer Oct 13 '17 at 12:18
  • @Jens Gustedt Actually sir I know correct format specifier of sizeof. And also I mentioned in my answer , but some peoples commented my answer, it's not UB?? My answer : https://stackoverflow.com/a/46705515/6935629 – msc Oct 13 '17 at 12:27
  • @rsp, reopened. But this was not at all clear from your question, please improve it. The contents should not be hidden behind links. – Jens Gustedt Oct 13 '17 at 12:47
  • Wait a sec, you did not find any dupe of this? That's a little surprising... – Sourav Ghosh Oct 13 '17 at 12:50
  • @SouravGhosh, as rsp remarked I search for the wrong fact for the duplicate. But you are right for the question "is the behavior of this undefined" there should be one, too. – Jens Gustedt Oct 13 '17 at 12:52
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    @SouravGhosh, so what you found is again wrong, it does not answer the question that rsp wanted to ask. – Jens Gustedt Oct 13 '17 at 12:53
  • @JensGustedt sir, I guess I understand the question sir, but the point is, as already known by RSP, that the correct specifier is `%zu` and anything else is UB. If any specific info is to be added for any other environment, then the question has to be updated with that info, don't you recommend that? – Sourav Ghosh Oct 13 '17 at 12:59
  • @rsp If it feels any better, you are theoretically correct and if you stick to that, you'll be writing much better and portable and conforming code, just my two cents. – Sourav Ghosh Oct 13 '17 at 13:04

2 Answers2

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The sizeof operator returns a type of size_t. The proper format specifier for that type is %zu.

If you use %lu to print a size_t, it may or may not work, depending on whether or not size_t is larger than a long. If size_t is larger, you have undefined behavior, if not the behavior is well defined. Of course, you can't really know for sure.

Better to use the proper format specifier specific to that type.

dbush
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Unix systems (nearly?) always have size_t == unsigned long == uintptr_t, and there is a lot of code that relies on the fact.

The only system in common use that's different is 64-bit Windows, where unsigned long is 32-bit but size_t and uintptr_t are 64-bit.

(Does windows even have a support for %zu in printf?)

o11c
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    "*Does windows even have a support `%zu` ...*" that not a question of the OS, but of the compiler. VC hasn't, others have. – alk Oct 13 '17 at 12:26