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proposal: fmt: %p should accept uintptr #41495

seebs opened this issue Sep 19, 2020 · 4 comments

proposal: fmt: %p should accept uintptr #41495

seebs opened this issue Sep 19, 2020 · 4 comments


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@seebs seebs commented Sep 19, 2020

What version of Go are you using (go version)?


Does this issue reproduce with the latest release?


What operating system and processor architecture are you using (go env)?


What did you do?

fmt.Printf("%p", stringHeader.Data)

What did you expect to see?

A pointer value.

What did you see instead?

A fmt warning that %p got a non-pointer.

Rationale: It's true that uintptr is an integer-like thing rather than a pointer-like thing, but it's explicitly the type of integer which is used to represent pointers. It's certainly possible to just add the extra conversion everywhere, but I don't think it makes anything clearer, safer, or better. Allowing %p for uintptr shouldn't break any code except possibly "tests that check for getting that diagnostic" which seems like an edge case.

In the absence of moving GC, etc, I'd want %p to print a uintptr and the corresponding unsafe.Pointer identically.

(As a secondary issue, I would also sort of like %p to work on strings, printing the address of their backing store, but I'm enough less convinced of that that I'm leaving it as a parenthetical.)

@ianlancetaylor ianlancetaylor changed the title %p should accept uintptr proposal: fmt: %p should accept uintptr Sep 19, 2020
@gopherbot gopherbot added this to the Proposal milestone Sep 19, 2020
@gopherbot gopherbot added the Proposal label Sep 19, 2020
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@ianlancetaylor ianlancetaylor commented Sep 19, 2020

@ianlancetaylor ianlancetaylor added this to Incoming in Proposals Sep 19, 2020
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@mvdan mvdan commented Sep 20, 2020

#28865 is somewhat related, I think.

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@robpike robpike commented Sep 21, 2020

I'd rather that this not happen because uintptr is not a pointer, and the use of %p provides a modicum of type safety. If you want the hex you can always ask for it with %#x.

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@seebs seebs commented Sep 21, 2020

Yeah. This ties in somewhat with the occasional proposals for a slice-header-like-thing which contains an unsafe.Pointer instead of a uintptr. That said, I don't think there's a format specifier for uintptr that produces the same results as %p -- because %p can print "nil".

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