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Adds missing error to writev, prevents infinite recursion reporting unexpected errnos #35574

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andrewrk merged 1 commit from MasonRemaley/zig:writev into master 2026年06月11日 06:01:41 +02:00

Previously, writev was missing handling for the E.ACCES error. Triggering E.ACCES therefore resulted in a call to posix.unexpectedErrno, which internally tries to write information on the unexpected errno to stderr. Since in my case writing to stderr was triggering E.ACCES, this resulted in infinite recursion and eventually a crash.

This PR adds the missing errno and also prevents infinite recursion in case a different missing errno fails when writing to stderr or dumping the stacktrace from unexpectedErrno.

I also clarified the docs on E.ACCES. Referring to this error as "permission" denied is misleading since there is also E.PERM.

Previously, writev was missing handling for the `E.ACCES` error. Triggering `E.ACCES` therefore resulted in a call to `posix.unexpectedErrno`, which internally tries to write information on the unexpected errno to stderr. Since in my case writing to stderr was triggering `E.ACCES`, this resulted in infinite recursion and eventually a crash. This PR adds the missing errno and also prevents infinite recursion in case a different missing errno fails when writing to stderr or dumping the stacktrace from `unexpectedErrno`. I also clarified the docs on `E.ACCES`. Referring to this error as "permission" denied is misleading since there is also `E.PERM`.
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for posterity, the existing names come from https://man7.org/linux/man-pages/man3/errno.3.html
and https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/include/uapi/asm-generic/errno-base.h
and https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/include/uapi/asm-generic/errno.h 

$ errno 13
EACCES 13 Permission denied
$ errno 1
EPERM 1 Operation not permitted
for posterity, the existing names come from https://man7.org/linux/man-pages/man3/errno.3.html and https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/include/uapi/asm-generic/errno-base.h and https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/include/uapi/asm-generic/errno.h ``` $ errno 13 EACCES 13 Permission denied $ errno 1 EPERM 1 Operation not permitted ```
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Ha, I didn't realize that the kernel docs also used this language. I'm still in favor of the doc comment change, but open to reverting it if others feel strongly against.

Ha, I didn't realize that the kernel docs also used this language. I'm still in favor of the doc comment change, but open to reverting it if others feel strongly against.
andrewrk requested changes 2026年06月07日 06:06:17 +02:00
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The missing error fix is very welcome however the other stuff tossed in here I think should be considered separately, and shouldn't be merged as-is.

The missing error fix is very welcome however the other stuff tossed in here I think should be considered separately, and shouldn't be merged as-is.
lib/std/c.zig Outdated
@ -507,3 +507,3 @@
/// Not enough core
NOMEM=12,
/// Permission denied
/// Access denied
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This code is inside the libc bindings; this intentionally matches result of strerror:

E(EACCES, "Permission denied")
This code is inside the libc bindings; this intentionally matches result of `strerror`: ```c E(EACCES, "Permission denied") ```
MasonRemaley marked this conversation as resolved
@ -1665,2 +1665,4 @@
pubconstUnexpectedError=std.Io.UnexpectedError;
/// Give up on printing unexpected errnos after it recurses once to prevent infinite recursion.
threadlocalvarunexpected_errno_stage:u8=0;
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introducing a threadlocal gives me pause... I think it would be better to accept the risk of infinite loop.

std.options.unexpected_error_tracing is only on in debug mode by default.

atomic global var would be reasonable here as an alternative to threadlocal.

threadlocal variables introduce dependency on the linker being involved in more ways; the machine code emitted is less pure, and this function will be dragged in in almost all applications.

introducing a threadlocal gives me pause... I think it would be better to accept the risk of infinite loop. `std.options.unexpected_error_tracing` is only on in debug mode by default. atomic global var would be reasonable here as an alternative to threadlocal. threadlocal variables introduce dependency on the linker being involved in more ways; the machine code emitted is less pure, and this function will be dragged in in almost all applications.
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Thinking this over more I agree. My debugger was crashing and printf debugging wasn't an option given the nature of the problem which made this crash hard to understand. If my debugger had been functioning properly or I had a stack trace this would have been easy to track down.

Thinking this over more I agree. My debugger was crashing and printf debugging wasn't an option given the nature of the problem which made this crash hard to understand. If my debugger had been functioning properly or I had a stack trace this would have been easy to track down.
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Codeberg is bugged and not seeing my commit that resolves the review feedback. I'm going to close this PR and open a new one to see if that resolves it.

Codeberg is bugged and not seeing my commit that resolves the review feedback. I'm going to close this PR and open a new one to see if that resolves it.
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Welp I guess it's impossible to create PRs on codeberg right now. I just get 500s every time. Reopening to remind myself to try again tomorrow.

[EDIT] Nope, can't even reopen the PR without getting a 500...

Welp I guess it's impossible to create PRs on codeberg right now. I just get 500s every time. Reopening to remind myself to try again tomorrow. [EDIT] Nope, can't even reopen the PR without getting a 500...
ad-gusted reopened this pull request 2026年06月07日 15:53:33 +02:00
alexrp force-pushed writev from fd2232446f
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2026年06月07日 18:44:29 +02:00 Compare
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There is one more place where posix writev (pwritev) is used in Threaded:

pwritev_sym(file.handle,&iovecs,@intCast(iovlen),@bitCast(offset));

Should we handle E.ACCESS there also?

There is one more place where posix writev (pwritev) is used in Threaded: https://codeberg.org/ziglang/zig/src/commit/a85cb728775375825afe4ebd62c60ae0b361d1e9/lib/std/Io/Threaded.zig#L10700 Should we handle E.ACCESS there also?
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Good catch I missed that because of the alias, I think it needs to be handled there as well.

Good catch I missed that because of the alias, I think it needs to be handled there as well.
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