Re: awk: CONVFMT format string causes crash

On 1 May 2026 at 11:08, Atle Østrem Tøge via busyb wrote:

To:	Busybox <[email protected]>
Copies to:	Sebastian Ellingsen <[email protected]>
Subject:	awk: CONVFMT format string causes crash
Date sent:	Fri, 1 May 2026 11:08:59 +0000
From:	Atle Østrem Tøge via busybox <[email protected]>
Send reply to:	Atle Østrem Tøge <[email protected]>

> 
> Package: busybox
> Version: 1.36.1 (also reproduced on current master)
> 
> When I set CONVFMT to a multi-specifier format string ending in a valid
> float specifier and perform a float-to-string conversion, busybox awk crashes.
> 
> With a standard CONVFMT I get:
> 
>   $ busybox awk 'BEGIN { CONVFMT="%.6g"; x=3.14; print x "" }'
>   3.14
> 
> But with the following input I get a segfault:
> 
>   $ busybox awk 'BEGIN { CONVFMT="%nf"; x=3.14; print x "" }'
>   Segmentation fault
> 

This is with latest 1.38 git snapshot build.
Belief the %nf should have the n replaced with a number?
./busybox awk 'BEGIN { CONVFMT="%.2f"; x=3.14; print x "" }'
3.14

With Fedora 43 awk get this with %nf
awk 'BEGIN { CONVFMT="%nf"; x=3.14; print x "" }'
%nf

So doesn't get a segment fault, but print %nf instead of number?


> Note that CONVFMT="%n" alone is correctly rejected with "Invalid format
> specifier", so the crash is specific to multi-specifier strings where the
> terminal character is otherwise valid.
> 
> Tested on x86_64 Linux. Crash confirmed with AddressSanitizer (SEGV,
> WRITE access in fmt_num() -> getvar_s() -> evaluate()).
> 
> - Sebastian Ellingsen & Atle Tøge, NTNU


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