rfc: free and errno

rfc: free and errno

[Walter Harms]

Hi List

free() is generaly assumed not to modify errno. 

Lately there was a bugfix posting on the busybox mailing list, because:

"musl libc's mallocing free() may modify errno if kernel does not support
MADV_FREE which causes echo to echo with error when it shouldn't."

In future  it seems that POSIX with require free() not to change errno.

after much soul searching i have still no idea if that should be documented in
the free(3) man page. Any comments ?

re,
 wh

Re: rfc: free and errno

[Michael Kerrisk (man-pages)]

Hello Walter,

Thanks for raising this.

On Mon, 1 Feb 2021 at 12:19, Walter Harms <wharms@bfs.de> wrote:
>
> Hi List
>
> free() is generaly assumed not to modify errno.
>
> Lately there was a bugfix posting on the busybox mailing list, because:
>
> "musl libc's mallocing free() may modify errno if kernel does not support
> MADV_FREE which causes echo to echo with error when it shouldn't."

URL please.

> In future  it seems that POSIX with require free() not to change errno.

The statement you make in the preceding sentence is useful
information, but you could save other people a lot of time searching
if you provided a reference:

https://www.austingroupbugs.net/view.php?id=385#c713

> after much soul searching i have still no idea if that should be documented in
> the free(3) man page. Any comments ?

Probably it should be documented. Did you have a suggestion for the text?

Thanks,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

Re: rfc: free and errno

[Jakub Wilk]

* Michael Kerrisk <mtk.manpages@gmail.com>, 2021-02-06, 10:49:
>On Mon, 1 Feb 2021 at 12:19, Walter Harms <wharms@bfs.de> wrote:
>>free() is generaly assumed not to modify errno.
>>
>>Lately there was a bugfix posting on the busybox mailing list, because:
>>
>>"musl libc's mallocing free() may modify errno if kernel does not 
>>support MADV_FREE which causes echo to echo with error when it 
>>shouldn't."
>
>URL please.

http://lists.busybox.net/pipermail/busybox/2021-January/088491.html 
probably.

glibc's free() also can set errno in some circumstances:
https://sourceware.org/bugzilla/show_bug.cgi?id=17924

-- 
Jakub Wilk