From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on inbox.vuxu.org
X-Spam-Level: 
X-Spam-Status: No, score=-3.3 required=5.0 tests=MAILING_LIST_MULTI,
	RCVD_IN_DNSWL_MED,RCVD_IN_MSPIKE_H3,RCVD_IN_MSPIKE_WL autolearn=ham
	autolearn_force=no version=3.4.4
Received: (qmail 18739 invoked from network); 15 Mar 2021 20:06:37 -0000
Received: from mother.openwall.net (195.42.179.200)
  by inbox.vuxu.org with ESMTPUTF8; 15 Mar 2021 20:06:37 -0000
Received: (qmail 9427 invoked by uid 550); 15 Mar 2021 20:06:35 -0000
Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm
Precedence: bulk
List-Post: <mailto:musl@lists.openwall.com>
List-Help: <mailto:musl-help@lists.openwall.com>
List-Unsubscribe: <mailto:musl-unsubscribe@lists.openwall.com>
List-Subscribe: <mailto:musl-subscribe@lists.openwall.com>
List-ID: <musl.lists.openwall.com>
Reply-To: musl@lists.openwall.com
Received: (qmail 9409 invoked from network); 15 Mar 2021 20:06:35 -0000
Date: Mon, 15 Mar 2021 16:06:23 -0400
From: Rich Felker <dalias@libc.org>
To: Yossi Gottlieb <yossigo@gmail.com>
Cc: musl@lists.openwall.com
Message-ID: <20210315200622.GE32655@brightrain.aerifal.cx>
References: <CAMquMNTmDZnGvtPo3H0Uo-3ytGDO5EHa+RHCBTZTph6c3WFsYA@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <CAMquMNTmDZnGvtPo3H0Uo-3ytGDO5EHa+RHCBTZTph6c3WFsYA@mail.gmail.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
Subject: Re: [musl] strftime() unexpectedly modifies errno

On Mon, Mar 15, 2021 at 09:43:36PM +0200, Yossi Gottlieb wrote:
> It seems like strftime() unexpectedly modifies errno, which is always set
> to EINVAL when it returns. Looks like it's not related to any specific
> format.
> 
> Here's an example:
> 
> #include <stdio.h>
> #include <sys/time.h>
> #include <time.h>
> #include <errno.h>
> 
> int main(int argc, char *argv[])
> {
>     time_t now = time(NULL);
>     struct tm *tm = localtime(&now);
>     char buf[100];
> 
>     errno = 0;
>     size_t len = strftime(buf, sizeof(buf), "%d %b %Y %H:%M:%S.", tm);
> 
>     printf("len=%zu\n", len);
>     printf("buf=%s\n", buf);
>     printf("errno=%d\n", errno);
> }

This isn't unexpected. Any function except for a few specific ones
documented not to can modify errno as a side effect of success.
Inspecting errno is only meaningful immediately after failure.

The relevant text in the C language (C11) is 7.5 Errors <errno.h>, ¶3:

"The value of errno may be set to nonzero by a library function call
whether or not there is an error, provided the use of errno is not
documented in the description of the function in this International
Standard."

There's equivalent text in POSIX as well.

Rich