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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: "undefined behavior"?
Date: Fri, 14 Jun 2024 13:43:28 -0700
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Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
[...]
> I don't know.  The default gcc on my system diagnoses it by default, but
> various versions of gcc I've built from source do not.  Perhaps Ubuntu
> configures gcc differently.  (Ubuntu 22.04.4, gcc 11.4.0.)  I'm building
> gcc 11.4.0 from source, and I'll compare its behavior to that of
> Ubuntu's gcc 11.4.0-1ubuntu1~22.04.
[...]

Context: Warning about incorrect printf format strings, such as
`printf("%d\n", strlen(s));` ("%d" requires an int argument but strlen()
returns a result of type size_t).

I've confirmed that, on my Ubuntu 22.04 system, the system-provided
gcc ("Ubuntu 11.4.0-1ubuntu1~22.04") warns about the mismatch, but
gcc 11.4.0 built from source does not.

So Ubuntu (or its upstream Debian) does a custom build of gcc that
enables "-Wformat" by default.

Confirmed by this answer on Stack Overflow:
<https://stackoverflow.com/a/50112401/827263>
"""
This is not caused by a difference in GCC versions. Rather, Ubuntu has
modified GCC to enable -Wformat -Wformat-security by default. If you
pass those options on Arch Linux, you should see the same behaviour
there.
"""

(The answer contains a link to a web page that no longer exists.)

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */