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From: Szabolcs Nagy <nsz@port70.net>
To: Rui Ueyama <rui314@gmail.com>
Cc: musl@lists.openwall.com
Subject: Re: [musl] [PATCH] Explicitly pass the -fno-common flag
Date: Sun, 9 Jan 2022 00:25:36 +0100	[thread overview]
Message-ID: <20220108232536.GC1320090@port70.net> (raw)
In-Reply-To: <CACKH++a5HJtKmXzYwoZk36m4ykAq_QSOby+ytK=2HKjqxgX3EQ@mail.gmail.com>

* Rui Ueyama <rui314@gmail.com> [2022-01-08 16:45:51 +0900]:
> Common symbol is a special type of symbol that allows a linker to merge
> multiple common symbols into one and turn it into a defined symbol.
> This feature was used in C to allow tentative definitions. That is,
> you can write `int foo;` instead of `extern int foo;` in a header file.
> 
> Common symbols are discouraged these days because they can easily
> hide unintentional duplicate symbol errors. For that reason, GCC
> (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85678)
> and Clang (https://github.com/llvm/llvm-project/commit/0a9fc9233e172601e26381810d093e02ef410f65)
> now default to `-fno-common`.
> 
> That means, musl libc's global variables are compiled to common symbols
> or regular defined symbols depending on the compiler. That's not an issue
> per se, but it's unnecessary churn.
> 
> This patch always passes the `-fno-common` flag to the compiler.

building x86_64 musl with -fcommon i see these symbols in COM section:

obj/src/malloc/replaced.lo:
 __aligned_alloc_replaced
 __malloc_replaced
obj/src/malloc/mallocng/malloc.lo:
 __malloc_lock
obj/src/misc/getopt.lo:
 __optpos
 optarg
 optopt
obj/src/env/__stack_chk_fail.lo:
 __stack_chk_guard
obj/src/env/__init_tls.lo:
 __thread_list_lock
obj/src/aio/aio.lo:
 __aio_fut
obj/src/locale/locale_map.lo:
 __locale_lock
obj/src/internal/libc.lo:
 __hwcap
 __libc
obj/src/internal/defsysinfo.lo:
 __sysinfo
obj/src/signal/sigaction.lo:
 __eintr_valid_flag
obj/src/exit/abort_lock.lo:
 __abort_lock
obj/src/network/h_errno.lo:
 h_errno
obj/src/time/getdate.lo:
 getdate_err

i'm not sure how this can cause trouble (maybe static linking?), but
if we care then another solution is to change

 int x;

to

 int x = 0;

instead of forcing -fno-common.

  reply	other threads:[~2022-01-08 23:25 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-01-08  7:45 Rui Ueyama
2022-01-08 23:25 ` Szabolcs Nagy [this message]
2022-01-09  1:49   ` Rich Felker
2022-01-10 16:50     ` Markus Wichmann
2022-01-10 18:45       ` Szabolcs Nagy

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