[PATCH] pkt-line: initialize packet_buffer to avoid macOS linker warning

[PATCH] pkt-line: initialize packet_buffer to avoid macOS linker warning

[Harald Nordgren via GitGitGadget]

From: Harald Nordgren <haraldnordgren@gmail.com>

Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>
---
    pkt-line: initialize packet_buffer to avoid macOS linker warning
    
    Removes this warning:
    
    $ make -s -j8
    GIT_VERSION=2.54.0.380.gc69baaf57b
    ld: warning: reducing alignment of section __DATA,__common from 0x8000 to 0x4000 because it exceeds segment maximum alignment
    ld: warning: reducing alignment of section __DATA,__common from 0x8000 to 0x4000 because it exceeds segment maximum alignment
    ld: warning: reducing alignment of section __DATA,__common from 0x8000 to 0x4000 because it exceeds segment maximum alignment
    ld: warning: reducing alignment of section __DATA,__common from 0x8000 to 0x4000 because it exceeds segment maximum alignment
    ld: warning: reducing alignment of section __DATA,__common from 0x8000 to 0x4000 because it exceeds segment maximum alignment
    ld: warning: reducing alignment of section __DATA,__common from 0x8000 to 0x4000 because it exceeds segment maximum alignment
    ld: warning: reducing alignment of section __DATA,__common from 0x8000 to 0x4000 because it exceeds segment maximum alignment
    ld: warning: reducing alignment of section __DATA,__common from 0x8000 to 0x4000 because it exceeds segment maximum alignment
    

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-2313%2FHaraldNordgren%2Fpkt-line-init-buffer-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-2313/HaraldNordgren/pkt-line-init-buffer-v1
Pull-Request: https://github.com/git/git/pull/2313

 pkt-line.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pkt-line.c b/pkt-line.c
index 3fc3e9ea70..cfd2799677 100644
--- a/pkt-line.c
+++ b/pkt-line.c
@@ -8,7 +8,7 @@
 #include "trace.h"
 #include "write-or-die.h"
 
-char packet_buffer[LARGE_PACKET_MAX];
+char packet_buffer[LARGE_PACKET_MAX] = {0};
 static const char *packet_trace_prefix = "git";
 static struct trace_key trace_packet = TRACE_KEY_INIT(PACKET);
 static struct trace_key trace_pack = TRACE_KEY_INIT(PACKFILE);

base-commit: c69baaf57ba26cf117c2b6793802877f19738b0d
-- 
gitgitgadget

Re: [PATCH] pkt-line: initialize packet_buffer to avoid macOS linker warning

[Junio C Hamano]

"Harald Nordgren via GitGitGadget" <gitgitgadget@gmail.com> writes:

> From: Harald Nordgren <haraldnordgren@gmail.com>
>
> Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>
> ---
>     pkt-line: initialize packet_buffer to avoid macOS linker warning
>     
>     Removes this warning:
>     
>     $ make -s -j8
>     GIT_VERSION=2.54.0.380.gc69baaf57b
>     ld: warning: reducing alignment of section __DATA,__common from 0x8000 to 0x4000 because it exceeds segment maximum alignment

This sounds nuts.

Not complaining at you, but we are talking about char[]; what
alignment constraints are they talking about?

> diff --git a/pkt-line.c b/pkt-line.c
> index 3fc3e9ea70..cfd2799677 100644
> --- a/pkt-line.c
> +++ b/pkt-line.c
> @@ -8,7 +8,7 @@
>  #include "trace.h"
>  #include "write-or-die.h"
>  
> -char packet_buffer[LARGE_PACKET_MAX];
> +char packet_buffer[LARGE_PACKET_MAX] = {0};

I do not like this; it sounds more like a workaround for broken
linker (and compiler to certain degree).

This, compiled with a stupid compiler that is too faithful to the
source text, may make the resulting object file on disk larger by
64kB, since the original said "I need 64kB area in BSS with its
starting address recorded as 'packet_buffer'" (which costs almost
nothing) and the updated says "Here is a 64kB of literal data"
(which would record the literal data, even if its bytes happen to be
all NUL).  Luckily both versions of GCC and Clang I have notices
that the literal data is all NUL and still keeps the area in BSS
with no change in the object file size or output from "size
packet-line.o", so to me and others on similar systems as I use,
this probably is a benign no-op, but not everywhere.

Are there different versions of C compiler available on macOS for
you to try?  I am hoping that even though vendor compilers tend to
lag a bit behind from the public upstream, the problems may have
already been fixed in more fresher versions.

    ... goes and looks ...

According to Internet, Xcode 16.3 or newer introduced this insanity,
it seems.  How about adding -fno-common to your CFLAGS?  If it
solves the issue, then we can think about teaching config.mak.uname
to detect macOS with problematic versions of compilers and add the
flag as workaround.


>  static const char *packet_trace_prefix = "git";
>  static struct trace_key trace_packet = TRACE_KEY_INIT(PACKET);
>  static struct trace_key trace_pack = TRACE_KEY_INIT(PACKFILE);
>
> base-commit: c69baaf57ba26cf117c2b6793802877f19738b0d

Re: [PATCH] pkt-line: initialize packet_buffer to avoid macOS linker warning

[Harald Nordgren]

> According to Internet, Xcode 16.3 or newer introduced this insanity,
> it seems.  How about adding -fno-common to your CFLAGS?  If it
> solves the issue, then we can think about teaching config.mak.uname
> to detect macOS with problematic versions of compilers and add the
> flag as workaround.

Yes, this works:

```
make -s CFLAGS_APPEND="-fno-common"
```


Harald

Re: [PATCH] pkt-line: initialize packet_buffer to avoid macOS linker warning

[Harald Nordgren]

So maybe we can do something like this then?

```
+       # Silence Xcode 16.3+ linker warning about __DATA,__common alignment.
+       LD_MAJOR_VERSION = $(shell ld -v 2>&1 | sed -n
's/.*PROJECT:ld-\([0-9]*\).*/\1/p')
+        ifeq ($(shell test "$(LD_MAJOR_VERSION)" -ge 1167 && echo 1),1)
+               BASIC_CFLAGS += -fno-common
+        endif
```

Harald

On Thu, May 28, 2026 at 9:40 AM Harald Nordgren
<haraldnordgren@gmail.com> wrote:
>
> > According to Internet, Xcode 16.3 or newer introduced this insanity,
> > it seems.  How about adding -fno-common to your CFLAGS?  If it
> > solves the issue, then we can think about teaching config.mak.uname
> > to detect macOS with problematic versions of compilers and add the
> > flag as workaround.
>
> Yes, this works:
>
> ```
> make -s CFLAGS_APPEND="-fno-common"
> ```
>
>
> Harald

Re: [PATCH] pkt-line: initialize packet_buffer to avoid macOS linker warning

[Junio C Hamano]

Harald Nordgren <haraldnordgren@gmail.com> writes:

> So maybe we can do something like this then?
>
> ```
> +       # Silence Xcode 16.3+ linker warning about __DATA,__common alignment.
> +       LD_MAJOR_VERSION = $(shell ld -v 2>&1 | sed -n
> 's/.*PROJECT:ld-\([0-9]*\).*/\1/p')
> +        ifeq ($(shell test "$(LD_MAJOR_VERSION)" -ge 1167 && echo 1),1)
> +               BASIC_CFLAGS += -fno-common
> +        endif
> ```
>
> Harald

I do not exactly know where these magic numbers and patterns for
"ld" comes from, but yes, something like that in macOS specific
section would be what I had in mind.

Thanks.

[PATCH v2] config.mak.uname: avoid macOS linker warning on Xcode 16.3+

[Harald Nordgren via GitGitGadget]

From: Harald Nordgren <haraldnordgren@gmail.com>

Building on macOS with Xcode 16.3 or newer emits:

    ld: warning: reducing alignment of section __DATA,__common
    from 0x8000 to 0x4000 because it exceeds segment maximum
    alignment

Pass -fno-common when "ld -v" reports ld-1167 or newer, so tentative
definitions of large arrays go into BSS instead of __DATA,__common.

Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>
---
    pkt-line: initialize packet_buffer to avoid macOS linker warning
    
     * Check MacOS ld version instead
       (https://en.wikipedia.org/wiki/Xcode#Xcode_15.0_-_16.x_(since_visionOS_support)_2)
    
    Parsing output of
    
    ❯ ld -v
    @(#)PROGRAM:ld PROJECT:ld-1267
    BUILD 18:30:29 Apr 22 2026
    configured to support archs: armv6 armv7 armv7s arm64 arm64e arm64_32 i386 x86_64 x86_64h armv6m armv7k armv7m armv7em armv8m.main armv8.1m.main
    will use ld-classic for: armv6 armv7 armv7s i386 armv6m armv7k armv7m armv7em
    LTO support using: LLVM version 21.0.0 (static support for 30, runtime is 30)
    TAPI support using: Apple TAPI version 21.0.0 (tapi-2100.0.2.6)
    

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-2313%2FHaraldNordgren%2Fpkt-line-init-buffer-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-2313/HaraldNordgren/pkt-line-init-buffer-v2
Pull-Request: https://github.com/git/git/pull/2313

Range-diff vs v1:

 1:  1c1c66d85b < -:  ---------- pkt-line: initialize packet_buffer to avoid macOS linker warning
 -:  ---------- > 1:  0e660a346e config.mak.uname: avoid macOS linker warning on Xcode 16.3+


 config.mak.uname | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/config.mak.uname b/config.mak.uname
index ce5e7de779..d4d55cb324 100644
--- a/config.mak.uname
+++ b/config.mak.uname
@@ -163,6 +163,12 @@ ifeq ($(uname_S),Darwin)
 		NEEDS_GOOD_LIBICONV = UnfortunatelyYes
         endif
 
+	# Silence Xcode 16.3+ linker warning about __DATA,__common alignment.
+	LD_MAJOR_VERSION = $(shell ld -v 2>&1 | sed -n 's/.*PROJECT:ld-\([0-9]*\).*/\1/p')
+        ifeq ($(shell test "$(LD_MAJOR_VERSION)" -ge 1167 && echo 1),1)
+		BASIC_CFLAGS += -fno-common
+        endif
+
 	# The builtin FSMonitor on MacOS builds upon Simple-IPC.  Both require
 	# Unix domain sockets and PThreads.
         ifndef NO_PTHREADS

base-commit: c69baaf57ba26cf117c2b6793802877f19738b0d
-- 
gitgitgadget