[RFC] btrfs-progs: Support for musl libc (and perhaps also uclibc)

[Merlijn Wajer <merlijn@wizzup.org>]

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Hi,

I've been experimenting with musl-libc Gentoo systems. I used the HEAD
of git://git.kernel.org/pub/scm/linux/kernel/git/kdave/btrfs-progs.git

I found that btrfs-progs does not compile with musl-libc, for a few reasons:

* It makes use of the private glibc __always_inline macro.

* Various headers that should be included are not included:
linux/limits.h and limits.h. (for XATTR_SIZE_MAX and PATH_MAX)

* backtrace() using execinfo.h is enabled by default; execinfo.h is
glibc-specific and thus does not work on other libc's. musl does not
support it, and I think uclibc also does not support it.

I have worked around the problems in the following way:

* Define __always_inline if __glibc__ is not defined. This is arguably
the most clean solution. It would be better to simply not use the
__always_inline macro (instead, use __attribute__) throughout
btrfs-progs, but I was not sure what the developers would prefer. This
is currently done in kerncompat.h, but you may want to move that to
another file.

* Include various headers where required.

* If __glibc__ is not defined, define BTRFS_DISABLE_BACKTRACE. Currently
the define magic happens in kerncompat, because that also where BTRFS
includes execinfo. Personally, I think it would make more sense to
always disable backtrace instead of enabling it by default -- but
perhaps in a testing phase, enabling it by default in the sensible choice.

Attached are the two patches generated with git format-patch. I am aware
that this may not be required format for submitting patches -- but
please give me some time to get used to the etiquette. :-)

Please let me know if musl-libc (or any other libc) is a supported
platform, and if so, if and how I can improve on said patches.

Regards,
Merlijn

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From da43021732fab3f70c75f155bded8e5f35fdffe3 Mon Sep 17 00:00:00 2001
From: Merlijn Wajer <merlijn@wizzup.org>
Date: Sat, 13 Dec 2014 15:07:25 +0100
Subject: [PATCH 1/2] Include headers required for musl-libc.

This fixes various compilation errors where PATH_MAX and XATTR_SIZE_MAX
were missing. To my knowledge, this should have no bad side effects.
---
 btrfs-convert.c | 1 +
 help.c          | 1 +
 mkfs.c          | 2 ++
 3 files changed, 4 insertions(+)

diff --git a/btrfs-convert.c b/btrfs-convert.c
index 02c5e94..7b69a13 100644
--- a/btrfs-convert.c
+++ b/btrfs-convert.c
@@ -31,6 +31,7 @@
 #include <fcntl.h>
 #include <unistd.h>
 #include <uuid/uuid.h>
+#include <linux/limits.h>
 
 #include "ctree.h"
 #include "disk-io.h"
diff --git a/help.c b/help.c
index fab942b..56aaf9c 100644
--- a/help.c
+++ b/help.c
@@ -17,6 +17,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <limits.h>
 
 #include "commands.h"
 #include "utils.h"
diff --git a/mkfs.c b/mkfs.c
index e10e62d..6343831 100644
--- a/mkfs.c
+++ b/mkfs.c
@@ -35,6 +35,8 @@
 #include <uuid/uuid.h>
 #include <ctype.h>
 #include <sys/xattr.h>
+#include <limits.h>
+#include <linux/limits.h>
 #include <blkid/blkid.h>
 #include <ftw.h>
 #include "ctree.h"
-- 
2.0.4


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From 01d0bfe48dc78b66b6e86d4935d9b9d20194b135 Mon Sep 17 00:00:00 2001
From: Merlijn Wajer <merlijn@wizzup.org>
Date: Sat, 13 Dec 2014 15:08:43 +0100
Subject: [PATCH 2/2] Disable backtrace and define __always_inline

Disable backtrace and define __always_inline when glibc is not used as
libc. This, together with some header changes allows btrfs-progs to
compile with musl-libc.
---
 kerncompat.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kerncompat.h b/kerncompat.h
index 8afadc8..05823a7 100644
--- a/kerncompat.h
+++ b/kerncompat.h
@@ -29,6 +29,12 @@
 #include <stddef.h>
 #include <linux/types.h>
 #include <stdint.h>
+
+#ifndef __glibc__
+#define BTRFS_DISABLE_BACKTRACE
+#define __always_inline __inline __attribute__ ((__always_inline__))
+#endif
+
 #ifndef BTRFS_DISABLE_BACKTRACE
 #include <execinfo.h>
 #endif
-- 
2.0.4


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