diff --git a/Documentation/config.txt b/Documentation/config.txt
index c6ff15e594..7bc0e53848 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -211,6 +211,15 @@ The default is false, except linkgit:git-clone[1] or linkgit:git-init[1]
 will probe and set core.ignorecase true if appropriate when the repository
 is created.
 
+core.precomposeunicode::
+	This option is only used by Mac OS implementation of git.
+	When core.precomposeunicode=true, git reverts the unicode decomposition
+	of filenames done by Mac OS. This is useful when sharing a repository
+	between Mac OS and Linux or Windows.
+	(Git for Windows 1.7.10 or higher is needed, or git under cygwin 1.7).
+	When false, file names are handled fully transparent by git,
+	which is backward compatible with older versions of git.
+
 core.trustctime::
 	If false, the ctime differences between the index and the
 	working tree are ignored; useful when the inode change time
diff --git a/Makefile b/Makefile
index 169dda5453..dc70902963 100644
--- a/Makefile
+++ b/Makefile
@@ -607,6 +607,7 @@ LIB_H += compat/bswap.h
 LIB_H += compat/cygwin.h
 LIB_H += compat/mingw.h
 LIB_H += compat/obstack.h
+LIB_H += compat/precompose_utf8.h
 LIB_H += compat/terminal.h
 LIB_H += compat/win32/dirent.h
 LIB_H += compat/win32/poll.h
@@ -1001,6 +1002,8 @@ ifeq ($(uname_S),Darwin)
 	NO_MEMMEM = YesPlease
 	USE_ST_TIMESPEC = YesPlease
 	HAVE_DEV_TTY = YesPlease
+	COMPAT_OBJS += compat/precompose_utf8.o
+	BASIC_CFLAGS += -DPRECOMPOSE_UNICODE
 endif
 ifeq ($(uname_S),SunOS)
 	NEEDS_SOCKET = YesPlease
diff --git a/builtin/init-db.c b/builtin/init-db.c
index 0dacb8b79c..244fb7fc32 100644
--- a/builtin/init-db.c
+++ b/builtin/init-db.c
@@ -290,6 +290,7 @@ static int create_default_files(const char *template_path)
 		strcpy(path + len, "CoNfIg");
 		if (!access(path, F_OK))
 			git_config_set("core.ignorecase", "true");
+		probe_utf8_pathname_composition(path, len);
 	}
 
 	return reinit;
diff --git a/cache.h b/cache.h
index c22b928980..6a1aff5e2a 100644
--- a/cache.h
+++ b/cache.h
@@ -563,6 +563,7 @@ extern int read_replace_refs;
 extern int fsync_object_files;
 extern int core_preload_index;
 extern int core_apply_sparse_checkout;
+extern int precomposed_unicode;
 
 enum branch_track {
 	BRANCH_TRACK_UNSPECIFIED = -1,
diff --git a/compat/precompose_utf8.c b/compat/precompose_utf8.c
new file mode 100644
index 0000000000..d40d1b3807
--- /dev/null
+++ b/compat/precompose_utf8.c
@@ -0,0 +1,190 @@
+/*
+ * Converts filenames from decomposed unicode into precomposed unicode.
+ * Used on MacOS X.
+*/
+
+
+#define PRECOMPOSE_UNICODE_C
+
+#include "cache.h"
+#include "utf8.h"
+#include "precompose_utf8.h"
+
+typedef char *iconv_ibp;
+const static char *repo_encoding = "UTF-8";
+const static char *path_encoding = "UTF-8-MAC";
+
+
+static size_t has_utf8(const char *s, size_t maxlen, size_t *strlen_c)
+{
+	const uint8_t *utf8p = (const uint8_t*) s;
+	size_t strlen_chars = 0;
+	size_t ret = 0;
+
+	if ((!utf8p) || (!*utf8p)) {
+		return 0;
+	}
+
+	while((*utf8p) && maxlen) {
+		if (*utf8p & 0x80)
+			ret++;
+		strlen_chars++;
+		utf8p++;
+		maxlen--;
+	}
+	if (strlen_c)
+		*strlen_c = strlen_chars;
+
+	return ret;
+}
+
+
+void probe_utf8_pathname_composition(char *path, int len)
+{
+	const static char *auml_nfc = "\xc3\xa4";
+	const static char *auml_nfd = "\x61\xcc\x88";
+	int output_fd;
+	if (precomposed_unicode != -1)
+		return; /* We found it defined in the global config, respect it */
+	path[len] = 0;
+	strcpy(path + len, auml_nfc);
+	output_fd = open(path, O_CREAT|O_EXCL|O_RDWR, 0600);
+	if (output_fd >=0) {
+		close(output_fd);
+		path[len] = 0;
+		strcpy(path + len, auml_nfd);
+		/* Indicate to the user, that we can configure it to true */
+		if (0 == access(path, R_OK))
+			git_config_set("core.precomposeunicode", "false");
+			/* To be backward compatible, set precomposed_unicode to 0 */
+		precomposed_unicode = 0;
+		path[len] = 0;
+		strcpy(path + len, auml_nfc);
+		unlink(path);
+	}
+}
+
+
+void precompose_argv(int argc, const char **argv)
+{
+	int i = 0;
+	const char *oldarg;
+	char *newarg;
+	iconv_t ic_precompose;
+
+	if (precomposed_unicode != 1)
+		return;
+
+	ic_precompose = iconv_open(repo_encoding, path_encoding);
+	if (ic_precompose == (iconv_t) -1)
+		return;
+
+	while (i < argc) {
+		size_t namelen;
+		oldarg = argv[i];
+		if (has_utf8(oldarg, (size_t)-1, &namelen)) {
+			newarg = reencode_string_iconv(oldarg, namelen, ic_precompose);
+			if (newarg)
+				argv[i] = newarg;
+		}
+		i++;
+	}
+	iconv_close(ic_precompose);
+}
+
+
+PREC_DIR *precompose_utf8_opendir(const char *dirname)
+{
+	PREC_DIR *prec_dir = xmalloc(sizeof(PREC_DIR));
+	prec_dir->dirent_nfc = xmalloc(sizeof(dirent_prec_psx));
+	prec_dir->dirent_nfc->max_name_len = sizeof(prec_dir->dirent_nfc->d_name);
+
+	prec_dir->dirp = opendir(dirname);
+	if (!prec_dir->dirp) {
+		free(prec_dir->dirent_nfc);
+		free(prec_dir);
+		return NULL;
+	} else {
+		int ret_errno = errno;
+		prec_dir->ic_precompose = iconv_open(repo_encoding, path_encoding);
+		/* if iconv_open() fails, die() in readdir() if needed */
+		errno = ret_errno;
+	}
+
+	return prec_dir;
+}
+
+struct dirent_prec_psx *precompose_utf8_readdir(PREC_DIR *prec_dir)
+{
+	struct dirent *res;
+	res = readdir(prec_dir->dirp);
+	if (res) {
+		size_t namelenz = strlen(res->d_name) + 1; /* \0 */
+		size_t new_maxlen = namelenz;
+
+		int ret_errno = errno;
+
+		if (new_maxlen > prec_dir->dirent_nfc->max_name_len) {
+			size_t new_len = sizeof(dirent_prec_psx) + new_maxlen -
+				sizeof(prec_dir->dirent_nfc->d_name);
+
+			prec_dir->dirent_nfc = xrealloc(prec_dir->dirent_nfc, new_len);
+			prec_dir->dirent_nfc->max_name_len = new_maxlen;
+		}
+
+		prec_dir->dirent_nfc->d_ino  = res->d_ino;
+		prec_dir->dirent_nfc->d_type = res->d_type;
+
+		if ((precomposed_unicode == 1) && has_utf8(res->d_name, (size_t)-1, NULL)) {
+			if (prec_dir->ic_precompose == (iconv_t)-1) {
+				die("iconv_open(%s,%s) failed, but needed:\n"
+						"    precomposed unicode is not supported.\n"
+						"    If you wnat to use decomposed unicode, run\n"
+						"    \"git config core.precomposeunicode false\"\n",
+						repo_encoding, path_encoding);
+			} else {
+				iconv_ibp	cp = (iconv_ibp)res->d_name;
+				size_t inleft = namelenz;
+				char *outpos = &prec_dir->dirent_nfc->d_name[0];
+				size_t outsz = prec_dir->dirent_nfc->max_name_len;
+				size_t cnt;
+				errno = 0;
+				cnt = iconv(prec_dir->ic_precompose, &cp, &inleft, &outpos, &outsz);
+				if (errno || inleft) {
+					/*
+					 * iconv() failed and errno could be E2BIG, EILSEQ, EINVAL, EBADF
+					 * MacOS X avoids illegal byte sequemces.
+					 * If they occur on a mounted drive (e.g. NFS) it is not worth to
+					 * die() for that, but rather let the user see the original name
+					*/
+					namelenz = 0; /* trigger strlcpy */
+				}
+			}
+		}
+		else
+			namelenz = 0;
+
+		if (!namelenz)
+			strlcpy(prec_dir->dirent_nfc->d_name, res->d_name,
+							prec_dir->dirent_nfc->max_name_len);
+
+		errno = ret_errno;
+		return prec_dir->dirent_nfc;
+	}
+	return NULL;
+}
+
+
+int precompose_utf8_closedir(PREC_DIR *prec_dir)
+{
+	int ret_value;
+	int ret_errno;
+	ret_value = closedir(prec_dir->dirp);
+	ret_errno = errno;
+	if (prec_dir->ic_precompose != (iconv_t)-1)
+		iconv_close(prec_dir->ic_precompose);
+	free(prec_dir->dirent_nfc);
+	free(prec_dir);
+	errno = ret_errno;
+	return ret_value;
+}
diff --git a/compat/precompose_utf8.h b/compat/precompose_utf8.h
new file mode 100644
index 0000000000..3b73585fc5
--- /dev/null
+++ b/compat/precompose_utf8.h
@@ -0,0 +1,45 @@
+#ifndef PRECOMPOSE_UNICODE_H
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <dirent.h>
+#include <iconv.h>
+
+
+typedef struct dirent_prec_psx {
+	ino_t d_ino;            /* Posix */
+	size_t max_name_len;    /* See below */
+	unsigned char d_type;   /* available on all systems git runs on */
+
+	/*
+	 * See http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/dirent.h.html
+	 * NAME_MAX + 1 should be enough, but some systems have
+	 * NAME_MAX=255 and strlen(d_name) may return 508 or 510
+	 * Solution: allocate more when needed, see precompose_utf8_readdir()
+	 */
+	char   d_name[NAME_MAX+1];
+} dirent_prec_psx;
+
+
+typedef struct {
+	iconv_t ic_precompose;
+	DIR *dirp;
+	struct dirent_prec_psx *dirent_nfc;
+} PREC_DIR;
+
+void precompose_argv(int argc, const char **argv);
+void probe_utf8_pathname_composition(char *, int);
+
+PREC_DIR *precompose_utf8_opendir(const char *dirname);
+struct dirent_prec_psx *precompose_utf8_readdir(PREC_DIR *dirp);
+int precompose_utf8_closedir(PREC_DIR *dirp);
+
+#ifndef PRECOMPOSE_UNICODE_C
+#define dirent dirent_prec_psx
+#define opendir(n) precompose_utf8_opendir(n)
+#define readdir(d) precompose_utf8_readdir(d)
+#define closedir(d) precompose_utf8_closedir(d)
+#define DIR PREC_DIR
+#endif /* PRECOMPOSE_UNICODE_C */
+
+#define  PRECOMPOSE_UNICODE_H
+#endif /* PRECOMPOSE_UNICODE_H */
diff --git a/config.c b/config.c
index d28a499b0b..40818e872f 100644
--- a/config.c
+++ b/config.c
@@ -758,6 +758,11 @@ static int git_default_core_config(const char *var, const char *value)
 		return 0;
 	}
 
+	if (!strcmp(var, "core.precomposeunicode")) {
+		precomposed_unicode = git_config_bool(var, value);
+		return 0;
+	}
+
 	/* Add other config variables here and to Documentation/config.txt. */
 	return 0;
 }
diff --git a/environment.c b/environment.c
index 669e498f5a..85edd7f95a 100644
--- a/environment.c
+++ b/environment.c
@@ -58,6 +58,7 @@ char *notes_ref_name;
 int grafts_replace_parents = 1;
 int core_apply_sparse_checkout;
 int merge_log_config = -1;
+int precomposed_unicode = -1; /* see probe_utf8_pathname_composition() */
 struct startup_info *startup_info;
 unsigned long pack_size_limit_cfg;
 
diff --git a/git-compat-util.h b/git-compat-util.h
index 5bd9ad7d2a..35b095e8ae 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -153,6 +153,15 @@
 #endif
 #endif
 
+/* used on Mac OS X */
+#ifdef PRECOMPOSE_UNICODE
+#include "compat/precompose_utf8.h"
+#else
+#define precompose_str(in,i_nfd2nfc)
+#define precompose_argv(c,v)
+#define probe_utf8_pathname_composition(a,b)
+#endif
+
 #ifndef NO_LIBGEN_H
 #include <libgen.h>
 #else
diff --git a/parse-options.c b/parse-options.c
index ab70c29c49..c1c66bd408 100644
--- a/parse-options.c
+++ b/parse-options.c
@@ -476,6 +476,7 @@ int parse_options(int argc, const char **argv, const char *prefix,
 		usage_with_options(usagestr, options);
 	}
 
+	precompose_argv(argc, argv);
 	return parse_options_end(&ctx);
 }
 
diff --git a/t/t3910-mac-os-precompose.sh b/t/t3910-mac-os-precompose.sh
new file mode 100755
index 0000000000..88b7a20c11
--- /dev/null
+++ b/t/t3910-mac-os-precompose.sh
@@ -0,0 +1,164 @@
+#!/bin/sh
+#
+# Copyright (c) 2012 Torsten Bögershausen
+#
+
+test_description='utf-8 decomposed (nfd) converted to precomposed (nfc)'
+
+. ./test-lib.sh
+
+Adiarnfc=`printf '\303\204'`
+Adiarnfd=`printf 'A\314\210'`
+
+# check if the feature is compiled in
+mkdir junk &&
+>junk/"$Adiarnfc" &&
+case "$(cd junk && echo *)" in
+	"$Adiarnfd")
+	test_nfd=1
+	;;
+	*)	;;
+esac
+rm -rf junk
+
+
+if test "$test_nfd"
+then
+	# create more utf-8 variables
+	Odiarnfc=`printf '\303\226'`
+	Odiarnfd=`printf 'O\314\210'`
+	AEligatu=`printf '\303\206'`
+	Invalidu=`printf '\303\377'`
+
+
+	#Create a string with 255 bytes (decomposed)
+	Alongd=$Adiarnfd$Adiarnfd$Adiarnfd$Adiarnfd$Adiarnfd$Adiarnfd$Adiarnfd #21 Byte
+	Alongd=$Alongd$Alongd$Alongd                                           #63 Byte
+	Alongd=$Alongd$Alongd$Alongd$Alongd$Adiarnfd                           #255 Byte
+
+	#Create a string with 254 bytes (precomposed)
+	Alongc=$AEligatu$AEligatu$AEligatu$AEligatu$AEligatu #10 Byte
+	Alongc=$Alongc$Alongc$Alongc$Alongc$Alongc           #50 Byte
+	Alongc=$Alongc$Alongc$Alongc$Alongc$Alongc           #250 Byte
+	Alongc=$Alongc$AEligatu$AEligatu                     #254 Byte
+
+	test_expect_success "detect if nfd needed" '
+		precomposeunicode=`git config core.precomposeunicode` &&
+		test "$precomposeunicode" = false &&
+		git config core.precomposeunicode true
+	'
+	test_expect_success "setup" '
+		>x &&
+		git add x &&
+		git commit -m "1st commit" &&
+		git rm x &&
+		git commit -m "rm x"
+	'
+	test_expect_success "setup case mac" '
+		git checkout -b mac_os
+	'
+	# This will test nfd2nfc in readdir()
+	test_expect_success "add file Adiarnfc" '
+		echo f.Adiarnfc >f.$Adiarnfc &&
+		git add f.$Adiarnfc &&
+		git commit -m "add f.$Adiarnfc"
+	'
+	# This will test nfd2nfc in git stage()
+	test_expect_success "stage file d.Adiarnfd/f.Adiarnfd" '
+		mkdir d.$Adiarnfd &&
+		echo d.$Adiarnfd/f.$Adiarnfd >d.$Adiarnfd/f.$Adiarnfd &&
+		git stage d.$Adiarnfd/f.$Adiarnfd &&
+		git commit -m "add d.$Adiarnfd/f.$Adiarnfd"
+	'
+	test_expect_success "add link Adiarnfc" '
+		ln -s d.$Adiarnfd/f.$Adiarnfd l.$Adiarnfc &&
+		git add l.$Adiarnfc &&
+		git commit -m "add l.Adiarnfc"
+	'
+	# This will test git log
+	test_expect_success "git log f.Adiar" '
+		git log f.$Adiarnfc > f.Adiarnfc.log &&
+		git log f.$Adiarnfd > f.Adiarnfd.log &&
+		test -s f.Adiarnfc.log &&
+		test -s f.Adiarnfd.log &&
+		test_cmp f.Adiarnfc.log f.Adiarnfd.log &&
+		rm f.Adiarnfc.log f.Adiarnfd.log
+	'
+	# This will test git ls-files
+	test_expect_success "git lsfiles f.Adiar" '
+		git ls-files f.$Adiarnfc > f.Adiarnfc.log &&
+		git ls-files f.$Adiarnfd > f.Adiarnfd.log &&
+		test -s f.Adiarnfc.log &&
+		test -s f.Adiarnfd.log &&
+		test_cmp f.Adiarnfc.log f.Adiarnfd.log &&
+		rm f.Adiarnfc.log f.Adiarnfd.log
+	'
+	# This will test git mv
+	test_expect_success "git mv" '
+		git mv f.$Adiarnfd f.$Odiarnfc &&
+		git mv d.$Adiarnfd d.$Odiarnfc &&
+		git mv l.$Adiarnfd l.$Odiarnfc &&
+		git commit -m "mv Adiarnfd Odiarnfc"
+	'
+	# Files can be checked out as nfc
+	# And the link has been corrected from nfd to nfc
+	test_expect_success "git checkout nfc" '
+		rm f.$Odiarnfc &&
+		git checkout f.$Odiarnfc
+	'
+	# Make it possible to checkout files with their NFD names
+	test_expect_success "git checkout file nfd" '
+		rm -f f.* &&
+		git checkout f.$Odiarnfd
+	'
+	# Make it possible to checkout links with their NFD names
+	test_expect_success "git checkout link nfd" '
+		rm l.* &&
+		git checkout l.$Odiarnfd
+	'
+	test_expect_success "setup case mac2" '
+		git checkout master &&
+		git reset --hard &&
+		git checkout -b mac_os_2
+	'
+	# This will test nfd2nfc in git commit
+	test_expect_success "commit file d2.Adiarnfd/f.Adiarnfd" '
+		mkdir d2.$Adiarnfd &&
+		echo d2.$Adiarnfd/f.$Adiarnfd >d2.$Adiarnfd/f.$Adiarnfd &&
+		git add d2.$Adiarnfd/f.$Adiarnfd &&
+		git commit -m "add d2.$Adiarnfd/f.$Adiarnfd" -- d2.$Adiarnfd/f.$Adiarnfd
+	'
+	test_expect_success "setup for long decomposed filename" '
+		git checkout master &&
+		git reset --hard &&
+		git checkout -b mac_os_long_nfd_fn
+	'
+	test_expect_success "Add long decomposed filename" '
+		echo longd >$Alongd &&
+		git add * &&
+		git commit -m "Long filename"
+	'
+	test_expect_success "setup for long precomposed filename" '
+		git checkout master &&
+		git reset --hard &&
+		git checkout -b mac_os_long_nfc_fn
+	'
+	test_expect_success "Add long precomposed filename" '
+		echo longc >$Alongc &&
+		git add * &&
+		git commit -m "Long filename"
+	'
+	# Test if the global core.precomposeunicode stops autosensing
+	# Must be the last test case
+	test_expect_success "respect git config --global core.precomposeunicode" '
+		git config --global core.precomposeunicode true &&
+		rm -rf .git &&
+		git init &&
+		precomposeunicode=`git config core.precomposeunicode` &&
+		test "$precomposeunicode" = "true"
+	'
+else
+	 say "Skipping nfc/nfd tests"
+fi
+
+test_done
diff --git a/utf8.c b/utf8.c
index 8acbc660d3..a544f15456 100644
--- a/utf8.c
+++ b/utf8.c
@@ -433,19 +433,12 @@ int is_encoding_utf8(const char *name)
 #else
 	typedef char * iconv_ibp;
 #endif
-char *reencode_string(const char *in, const char *out_encoding, const char *in_encoding)
+char *reencode_string_iconv(const char *in, size_t insz, iconv_t conv)
 {
-	iconv_t conv;
-	size_t insz, outsz, outalloc;
+	size_t outsz, outalloc;
 	char *out, *outpos;
 	iconv_ibp cp;
 
-	if (!in_encoding)
-		return NULL;
-	conv = iconv_open(out_encoding, in_encoding);
-	if (conv == (iconv_t) -1)
-		return NULL;
-	insz = strlen(in);
 	outsz = insz;
 	outalloc = outsz + 1; /* for terminating NUL */
 	out = xmalloc(outalloc);
@@ -459,7 +452,6 @@ char *reencode_string(const char *in, const char *out_encoding, const char *in_e
 			size_t sofar;
 			if (errno != E2BIG) {
 				free(out);
-				iconv_close(conv);
 				return NULL;
 			}
 			/* insz has remaining number of bytes.
@@ -478,6 +470,20 @@ char *reencode_string(const char *in, const char *out_encoding, const char *in_e
 			break;
 		}
 	}
+	return out;
+}
+
+char *reencode_string(const char *in, const char *out_encoding, const char *in_encoding)
+{
+	iconv_t conv;
+	char *out;
+
+	if (!in_encoding)
+		return NULL;
+	conv = iconv_open(out_encoding, in_encoding);
+	if (conv == (iconv_t) -1)
+		return NULL;
+	out = reencode_string_iconv(in, strlen(in), conv);
 	iconv_close(conv);
 	return out;
 }
diff --git a/utf8.h b/utf8.h
index 81f2c82fab..3c0ae7624e 100644
--- a/utf8.h
+++ b/utf8.h
@@ -14,6 +14,7 @@ int strbuf_add_wrapped_bytes(struct strbuf *buf, const char *data, int len,
 			     int indent, int indent2, int width);
 
 #ifndef NO_ICONV
+char *reencode_string_iconv(const char *in, size_t insz, iconv_t conv);
 char *reencode_string(const char *in, const char *out_encoding, const char *in_encoding);
 #else
 #define reencode_string(a,b,c) NULL