Pxattr provide a Linux/FreeBSD/Mac OS X cross-platform uniform interface to 
user file extended attributes, with an attributes backup/restore tool.

Full documentation: http://www.lesbonscomptes.com/pxattr/ 
or doc/html/index.html inside the source tree.
- Sur linux, il faut explicitement enabler les xattr:
mount -o remount,user_xattr /home
ou
/dev/sda7       /home   reiserfs        acl,user_xattr 1 2


FreeBSD
--------------------------
 #include <sys/types.h>
 #include <sys/extattr.h>
 #include <sys/uio.h>

 ssize_t extattr_get_fd(int fd, int attrnamespace, const char *attrname,
                        void *data, size_t nbytes);
 int extattr_set_fd(int fd, int attrnamespace, const char *attrname,
                    const void *data, size_t nbytes);
 int extattr_delete_fd(int fd, int attrnamespace, const char *attrname);
 ssize_t extattr_list_fd(int fd, int attrnamespace, void *data, size_t nbytes);

 ssize_t extattr_get_file(const char *path, int attrnamespace,
                          const char *attrname, void *data, size_t nbytes);
 etc.
 ssize_t extattr_get_link(const char *path, int attrnamespace,
                          const char *attrname, void *data, size_t nbytes);
 etc.
- Namespaces are separate, not part of the name. 2 defined by default:
  EXTATTR_NAMESPACE_USER, EXTATTR_NAMESPACE_SYSTEM
- _get_() can get size by passing null buffer. Can also apparently read in
  multiple chunks ?? to be checked.

Linux
--------------------------------
  #include <sys/types.h>
  #include <attr/xattr.h>

 ssize_t fgetxattr(int fd, const char *name, void *value, size_t size);
 ssize_t flistxattr(int fd, char *list, size_t size);
 int fsetxattr(int fd, const char *name, const void *value, size_t size, 
               int flags);
 int fremovexattr(int fd, const char *name);

 ssize_t getxattr(const char *path, const char *name, void *value, size_t size);
etc.
 int lsetxattr(const char *path, const char *name, const void *value, 
   size_t size, int flags);
etc.

- The namespace is part of the name "user.xxx.yy". Four predefined spaces:
  security, system, trusted, user. So permissions depend on the name prefix.
  The system namespace is used for implementing acls

- get() can get size by passing buffer of size 0

- The flags to "set" are XATTR_CREATE or XATTR_REPLACE. Default 0 means
  create or replace as appropriate.


MacOSX
-----------------------------
 #include <sys/xattr.h>

 ssize_t fgetxattr(int fd, const char *name, void *value, size_t size,
                   u_int32_t position, int options);
 int fsetxattr(int fd, const char *name, void *value, size_t size,
               u_int32_t position, int options);
 int fremovexattr(int fd, const char *name, int options);
 ssize_t flistxattr(int fd, char *namebuf, size_t size, int options);

 ssize_t getxattr(const char *path, const char *name, void *value, size_t size,
                  u_int32_t position, int options);
etc.

 - position is only used with the resource fork, else set to 0
 - names are UTF-8. Max size 
 - get with size 0 returns actual size.
 - options: XATTR_NOFOLLOW (all), XATTR_CREATE/XATTR_REPLACE (setxattr()
   not set->either)

http://arstechnica.com/reviews/os/macosx-10-4.ars/7:
    Apple recommends the use of a reverse-DNS naming scheme (e.g.,
    com.apple.itunes.artist), but with the dots replaced with underscores
    in order to avoid conflicting with the namespace convention used by
    Cocoa's key/value coding system.  
    - Max name size 128 ?

There are apparently no predefined namespaces and no restrictions on names

Solaris
---------