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