ATTR_LIST(2) ATTR_LIST(2)
attr_list, attr_listf - list the names of the user attributes of a
filesystem object
#include <sys/attributes.h>
int attr_list (const char *path, char *buffer,
const int buffersize, int flags,
attrlist_cursor_t *cursor);
int attr_listf (int fd, char *buffer,
const int buffersize, int flags,
attrlist_cursor_t *cursor);
The attr group of system calls implement the ability for a user to attach
name/value pairs to objects within the filesystem.
They could be used to store meta-information about the file. For example
"character-set=kanji" could tell a document browser to use the Kanji
character set when displaying that document and "thumbnail=..." could
provide a reduced resolution overview of a high resolution graphic image.
The names can be up to MAXNAMELEN bytes in length, terminated by the
first 0 byte. The intent is that they be printable ASCII (or other
character set) names for the attribute.
The values can be up to ATTR_MAX_VALUELEN (currently 64KB) of arbitrary
binary data.
Attributes can be attached to all types of inodes: regular files,
directories, symbolic links, device nodes, etc.
There are 2 disjoint attribute name spaces associated with every
filesystem object. They are the root and user address spaces. The root
address space is accessible only to privileged users, and only then by
specifying a flag argument to the function call. A privileged user can
be either the superuser in an IRIX environment, or a user with
CAP_DEVICE_MGT capability. Other users will not see or be able to modify
attributes in the root address space. The user address space is
protected by the normal file permissions mechanism, so the owner of the
file can decide who is able to see and/or modify the value of attributes
on any particular file.
Attributes are currently fully supported only in the XFS and CXFS
filesystem types. Other filesystem types may provide a partial
implementation.
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ATTR_LIST(2) ATTR_LIST(2)
DESCRIPTION
The attr_list and attr_listf functions provide a way to list the existing
attributes of a filesystem object.
Path points to a path name for a filesystem object, and fd refers to the
file descriptor associated with a file. The buffer will be filled with a
structure describing at least a portion of the attributes associated with
the given filesystem object. Buffer will be overwritten with an
attrlist_t structure containing a list of the attributes associated with
that filesystem object, up to a maximum of buffersize bytes. The buffer
must be sufficiently large to hold the appropriate data structures plus
at least one maximally sized attribute name, but cannot be more than
ATTR_MAX_VALUELEN (currently 64KB) bytes in length.
The contents of an attrlist_t structure include the following members:
__int32_t al_count; /* number of entries in attrlist */
__int32_t al_more; /* T/F: more attrs (do syscall again) */
__int32_t al_offset[1]; /* byte offsets of attrs [var-sized] */
The al_count field shows the number of attributes represented in this
buffer, which is also the number of elements in the al_offset array. The
al_more field will be non-zero if another attr_list call would result in
more attributes. The al_offset array contains the byte offset within the
buffer of the structure describing each of the attributes, an
attrlist_ent_t structure. The ATTR_ENTRY(buffer, index) macro will help
with decoding the list. It takes a pointer to the buffer and an index
into the al_offset array and returns a pointer to the corresponding
attrlist_ent_t structure.
The contents of an attrlist_ent_t structure include the following
members:
u_int32_t a_valuelen; /* number bytes in value of attr */
char a_name[]; /* attr name (NULL terminated) */
The a_valuelen field shows the size in bytes of the value associated with
the attribute whose name is stored in the a_name field. The name is a
NULL terminated string.
Note that the value of the attribute cannot be obtained through this
interface, the attr_get call should be used to get the value. The
attr_list interface tells the calling process how large of a buffer it
must have in order to get the attribute's value.
The flags argument can contain the following symbols bitwise OR'ed
together:
ATTR_ROOT
List the attributes that are in the root address space, not in the
user address space. (limited to use by super-user only)
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ATTR_LIST(2) ATTR_LIST(2)
ATTR_DONTFOLLOW
Do not follow symbolic links when resolving a path on an attr_list
function call. The default is to follow symbolic links.
The cursor argument is a pointer to an opaque data structure that the
kernel uses to track the calling process's position in the attribute
list. The only valid operations on a cursor are to pass it into an
attr_list function call or to zero it out. It should be zero'ed out
before the first attr_list call. Note that multi-threaded applications
may keep more than one cursor in order to serve multiple contexts, ie:
the attr_list call is "thread-safe".
attr_list will fail if one or more of the following are true:
[ENOENT] The named file does not exist.
[EPERM] The effective user ID does not match the owner of the
file and the effective user ID is not super-user.
[ENOTDIR] A component of the path prefix is not a directory.
[EACCES] Search permission is denied on a component of the path
prefix.
[EINVAL] A bit was set in the flag argument that is not defined
for this system call, or the buffer was too small or too
large.
[EFAULT] Either Path or buffer points outside the allocated
address space of the process, or buffer or bufsize are
not 32bit aligned.
[ELOOP] A path name lookup involved too many symbolic links.
[ENAMETOOLONG] The length of path exceeds {MAXPATHLEN}, or a pathname
component is longer than {MAXNAMELEN}.
[ENOATTR] attribute does not exist for this file.
attr_listf will fail if:
[EINVAL] A bit was set in the flag argument that is not defined for
this system call, or fd refers to a socket, not a file, or
the buffer was too small or too large.
[EFAULT] Either Path or buffer points outside the allocated address
space of the process, or buffer or bufsize are not 32bit
aligned.
[EBADF] Fd does not refer to a valid descriptor.
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ATTR_LIST(2) ATTR_LIST(2)
attr(1),
attr_get(2), attr_getf(2),
attr_multi(2), attr_multi
,
attr_set(2), attr_set(2)
Upon successful completion, a value of 0 is returned. Otherwise, a value
of -1 is returned and errno is set to indicate the error.
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