NAME
attr_list, attr_listf - list the names of the user attributes of a
filesystem object
C SYNOPSIS
#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);
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)
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.
DIAGNOSTICS
Upon successful completion, a value of 0 is returned. Otherwise, a
value of -1 is returned and errno is set to indicate the error.
SEE ALSO
attr(1), attr_multi(3), attr_remove(3), and attr_set(3).