ATTR_GET(2)ATTR_GET(2)NAME
attr_get, attr_getf - get the value of a user attribute of a filesystem
object
C SYNOPSIS
#include <sys/attributes.h>
int attr_get (const char *path, const char *attrname,
char *attrvalue, int *valuelength, int flags);
int attr_getf (int fd, const char *attrname,
char *attrvalue, int *valuelength, int flags);
OVERVIEW
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, CXFS and UDF
filesystem types. Other filesystem types may provide a partial
implementation.
DESCRIPTION
The attr_get and attr_getf functions provide a way to retrieve the value
of an attribute.
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ATTR_GET(2)ATTR_GET(2)
Path points to a path name for a filesystem object, and fd refers to the
file descriptor associated with a file. If the attribute attrname
exists, the value associated with it will be copied into the attrvalue
buffer. The valuelength argument is an input/output argument that on the
call to attr_get should contain the maximum size of attribute value the
process is willing to accept. On return, the valuelength will have been
modified to show the actual size of the attribute value returned. The
flags argument can contain the following symbols bitwise OR'ed together:
ATTR_ROOT
Look for attrname 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_get
function call. The default is to follow symbolic links.
attr_get will fail if one or more of the following are true:
[ENOATTR] The attribute name given is not associated with the
indicated filesystem object.
[E2BIG] The value of the given attribute is too large to fit
into the buffer. The integer that the valuelength
argument points to has been modified to show the actual
number of bytes that would be required to store the
value of that attribute.
[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.
[EFAULT] Path, attrname, attrvalue, or valuelength points outside
the allocated address space of the process.
[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}.
attr_getf will fail if:
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ATTR_GET(2)ATTR_GET(2)
[ENOATTR] The attribute name given is not associated with the
indicated filesystem object.
[E2BIG] The value of the given attribute is too large to fit into
the buffer. The integer that the valuelength argument
points to has been modified to show the actual number of
bytes that would be required to store the value of that
attribute.
[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.
[EFAULT] Attrname, attrvalue, or valuelength points outside the
allocated address space of the process.
[EBADF] Fd does not refer to a valid descriptor.
SEE ALSOattr(1),
attr_list(2), attr_listf(2)attr_multi(2), attr_multif(2)attr_remove(2), attr_removef(2),
attr_set(2), attr_setf(2),
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.
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