ATTRMULTI(2) ATTRMULTI(2)
NAME
attr_multi, attr_multif - manipulate multiple user attributes on a
filesystem object at once
C SYNOPSIS
#include <sys/attributes.h>
int attrmulti (const char *path, attrmultiopt *oplist,
int count, int flags);
int attrmultif (int fd, attrmultiopt *oplist,
int count, 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 accessable only to the super-user, and then only by
specifying a flag argument to the function call. 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 supported only in the XFS filesystem type.
DESCRIPTION
The attr_multi and attr_multif functions provide a way to operate on
multiple attributes of a filesystem object at once.
Path points to a path name for a filesystem object, and fd refers to the
file descriptor associated with a file. The oplist is an array of
attrmultiopt structures. Each element in that array describes a single
attribute operation and provides all the information required to carry
out that operation and to check for success or failure of that operation.
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Count tells how many elements are in the oplist array.
The contents of an attrmultiopt structure include the following
members:
int amopcode; /* which operation to perform (see below) */
int amerror; /* [out arg] result of this sub-op (an errno) */
char *amattrname; /* attribute name to work with */
char *amattrvalue; /* [in/out arg] attribute value (raw bytes) */
int amlength; /* [in/out arg] length of value */
int amflags; /* flags (bit-wise OR of #defines below) */
The am_opcode field defines how the remaining fields are to be
interpreted and can take on one of the following values:
ATTROPGET /* return the indicated attr's value */
ATTROPSET /* set/create the indicated attr/value pair */
ATTROPREMOVE /* remove the indicated attr */
The am_error field will contain the appropriate error result code if that
sub-operation fails. The result codes for a given sub-operation are a
subset of the result codes that are possible from the corresponding
single-attribute function call. For example, the result code possible
from an ATTROPGET sub-operation are a subset of those that can be
returned from an attr_get function call.
The am_attrname field is a pointer to a NULL terminated string giving the
attribute name that the sub-operation should operate on.
The am_attrvalue, am_length and am_flags fields are used to store the
value of the named attribute, and some control flags for that sub-
operation, respectively. Their use varies depending on the value of the
am_opcode field.
ATTROPGET
The am_attrvalue field is a pointer to a empty buffer that will be
overwritten with the value of the named attribute. The am_length
field is initially the total size of the memory buffer that the
am_attrvalue field points to. After the operation, the am_length
field contains the actual size of the attribute's value. The
am_flags field may be set to the ATTRROOT flag. If the process has
appropriate priviledges, the ROOT namespace will be searched for the
named attribute, otherwise the USER namespace will be searched.
ATTROPSET
The am_attrvalue and am_length fields contain the new value for the
given attribute name and its length. The ATTRROOT flag may be set
in the am_flags field. If the process has appropriate priviledges,
the ROOT namespace will be searched for the named attribute,
otherwise the USER namespace will be searched. The ATTRCREATE and
the ATTRREPLACE flags may also be set in the am_flags field (but
not simultaneously). If the ATTRCREATE flag is set, the sub-
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operation will set the am_error field to EEXIST if the named
attribute already exists. If the ATTRREPLACE flag is set, the
sub-operation will set the am_error field to ENOATTR if the named
attribute does not already exist. If neither of those two flags are
set and the attribute does not exist, then the attribute will be
created with the given value. If neither of those two flags are set
and the attribute already exists, then the value will be replaced
with the given value.
ATTROPREMOVE
The am_attrvalue and am_length fields are not used and are ignored.
The am_flags field may be set to the ATTRROOT flag. If the process
has appropriate priviledges, the ROOT namespace will be searched for
the named attribute, otherwise the USER namespace will be searched.
The flags argument to the attr_multi call is used to control following of
symbolic links in the path argument. The default is to follow symbolic
links, flags should be set to ATTR_DONTFOLLOW to not follow symbolic
links.
attr_multi 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 other than ATTR_DONTFOLLOW was set in the flag
argument.
[EFAULT] Path, or oplist 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_multif will fail if:
[EINVAL] A bit was set in the flag argument, or fd refers to a
socket, not a file.
[EFAULT] Oplist points outside the allocated address space of the
process.
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ATTRMULTI(2) ATTRMULTI(2)
[EBADF] Fd does not refer to a valid descriptor.
SEE ALSO
attr(1),
attr_get(2), attr_getf(2),
attr_list(2), attr_list(2)
attr_remove(2), attr_removef(2),
attr_set(2), attr_set(2)
DIAGNOSTICS
Upon successful completion of the attr_multi call, a value of 0 is
returned. Otherwise, a value of -1 is returned and errno is set to
indicate the error. Note that the individual operations listed in the
oplist array each have their own error return fields. The errno variable
only records the result of the attr_multi call itself, not the result of
any of the sub-operations.
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