attr_get, attr_getf - get the value of a user attribute of a filesystem object
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);
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 docu- ment 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.
The attr_get and attr_getf functions provide a way to retrieve the value of an attribute.
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 ORŽed together:
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ATTR_ROOT Look for attrname in the root address space, not in the user address space. |
(limited to use |
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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:
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[ENOATTR] [E2BIG] [ENOENT] [EPERM] [ENOTDIR] [EACCES] [EINVAL] [EFAULT] [ELOOP] |
The attribute name given is not associated with the indicated filesystem object. 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. The named file does not exist. The effective user ID does not match the owner of the file and the effective user ID is not super- user. A component of the path prefix is not a directory. Search permission is denied on a component of the path prefix. A bit was set in the flag argument that is not defined for this system call. Path, attrname, attrvalue, or valuelength points outside the allocated address space of the pro- cess. A path name lookup involved too many symbolic links. |
[ENAMETOOLONG]
The length of path exceeds {MAXPATHLEN}, or a pathname component is longer than {MAXNAME-
LEN}.
attr_getf will fail if:
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[ENOATTR] [E2BIG] [EINVAL] [EFAULT] [EBADF] |
The attribute name given is not associated with the indicated filesystem object. 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 numnber of bytes that would be required to store the value of that attribute. 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. Attrname, attrvalue, or valuelength points outside the allocated address space of the process. Fd does not refer to a valid descriptor. |
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attr(1), |
Upon successful completion, a value of 0 is returned. Otherwise, a value of -1 is returned and errno is set to indi- cate the error.