attr_set, attr_setf - set the value of a user attribute of a filesystem object
int attr_set (const char *path, const char *attrname,
const char *attrvalue, const int valuelength,
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int flags); int attr_setf (int fd, const char *attrname, |
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_set and attr_setf functions provide a way to create attributes and set/change their values.
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 does not exist, an attribute with the given name and value will be created and associated with that indicated filesystem object. If an attribute with that name already exists on that filesystem object, the existing value is replaced with the new value given in this call. The new attribute value is copied from the attrvalue buffer for a total of valuelength bytes. 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_set function call. The default
is to follow symbolic links.
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ATTR_CREATE ATTR_REPLACE |
Return an error (EEXIST) if an attribute of the given name already exists on the indicated filesystem object, otherwise create an attribute with the given name and value. This flag is used to implement a pure create operation, without this flag attr_set will create the attribute if it does not already exist. An error (EINVAL) will be returned if both ATTR_CREATE and ATTR_REPLACE are set in the same call. Return an error (ENOATTR) if an attribute of the given name does not already exist on the indicated filesystem object, otherwise replace the existing attributeŽs value with the given value. This flag is used to implement a pure replacement operation, without this flag attr_set will create the attribute if it does not already exist. An error (EINVAL) will be returned if both ATTR_CREATE and ATTR_REPLACE are set in the same call. |
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attr_set will fail if one or more of the following are true: [ENOATTR] The attribute name given is not associated with the indicated filesystem object and |
the |
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ATTR_REPLACE flag bit was set. [E2BIG] The value of the given attribute is too large, it exceeds the maximum allowable size of |
an |
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attribute value. [EEXIST] The attribute name given is already associated with the indicated filesystem object and |
the |
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ATTR_CREATE flag bit was set. |