attr_list, attr_listf - list the names of the user attributes of a filesystem object
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);
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_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 filesys- tem 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:
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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) |
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ATTR_DONTFOLLOW |
Do not follow symbolic links when resolving a path on an attr_list function call. The default |
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:
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[ENOENT] [EPERM] [ENOTDIR] [EACCES] [EINVAL] [EFAULT] [ELOOP] |
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, or the buffer was too small or too large. Either Path or buffer points outside the allocated address space of the process, or buffer or buf- size are not 32bit aligned. A path name lookup involved too many symbolic links. |
[ENAMETOOLONG]
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The length of path exceeds |
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{MAXPATHLEN}, or a pathname component is longer than {MAXNAME- |
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LEN}. |
[ENOATTR] attribute does not exist for this file.
attr_listf will fail if:
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[EINVAL] [EFAULT] [EBADF] |
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. Either Path or buffer points outside the allocated address space of the process, or buffer or buf- size are not 32bit aligned. 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.