NAME¶

attr_list, attr_listf - list the names of the user attributes of a filesystem object

C SYNOPSIS¶

#include <attr/attributes.h>

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);

DESCRIPTION¶

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 filesystem 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:

uint32_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:

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)

ATTR_DONTFOLLOW

Do not follow symbolic links when resolving a path on an attr_list function call. The default is to follow symbolic links.

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:

[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, or the buffer was too small or too large.

[EFAULT]

Either Path or buffer points outside the allocated address space of the process, or buffer or bufsize are not 32bit aligned.

[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}.

[ENOATTR]

attribute does not exist for this file. If the system libc does not define this error code, then libattr will define it to ENODATA in <attr/attributes.h>.

attr_listf will fail if:

[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, or the buffer was too small or too large.

[EFAULT]

Either Path or buffer points outside the allocated address space of the process, or buffer or bufsize are not 32bit aligned.

[EBADF]

Fd does not refer to a valid descriptor.

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.

SEE ALSO¶

attr(1), attr_multi(3), attr_remove(3), attr_set(3)