Meta Info Manipulation Methods

Methods to do various operations on Key metadata. More...

Collaboration diagram for Meta Info Manipulation Methods:

Functions
const Key *	keyNextMeta (Key *key)
	Get the next metadata entry of a Key. More...
int	keyCopyMeta (Key *dest, const Key *source, const char *metaName)
	Do a shallow copy of metadata with name metaName from source to dest. More...
int	keyCopyAllMeta (Key *dest, const Key *source)
	Do a shallow copy of all metadata from source to dest. More...
const Key *	keyGetMeta (const Key *key, const char *metaName)
	Returns the Key for a metadata entry with name metaName. More...
ssize_t	keySetMeta (Key *key, const char *metaName, const char *newMetaString)
	Set a new metadata Key. More...
KeySet *	keyMeta (Key *key)
	Returns the KeySet holding the given Key's metadata. More...

Detailed Description

Methods to do various operations on Key metadata.

To use them:

#include <kdb.h>

Next to Name (key and owner) and value (data and comment) there is the so called meta information inside every key.

Key meta information are an unlimited number of key/value pairs strongly related to a key. It main purpose is to give keys special semantics, so that plugins can treat them differently.

Metakey, as opposed to Key, deliberately has following limitations:

• no null values
• no binary data
• no modification of references (COW)
• no guarantee of ordering

Examples for metadata

File system information (see stat(2) for more information):

• uid: the user id (positive number)
• gid: the group id (positive number)
• mode: filesystem-like mode permissions (positive octal number)
• atime: When was the key accessed the last time.
• mtime: When was the key modified the last time.
• ctime: When the uid, gid or mode of a key changes. (times are represented through a positive number as unix timestamp)

The comment can contain userdata which directly belong to that key. The name of the meta information is "comment" for a general purpose comment about the key. Multi-Language comments are also supported by appending [LANG] to the name.

Validators are regular expressions which are tested against the key value. The metakey "validator" can hold a regular expression which will be matched against.

Types can be expressed with the meta information "type".

The relevance of the key can be tagged with a value from -20 to 20. Negative numbers are the more important and must be present in order to start the program.

A version of a key may be stored with "version". Its format is full.major.minor where all of these are integers.

The order inside a persistent storage can be described with the tag "order" which contains a positive number.

The metakey "app" describes to which application a key belongs. It can be used to remove keys from an application no longer installed.

The metakey "path" describes where the key is physically stored.

The "owner" is the user that owns the key. It only works for the user:/ hierarchy. It rather says where the key is stored and says nothing about the filesystem properties.

Function Documentation

keyCopyAllMeta()

int keyCopyAllMeta	(	Key *	dest,
		const Key *	source
	)

Do a shallow copy of all metadata from source to dest.

The key dest will additionally have all metadata the source had. Metadata not present in source will not be changed. Metadata which was present in source and dest will be overwritten. If the dest Key is read-only it will not be changed.

For example the metadata type is copied into the Key k:

void l (Key * k)
{
        // receive copy
        keyCopyAllMeta (k, copy);
        // the caller will see the changed key k
        // with all the metadata from copy
}

The main purpose of this function is for plugins or applications which want to add the same metadata to n keys. When you do that with keySetMeta() it will take n times the memory for the key. This can be considerable amount of memory for many keys with some metadata for each.

To avoid that problem you can use keyCopyAllMeta() or keyCopyMeta():

void o (KeySet * ks)
{
        Key * current;
        Key * shared = keyNew ("/", KEY_END);
        keySetMeta (shared, "shared1", "this metadata should be shared among many keys");
        keySetMeta (shared, "shared2", "this metadata should be shared among many keys also");
        keySetMeta (shared, "shared3", "this metadata should be shared among many keys too");
 
        for (elektraCursor it = 0; it < ksGetSize (ks); ++it)
        {
                current = ksAtCursor (ks, it);
                if (needsSharedData (current)) keyCopyAllMeta (current, shared);
        }
 
        keyDel (shared);
}

Precondition

dest's metadata is not read-only

Postcondition

for every metaName present in source: keyGetMeta(source, metaName) == keyGetMeta(dest, metaName)

Parameters

dest	the destination where the metadata should be copied too
source	the key where the metadata should be copied from

Return values

1	if metadata was successfully copied
0	if source did not have any metadata
-1	on null pointer of dest or source
-1	on memory problems

Since

1.0.0

See also

keyCopyMeta() for copying one metadata Key from dest to source

keyCopyMeta()

int keyCopyMeta	(	Key *	dest,
		const Key *	source,
		const char *	metaName
	)

Do a shallow copy of metadata with name metaName from source to dest.

Afterwards source and dest will have the same metadata referred with metaName. If the Key with name metaName doesn't exist in source - it gets deleted in dest.

For example the metadata type is copied into the Key k.

void l(Key *k)
{
        // receive c
        keyCopyMeta(k, c, "type");
        // the caller will see the changed key k
        // with the metadata "type" from c
}

The main purpose of this function is for plugins or applications, which want to add the same metadata to n keys. When you do that keySetMeta() will take n times the memory for the key. This can be a considerable amount of memory for many keys with some metadata for each.

To avoid that problem you can use keyCopyAllMeta() or keyCopyMeta().

void o(KeySet *ks)
{
        Key *shared = keyNew ("/", KEY_END);
        keySetMeta(shared, "shared", "this metadata should be shared among many keys");
 
        for (elektraCursor it = 0; it < ksGetSize (ks); ++it)
        {
                Key * current = ksAtCursor (ks, it);
                if (needs_shared_data(current)) keyCopyMeta(current, shared, "shared");
        }
}

Precondition

dest's metadata is not read-only

Postcondition

keyGetMeta(source, metaName) == keyGetMeta(dest, metaName)

Parameters

dest	the destination where the metadata should be copied to
source	the key where the metadata should be copied from
metaName	the name of the metadata Key which should be copied

Return values

1	if was successfully copied
0	if the metadata in dest was removed too
-1	on null pointers (source or dest)
-1	on memory problems
-1	if metadata is read-only

Since

1.0.0

See also

keyCopyAllMeta() copies all metadata from dest to src

keyGetMeta()

const Key* keyGetMeta	(	const Key *	key,
		const char *	metaName
	)

Returns the Key for a metadata entry with name metaName.

You are not allowed to modify the resulting key.

If metaName does not start with 'meta:/', it will be prefixed with 'meta:/'.

Key metaData = keyGetMeta(k, "type")
// keyType == "boolean"
char keyType[] = keyValue(metaData)

Note

You must not delete or change the returned key, use keySetMeta() if you want to delete or change it.

Precondition

key contains metadata

metaName is prefixed with "meta:/"

Parameters

key	the Key from which to get metadata
metaName	the name of the meta information you want the Key from.

Returns

value of meta-information if meta-information is found

Return values

0	if key or metaName is NULL
0	if no such metaName is found

Since

1.0.0

See also

keySetMeta() for setting metadata

keyMeta() for getting the KeySet containing metadata

keyMeta()

KeySet* keyMeta

(

Key *

key

)

Returns the KeySet holding the given Key's metadata.

Use keySetMeta() to populate the metadata KeySet of a Key.

        Key * key = keyNew ("user:/test/key", KEY_END);
 
        keySetMeta (key, "meta1", "value1");
        keySetMeta (key, "meta2", "value2");

Iterate the returned metadata KeySet like any other KeySet.

        Key * cur;
        KeySet * metaKeys;
        ssize_t ksSize;
 
        metaKeys = keyMeta (key);
        ksSize = ksGetSize (metaKeys);
 
        for (elektraCursor it = 0; it < ksSize; ++it)
        {
                cur = ksAtCursor (metaKeys, it);
                printf ("meta name: %s, meta value: %s\n", keyName (cur), keyString (cur));
        }

Use ksLookup() or keyGetMeta() to retrieve a single value for a given Key.

        Key * lookupKey = ksLookupByName (keyMeta (key), "meta2", 0);
        printf ("meta name: %s, meta value: %s\n", keyName (lookupKey), keyString (lookupKey));
        keyDel (key);

Note

You are not allowed to modify the name of KeySet's Keys or delete them.

You must not delete the returned KeySet.

Adding a key with metadata to the KeySet is an error.

If the key has not metadata keyset, it will be allocated

Postcondition

for the returned KeySet ks: keyGetMeta(key, metaName) == ksLookupByName(ks, metaName)

key contains a KeySet for the metadata

Parameters

key	the Key from which to get the metadata KeySet

Returns

the KeySet holding the metadata

Return values

0	if the Key is 0

Since

1.0.0

See also

keySetMeta() for setting a metadata Key

keyGetMeta() for getting a metadata Key

keyNextMeta()

const Key* keyNextMeta

(

Key *

key

)

Get the next metadata entry of a Key.

Keys have an internal cursor. Every time keyNextMeta() is called the cursor is incremented and the new current Name of Meta Information is returned.

You'll get a NULL pointer if the metadata after the end of the Key was reached. On subsequent calls of keyNextMeta() it will still return the NULL pointer.

The key internal cursor will be changed, so it is not const.

Note

That the resulting key is guaranteed to have a value, because meta information has no binary or null pointer semantics.

You must not delete or change the returned key, use keySetMeta() if you want to delete or change it.

Parameters

key	the Key object to work with

Returns

a key containing metadata

Return values

0	when the last Key has been reached
0	when Key is a NULL pointer

Since

1.0.0

keySetMeta()

ssize_t keySetMeta	(	Key *	key,
		const char *	metaName,
		const char *	newMetaString
	)

Set a new metadata Key.

Will set a new metadata pair with name metaName and value newMetaString.

Will add a new metadata Key, if metaName was unused until now.

It will modify an existing Pair of metadata if metaName was already present.

It will remove a metadata Key if newMetaString is 0.

If metaName does not start with 'meta:/', it will be prefixed with 'meta:/'.

Precondition

metaName is prefixed with "meta:/"

key's metadata is not read-only

Postcondition

The value in key's metadata Keyset for metaName is newMetaString

Parameters

key	Key whose metadata should be set
metaName	name of the metadata Key that should be set
newMetaString	new value for the metadata Key

Returns

size (>0) of newMetaString if metadata has been successfully added

Return values

0	if the meta-information for metaName was removed
-1	if key or metaName is 0
-1	if system is out of memory
-1	if metaName is not a valid metadata name

Since

1.0.0

See also

keyGetMeta() for getting the value of a metadata Key

keyMeta() for getting the KeySet containing metadata