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DESIGN

This document describes the design of Elektra's C-API and provides hints for binding writers. It is not aimed at plugin writers, since it does not talk about the implementation details of Elektra.

Elektra aims to fulfill the following design principles:

  1. To make the API future-proof so that it can remain compatible and stable over a long period of time,
  2. to make it hard to use the API the wrong way by making it simple & robust, and
  3. to make the API easy to use for programmers reading and writing configuration.

The C-API is suitable to be reimplemented, also in non-C-languages, like Rust. Elektra provides a full-blown architecture to support configuring systems, and the C-API is the core of this endeavour.

Data Structures

The Key, KeySet and KDB data structures are defined in kdbprivate.h to allow ABI compatibility. This means, it is not possible to put one of Elektra’s data structures on the stack. You must use the memory management facilities mentioned in the next section.

Memory Management

Elektra provides functions that create and free data. For example after you call:

KDB * kdbOpen();

you need to use:

int kdbClose(KDB *handle);

to get rid of the resources again. The second function may also shut down connections. Therefore, it must be called before the end of a program.

Key *keyNew(const char *keyName, ...);
int keyDel(Key *key);

KeySet *ksNew(int alloc, ...);
int ksDel(KeySet *ks);

In the above pairs, the first function reserves the necessary amount of memory. The second function frees the allocated data segment. There are more allocations happening, but they are invisible to the user of the API and happen implicitly within any of these 3 classes: KDB, Key and KeySet.

Key names and values cannot be handled as easy without helper libraries, because Elektra does not provide a string library. The function

const void *keyValue(const Key *key);

returns a value. You are not allowed to change the returned value. The life time is bound to the Key. The function

ssize_t keyValueSize(const Key *key);

gives the length of the value in bytes.

Variable Arguments

The constructors for Key and KeySet take a variable sized list of arguments. They can be used as an alternatives to the various keySet* methods and ksAppendKey. With them you are able to generate any Key or KeySet with a single C-statement. This can be done programmatically by the plugin c.

To just retrieve a key, use

Key *k = keyNew("/", KEY_END);

To obtain a keyset, use

KeySet *k = ksNew(0, KS_END);

Alternatively pass a list as described in the documentation. The idea of these variable arguments is, that one function call can create any KeySet. For binding writers keyVNew might be useful.

Off-by-one

We avoid off-by-one errors by starting all indices with 0, as usual in C. The size returned by the *GetSize functions (keyGetValueSize, keyGetCommentSize and keyGetOwnerSize) is exactly the size you need to allocate. So if you add 1 to it, too much space is allocated, but no error will occur.

The same is true for elektraStrLen which also already has the null byte included.

Minimal Set

kdb.h contains a minimal set of functions to fully work with a key database. The functions are implemented in src/libs/elektra in ANSI C.

Useful extensions are available in further libraries.

Return Values

Elektra’s function share common error codes. Every function must return -1 on error, if its return type is integer (like int, ssize_t). If the function returns a pointer, 0 (NULL) will indicate an error.

Elektra uses integers for the length of C strings, reference counting, KeySet length and internal KeySet allocations.

The interface always accepts ssize_t and internally uses size_t, which is able to store larger numbers than ssize_t.

The real size of C strings and buffers is limited to SSIZE_MAX. When a string exceeds that limit -1 or a NULL pointer (see above) must be returned.

The following functions return an internal string:

const char *keyName(const Key *key);
const char *keyBaseName(const Key *key);

and in the case that keyIsBinary(key)==0:

const void *keyValue(const Key *key);

does so, too. If in any of the functions above key is a NULL pointer, then they also return NULL.

If there is no string you will get back "", that is a pointer to the value '\0'. The function to determine the size will return 1 in that case. That means that an empty string – nothing except the NULL terminator – has size 1.

This is not true for keyValue in the case of binary data, because the value '\0' in the first byte is perfectly legal binary data. keyGetValueSize may also return 0 for that reason.

Error Handling

For KDB functions the user does not only get the return value but also a more elaborate error information, including an error message, in the metadata of the parentKey or errorKey. Furthermore, it is also possible to get warnings, even if the calls succeeded.

Using different error categories, the user of the API can have suitable reactions on specific error situations. Additional information about error handling is available here.

Elektra does not set errno. If a function you call sets errno, make sure to set it back to the old value again.

Naming

All function names begin with their class name, e.g. kdb, ks or key. We use capital letters to separate single words (CamelCase). This leads to short names, but might be not as readable as separating names by other means.

Get and Set are used for getters/and setters. We use Is to ask about a flag or state and Needs to ask about state related to databases. For allocation/deallocation we use C++ styled names (e.g *New, *Del).

Macros and Enums are written in capital letters. Flags start with KDB_, namespaces with KEY_NS_ and macros with ELEKTRA_.

Data structures start with a capital letter for every part of the word:

  • KDB ... Key Data Base Handle
  • KeySet ... Key Set
  • Key ... Key

We use singular for all names.

Function names not belonging to one of the three classes use the prefix elektra*.

const

Wherever possible functions should use the keyword const for parameters. The API uses this keyword for parameters, to show that a function does not modify a Key or a KeySet, e.g.:

const char *keyName(const Key *key);
const char *keyBaseName(const Key *key);
const void *keyValue(const Key *key);
const char *keyString(const Key *key);
const Key  *keyGetMeta(const Key *key, const char* metaName);

The reason behind this is, that the above functions – as their name suggest – only retrieve values. The returned value must not be modified directly.

Design Guidelines Checklist

On potential changes of the API/ABI as detected by the build server, please make sure the API has been reviewed according to the following 2 checklists:

Checklist for overall API

Consistency

  • Consistent naming schemes for enums, macros, typedefs and functions
  • Same things are named the same and included in Glossary
  • Different things are named differently
  • The order of arguments should be consistent across similar functions

Structural Clarity

  • Functions with similar functionality have the same prefix

Compatibility

  • All bindings have been updated to reflect the new API and work properly

Extensibility

  • New API is easily extensible with additional functionality
  • Components only depend on each other if needed

Checklist for each function

There are several checklists for functions, depending on the language in which the function is written: