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Elektra
0.9.8
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This document focuses on the advanced features of the high-level API code-generator template. We assume you already familiarized yourself with the basic features explained in `kdb-gen-highlevel(1)`.
The parameters that are relevant to the concepts described here are (for the rest see `kdb-gen-highlevel(1)):
embeddedSpec: allowed values: full (default), defaults, noneenumConv: allowed values: strcmp, switch, auto (default)Using embeddedSpec you can configure how much of the specification is embedded into your application. By default we use full. This means the full specification is embedded into your application's binary. Since this can drastically increase the size of the binary, you can also choose defaults or none. The defaults setting embeds a reduced version of the specification, which only contains the metadata required by elektraOpen. By setting embeddedSpec=none you can also remove this reduced specification.
The advantage of using full is that your application is contained in a single executable file. If you don't use full, the code-generator produces an additional .spec.eqd file and omits specload function (called exitForSpecload by default). This file contains the full specification in quickdump format. You can either mount it directly via quickdump, or if you want the features of specload use a specload configuration like this: app=/usr/bin/cat args=#0 args/#0="path-to-spec-output-file".
Setting embeddedSpec=none is only recommended, if you must have the minimal binary size and you know what you are doing. In this case no defaults are passed to elektraOpen and defaults are only handled via the spec plugin. If the specification/configuration isn't mounted, the getter functions may fail.
The case of a misconfigured mountpoint will be detected automatically and reported as an error. It will cause the initialization function (by default named loadConfiguration) to fail, if the specification is not mounted at the expected mountpoint or if the specification was not spec-mounted.
We support the mapping of a set of string values to a native C enum. To use this feature, you need to write your specification the same way that the enum part of the type plugin expects.
The above specification will generate the following C enum:
As you can see the integer values of the different enum values are taken from the indices of the check/enum/# array. You may also use e.g. gen/enum/#2/value=1 << 1 to set a different value. The gen/enum/#/value values are inserted literally into the C files, so the values must be valid C code. The name of the enum may be configured via gen/enum/type. If you want to use an existing enum and map its values to strings you can turn the generation of the enum off, by adding gen/enum/create=0. In this case you have to add a header that defines the enum or typedefs it, to the headers parameter of the code-generator invocation.
Like with any other key, the code-generator produces static inline getter and setter functions for the key. Since there are no generic functions for the conversion of the strings into the enum values, we also generate those:
These functions are not generated per key, but per enum type. If multiple keys use the same enum type (both need to define the full metadata, including the full set of values), we only generate one set of these functions.
The getter and setter functions won't be explained here, they work like any of the other getter and setter functions of the high-level API.
The other three functions are used to convert between the string values and the generated enum. You may find these useful in your application. You can call them via e.g. ELEKTRA_KEY_TO (EnumMyenum). The difference between ELEKTRA_TO_STRING and ELEKTRA_TO_CONST_STRING is that the first returns a char * allocated via elektraMalloc, while the second returns a static constchar *.
Both ELEKTRA_TO_STRING and ELEKTRA_TO_CONST_STRING are always implemented via a straightforward switch statement. The implementation of ELEKTRA_KEY_TO on the other hand can be changed via the enumConv parameter. If you set enumConv=strcmp, we will generate a code analogous to:
This code is not really optimal, since we really only need to look at the first character to determine the correct enum value. This is where enumConv=switch comes in. With this option, we generate a series of (nested, if necessary) switch/case statements:
Of course this version also has its own problems. Take for example the the enum with the values: blue, blueish and brown. With enumConv=switch this would generate the following code:
This is already quite hard to read and blueish isn't even that long.
To provide a compromise between readability and performance, we default to enumConv=auto. This options uses the switch version, if the depth is less than 3, and the strcmp version in all other cases. A depth of n means looking at the first n characters string[0], string[1], ..., string[n-1]. In other words a depth of n uses n switch statements.
The highlevel template also has support for structs. By setting type = struct on a key, you can enable the generation of a native C struct for the keys below it.
We will look at this simple example:
Note: That we set check/type=any and default="". This is to avoid problems with the type plugin, which doesn't know about structs.
The generated struct looks like this:
Similar to enums, you can customise the generated struct via additional metadata:
type=struct:gen/struct/type can be used to set the name of the generated struct.gen/struct/create=0 disables the struct generation and only generates the accessor functions. Use this to use structs defined elsewhere. Don't forget to include the needed header in the headers parameter.gen/struct/alloc (values 0, 1) sets whether the struct is allocating. This changes how the getter works and also has some other implications. By default structs are non-allocating.gen/struct/depth sets the how many levels below the type=struct key, we will include in the generated struct. Note that keys ending in /# (i.e. array keys) count as one level above. So mystruct/x/# would be included with the default gen/struct/depth=1.gen/struct/field sets the name of the field in the generated struct.gen/struct/field/ignore=1 ignores this key during struct generation, i.e. we don't create a field for it.gen/array/sizefield sets the name of the field used to store the size of arrays. Only useful on array keys. For example, by default the size of the array key mystruct/x/# is stored in xSize, while the array is accessed via the field x.We will also generate getter and setter functions:
The difference between ELEKTRA_GET_SIGNATURE and ELEKTRA_GET_OUT_PTR_SIGNATURE is explained in the next section. Both versions are called via ELEKTRA_GET (...) (...).
Allocating structs also generate ELEKTRA_STRUCT_FREE (/* struct name */), which is used to free the allocated memory.
The main difference between allocating and non-allocating structs, is how their getter function works.
Allocating structs use a getter similar to the one primitive types, strings and enums use. It returns a pointer to a newly allocated struct, which has to be freed using the generated ELEKTRA_STRUCT_FREE function.
Non-allocating structs meanwhile use a different kind of getter declared via ELEKTRA_GET_OUT_PTR_SIGNATURE instead of ELEKTRA_GET_SIGNATURE. This version doesn't return a pointer, instead it takes a pointer to an existing struct and only sets its fields. This is why you have to use the convenience macros elektraFillStruct and elektraFillStructV for these structs.
Non-allocating structs are also more limited than their allocating counterparts. They do not support arrays or struct references. They also cannot be for unions. Their main advantage is that you can use non-allocating structs without (additional) malloc/free, by providing a stack allocated pointer to the getter function.
Structs cannot be nested, but they can reference each other. This allows for complex and possibly recursive structures. Take for example:
This results in a struct like this:
As you can see an instance of ElektraStructPerson may reference different instances. To declare this we must add a key with type=struct_ref. We use the metakeys of the reference plugin (which should be mounted to validate reference) to define what struct we want to reference. We also again set check/type=any and default="" to please the type plugin.
Struct references are also supported as arrays, in which case the check/reference keys must be on a different key than the rest of the metadata, because of how the reference plugin works. The example above shows this with person/#/children and person/#/children/#.
If you access an element of the person/# array via the getter function, we will recursively read the references structs. Writing structs that contain struct references or setting struct_ref keys directly is not supported.
Struct references can also exist outside of structs and maybe accessed directly via the generated accessor functions. Please, be careful when handling struct references, since invalid references will cause fatal errors.
The most advanced feature of the code-generator are unions. Sometimes we want a reference inside a struct, but it is not always to the same struct. For example in a menu structure, we might have a list of entries that are either submenus or actual items that execute a command.
As you can see the unions feature requires quite a bit more setup. We will start with menu/#/entries/#. It is set to type=struct_ref like you would do for normal struct reference, but the accompanying menu/#/entries uses check/reference/restrict as an array. This tells the reference plugin that any of the given reference restrictions are allowed. Therefore we could be referencing one of several structs and the code-generator has to deal with that somehow.
To allow alternative references, we need to define gen/reference/discriminator/union and gen/reference/discriminator/enum on the key with type=struct_ref. The former of these defines the name of the native C union the code-generator creates:
The other required metakey defines which enum shall be used as a discriminator between the union values:
Each of the possibly referenced structs must have a discriminator key. This key must be part of the struct, it must have type=discriminator and should have check/type=enum. All the discriminator keys must also set gen/enum/type to the same value as chosen for gen/reference/discriminator/enum and all of them have to define the same enum, via the check/enum/# array. The values also have to match the values of the gen/reference/restrict/#/discriminator metakeys on the type=struct_ref key.
The generated structs will then look like this:
As you can see the discriminator field is excluded from the struct itself and stored in a separate array. We do generate getter and free functions for unions, but we don't recommend using them directly. There are no setter functions for unions, because they involve struct references.
1.9.1