All calls to functions that are written in a language other than the current “version 1” interface for compiled languages (this includes functions in user-defined procedural languages and functions written in SQL) go through a call handler function for the specific language. It is the responsibility of the call handler to execute the function in a meaningful way, such as by interpreting the supplied source text. This chapter outlines how a new procedural language's call handler can be written.
    The call handler for a procedural language is a
    “normal” function that must be written in a compiled
    language such as C, using the version-1 interface, and registered
    with PostgreSQL as taking no arguments
    and returning the type language_handler.  This
    special pseudo-type identifies the function as a call handler and
    prevents it from being called directly in SQL commands.
    For more details on C language calling conventions and dynamic loading,
    see Section 36.10.
   
    The call handler is called in the same way as any other function:
    It receives a pointer to a
    FunctionCallInfoBaseData struct containing
    argument values and information about the called function, and it
    is expected to return a Datum result (and possibly
    set the isnull field of the
    FunctionCallInfoBaseData structure, if it wishes
    to return an SQL null result).  The difference between a call
    handler and an ordinary callee function is that the
    flinfo->fn_oid field of the
    FunctionCallInfoBaseData structure will contain
    the OID of the actual function to be called, not of the call
    handler itself.  The call handler must use this field to determine
    which function to execute.  Also, the passed argument list has
    been set up according to the declaration of the target function,
    not of the call handler.
   
    It's up to the call handler to fetch the entry of the function from the
    pg_proc system catalog and to analyze the argument
    and return types of the called function. The AS clause from the
    CREATE FUNCTION command for the function will be found
    in the prosrc column of the
    pg_proc row. This is commonly source
    text in the procedural language, but in theory it could be something else,
    such as a path name to a file, or anything else that tells the call handler
    what to do in detail.
   
    Often, the same function is called many times per SQL statement.
    A call handler can avoid repeated lookups of information about the
    called function by using the
    flinfo->fn_extra field.  This will
    initially be NULL, but can be set by the call handler to point at
    information about the called function.  On subsequent calls, if
    flinfo->fn_extra is already non-NULL
    then it can be used and the information lookup step skipped.  The
    call handler must make sure that
    flinfo->fn_extra is made to point at
    memory that will live at least until the end of the current query,
    since an FmgrInfo data structure could be
    kept that long.  One way to do this is to allocate the extra data
    in the memory context specified by
    flinfo->fn_mcxt; such data will
    normally have the same lifespan as the
    FmgrInfo itself.  But the handler could
    also choose to use a longer-lived memory context so that it can cache
    function definition information across queries.
   
    When a procedural-language function is invoked as a trigger, no arguments
    are passed in the usual way, but the
    FunctionCallInfoBaseData's
    context field points at a
    TriggerData structure, rather than being NULL
    as it is in a plain function call.  A language handler should
    provide mechanisms for procedural-language functions to get at the trigger
    information.
   
    A template for a procedural-language handler written as a C extension is
    provided in src/test/modules/plsample.  This is a
    working sample demonstrating one way to create a procedural-language
    handler, process parameters, and return a value.
   
Although providing a call handler is sufficient to create a minimal procedural language, there are two other functions that can optionally be provided to make the language more convenient to use. These are a validator and an inline handler. A validator can be provided to allow language-specific checking to be done during CREATE FUNCTION. An inline handler can be provided to allow the language to support anonymous code blocks executed via the DO command.
    If a validator is provided by a procedural language, it
    must be declared as a function taking a single parameter of type
    oid.  The validator's result is ignored, so it is customarily
    declared to return void.  The validator will be called at
    the end of a CREATE FUNCTION command that has created
    or updated a function written in the procedural language.
    The passed-in OID is the OID of the function's pg_proc
    row.  The validator must fetch this row in the usual way, and do
    whatever checking is appropriate.
    First, call CheckFunctionValidatorAccess() to diagnose
    explicit calls to the validator that the user could not achieve through
    CREATE FUNCTION.  Typical checks then include verifying
    that the function's argument and result types are supported by the
    language, and that the function's body is syntactically correct
    in the language.  If the validator finds the function to be okay,
    it should just return.  If it finds an error, it should report that
    via the normal ereport() error reporting mechanism.
    Throwing an error will force a transaction rollback and thus prevent
    the incorrect function definition from being committed.
   
    Validator functions should typically honor the check_function_bodies parameter: if it is turned off then
    any expensive or context-sensitive checking should be skipped.  If the
    language provides for code execution at compilation time, the validator
    must suppress checks that would induce such execution.  In particular,
    this parameter is turned off by pg_dump so that it can
    load procedural language functions without worrying about side effects or
    dependencies of the function bodies on other database objects.
    (Because of this requirement, the call handler should avoid
    assuming that the validator has fully checked the function.  The point
    of having a validator is not to let the call handler omit checks, but
    to notify the user immediately if there are obvious errors in a
    CREATE FUNCTION command.)
    While the choice of exactly what to check is mostly left to the
    discretion of the validator function, note that the core
    CREATE FUNCTION code only executes SET clauses
    attached to a function when check_function_bodies is on.
    Therefore, checks whose results might be affected by GUC parameters
    definitely should be skipped when check_function_bodies is
    off, to avoid false failures when restoring a dump.
   
    If an inline handler is provided by a procedural language, it
    must be declared as a function taking a single parameter of type
    internal.  The inline handler's result is ignored, so it is
    customarily declared to return void.  The inline handler
    will be called when a DO statement is executed specifying
    the procedural language.  The parameter actually passed is a pointer
    to an InlineCodeBlock struct, which contains information
    about the DO statement's parameters, in particular the
    text of the anonymous code block to be executed.  The inline handler
    should execute this code and return.
   
    It's recommended that you wrap all these function declarations,
    as well as the CREATE LANGUAGE command itself, into
    an extension so that a simple CREATE EXTENSION
    command is sufficient to install the language.  See
    Section 36.17 for information about writing
    extensions.
   
    The procedural languages included in the standard distribution
    are good references when trying to write your own language handler.
    Look into the src/pl subdirectory of the source tree.
    The CREATE LANGUAGE
    reference page also has some useful details.