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36.10. C-Language Functions #

   36.10.1. Dynamic Loading
   36.10.2. Base Types in C-Language Functions
   36.10.3. Version 1 Calling Conventions
   36.10.4. Writing Code
   36.10.5. Compiling and Linking Dynamically-Loaded Functions
   36.10.6. Server API and ABI Stability Guidance
   36.10.7. Composite-Type Arguments
   36.10.8. Returning Rows (Composite Types)
   36.10.9. Returning Sets
   36.10.10. Polymorphic Arguments and Return Types
   36.10.11. Shared Memory
   36.10.12. LWLocks
   36.10.13. Custom Wait Events
   36.10.14. Injection Points
   36.10.15. Custom Cumulative Statistics
   36.10.16. Using C++ for Extensibility

   User-defined functions can be written in C (or a language that can be
   made compatible with C, such as C++). Such functions are compiled into
   dynamically loadable objects (also called shared libraries) and are
   loaded by the server on demand. The dynamic loading feature is what
   distinguishes “C language” functions from “internal” functions — the
   actual coding conventions are essentially the same for both. (Hence,
   the standard internal function library is a rich source of coding
   examples for user-defined C functions.)

   Currently only one calling convention is used for C functions (“version
   1”). Support for that calling convention is indicated by writing a
   PG_FUNCTION_INFO_V1() macro call for the function, as illustrated
   below.

36.10.1. Dynamic Loading #

   The first time a user-defined function in a particular loadable object
   file is called in a session, the dynamic loader loads that object file
   into memory so that the function can be called. The CREATE FUNCTION for
   a user-defined C function must therefore specify two pieces of
   information for the function: the name of the loadable object file, and
   the C name (link symbol) of the specific function to call within that
   object file. If the C name is not explicitly specified then it is
   assumed to be the same as the SQL function name.

   The following algorithm is used to locate the shared object file based
   on the name given in the CREATE FUNCTION command:
    1. If the name is an absolute path, the given file is loaded.
    2. If the name starts with the string $libdir, that part is replaced
       by the PostgreSQL package library directory name, which is
       determined at build time.
    3. If the name does not contain a directory part, the file is searched
       for in the path specified by the configuration variable
       dynamic_library_path.
    4. Otherwise (the file was not found in the path, or it contains a
       non-absolute directory part), the dynamic loader will try to take
       the name as given, which will most likely fail. (It is unreliable
       to depend on the current working directory.)

   If this sequence does not work, the platform-specific shared library
   file name extension (often .so) is appended to the given name and this
   sequence is tried again. If that fails as well, the load will fail.

   It is recommended to locate shared libraries either relative to $libdir
   or through the dynamic library path. This simplifies version upgrades
   if the new installation is at a different location. The actual
   directory that $libdir stands for can be found out with the command
   pg_config --pkglibdir.

   The user ID the PostgreSQL server runs as must be able to traverse the
   path to the file you intend to load. Making the file or a higher-level
   directory not readable and/or not executable by the postgres user is a
   common mistake.

   In any case, the file name that is given in the CREATE FUNCTION command
   is recorded literally in the system catalogs, so if the file needs to
   be loaded again the same procedure is applied.

Note

   PostgreSQL will not compile a C function automatically. The object file
   must be compiled before it is referenced in a CREATE FUNCTION command.
   See Section 36.10.5 for additional information.

   To ensure that a dynamically loaded object file is not loaded into an
   incompatible server, PostgreSQL checks that the file contains a “magic
   block” with the appropriate contents. This allows the server to detect
   obvious incompatibilities, such as code compiled for a different major
   version of PostgreSQL. To include a magic block, write this in one (and
   only one) of the module source files, after having included the header
   fmgr.h:
PG_MODULE_MAGIC;

   or
PG_MODULE_MAGIC_EXT(parameters);

   The PG_MODULE_MAGIC_EXT variant allows the specification of additional
   information about the module; currently, a name and/or a version string
   can be added. (More fields might be allowed in future.) Write something
   like this:
PG_MODULE_MAGIC_EXT(
    .name = "my_module_name",
    .version = "1.2.3"
);

   Subsequently the name and version can be examined via the
   pg_get_loaded_modules() function. The meaning of the version string is
   not restricted by PostgreSQL, but use of semantic versioning rules is
   recommended.

   After it is used for the first time, a dynamically loaded object file
   is retained in memory. Future calls in the same session to the
   function(s) in that file will only incur the small overhead of a symbol
   table lookup. If you need to force a reload of an object file, for
   example after recompiling it, begin a fresh session.

   Optionally, a dynamically loaded file can contain an initialization
   function. If the file includes a function named _PG_init, that function
   will be called immediately after loading the file. The function
   receives no parameters and should return void. There is presently no
   way to unload a dynamically loaded file.

36.10.2. Base Types in C-Language Functions #

   To know how to write C-language functions, you need to know how
   PostgreSQL internally represents base data types and how they can be
   passed to and from functions. Internally, PostgreSQL regards a base
   type as a “blob of memory”. The user-defined functions that you define
   over a type in turn define the way that PostgreSQL can operate on it.
   That is, PostgreSQL will only store and retrieve the data from disk and
   use your user-defined functions to input, process, and output the data.

   Base types can have one of three internal formats:
     * pass by value, fixed-length
     * pass by reference, fixed-length
     * pass by reference, variable-length

   By-value types can only be 1, 2, or 4 bytes in length (also 8 bytes, if
   sizeof(Datum) is 8 on your machine). You should be careful to define
   your types such that they will be the same size (in bytes) on all
   architectures. For example, the long type is dangerous because it is 4
   bytes on some machines and 8 bytes on others, whereas int type is 4
   bytes on most Unix machines. A reasonable implementation of the int4
   type on Unix machines might be:
/* 4-byte integer, passed by value */
typedef int int4;

   (The actual PostgreSQL C code calls this type int32, because it is a
   convention in C that intXX means XX bits. Note therefore also that the
   C type int8 is 1 byte in size. The SQL type int8 is called int64 in C.
   See also Table 36.2.)

   On the other hand, fixed-length types of any size can be passed
   by-reference. For example, here is a sample implementation of a
   PostgreSQL type:
/* 16-byte structure, passed by reference */
typedef struct
{
    double  x, y;
} Point;

   Only pointers to such types can be used when passing them in and out of
   PostgreSQL functions. To return a value of such a type, allocate the
   right amount of memory with palloc, fill in the allocated memory, and
   return a pointer to it. (Also, if you just want to return the same
   value as one of your input arguments that's of the same data type, you
   can skip the extra palloc and just return the pointer to the input
   value.)

   Finally, all variable-length types must also be passed by reference.
   All variable-length types must begin with an opaque length field of
   exactly 4 bytes, which will be set by SET_VARSIZE; never set this field
   directly! All data to be stored within that type must be located in the
   memory immediately following that length field. The length field
   contains the total length of the structure, that is, it includes the
   size of the length field itself.

   Another important point is to avoid leaving any uninitialized bits
   within data type values; for example, take care to zero out any
   alignment padding bytes that might be present in structs. Without this,
   logically-equivalent constants of your data type might be seen as
   unequal by the planner, leading to inefficient (though not incorrect)
   plans.

Warning

   Never modify the contents of a pass-by-reference input value. If you do
   so you are likely to corrupt on-disk data, since the pointer you are
   given might point directly into a disk buffer. The sole exception to
   this rule is explained in Section 36.12.

   As an example, we can define the type text as follows:
typedef struct {
    int32 length;
    char data[FLEXIBLE_ARRAY_MEMBER];
} text;

   The [FLEXIBLE_ARRAY_MEMBER] notation means that the actual length of
   the data part is not specified by this declaration.

   When manipulating variable-length types, we must be careful to allocate
   the correct amount of memory and set the length field correctly. For
   example, if we wanted to store 40 bytes in a text structure, we might
   use a code fragment like this:
#include "postgres.h"
...
char buffer[40]; /* our source data */
...
text *destination = (text *) palloc(VARHDRSZ + 40);
SET_VARSIZE(destination, VARHDRSZ + 40);
memcpy(destination->data, buffer, 40);
...


   VARHDRSZ is the same as sizeof(int32), but it's considered good style
   to use the macro VARHDRSZ to refer to the size of the overhead for a
   variable-length type. Also, the length field must be set using the
   SET_VARSIZE macro, not by simple assignment.

   Table 36.2 shows the C types corresponding to many of the built-in SQL
   data types of PostgreSQL. The “Defined In” column gives the header file
   that needs to be included to get the type definition. (The actual
   definition might be in a different file that is included by the listed
   file. It is recommended that users stick to the defined interface.)
   Note that you should always include postgres.h first in any source file
   of server code, because it declares a number of things that you will
   need anyway, and because including other headers first can cause
   portability issues.

   Table 36.2. Equivalent C Types for Built-in SQL Types
           SQL Type             C Type                 Defined In
   boolean                   bool          postgres.h (maybe compiler built-in)
   box                       BOX*          utils/geo_decls.h
   bytea                     bytea*        postgres.h
   "char"                    char          (compiler built-in)
   character                 BpChar*       postgres.h
   cid                       CommandId     postgres.h
   date                      DateADT       utils/date.h
   float4 (real)             float4        postgres.h
   float8 (double precision) float8        postgres.h
   int2 (smallint)           int16         postgres.h
   int4 (integer)            int32         postgres.h
   int8 (bigint)             int64         postgres.h
   interval                  Interval*     datatype/timestamp.h
   lseg                      LSEG*         utils/geo_decls.h
   name                      Name          postgres.h
   numeric                   Numeric       utils/numeric.h
   oid                       Oid           postgres.h
   oidvector                 oidvector*    postgres.h
   path                      PATH*         utils/geo_decls.h
   point                     POINT*        utils/geo_decls.h
   regproc                   RegProcedure  postgres.h
   text                      text*         postgres.h
   tid                       ItemPointer   storage/itemptr.h
   time                      TimeADT       utils/date.h
   time with time zone       TimeTzADT     utils/date.h
   timestamp                 Timestamp     datatype/timestamp.h
   timestamp with time zone  TimestampTz   datatype/timestamp.h
   varchar                   VarChar*      postgres.h
   xid                       TransactionId postgres.h

   Now that we've gone over all of the possible structures for base types,
   we can show some examples of real functions.

36.10.3. Version 1 Calling Conventions #

   The version-1 calling convention relies on macros to suppress most of
   the complexity of passing arguments and results. The C declaration of a
   version-1 function is always:
Datum funcname(PG_FUNCTION_ARGS)

   In addition, the macro call:
PG_FUNCTION_INFO_V1(funcname);

   must appear in the same source file. (Conventionally, it's written just
   before the function itself.) This macro call is not needed for
   internal-language functions, since PostgreSQL assumes that all internal
   functions use the version-1 convention. It is, however, required for
   dynamically-loaded functions.

   In a version-1 function, each actual argument is fetched using a
   PG_GETARG_xxx() macro that corresponds to the argument's data type. (In
   non-strict functions there needs to be a previous check about argument
   null-ness using PG_ARGISNULL(); see below.) The result is returned
   using a PG_RETURN_xxx() macro for the return type. PG_GETARG_xxx()
   takes as its argument the number of the function argument to fetch,
   where the count starts at 0. PG_RETURN_xxx() takes as its argument the
   actual value to return.

   To call another version-1 function, you can use
   DirectFunctionCalln(func, arg1, ..., argn). This is particularly useful
   when you want to call functions defined in the standard internal
   library, by using an interface similar to their SQL signature.

   These convenience functions and similar ones can be found in fmgr.h.
   The DirectFunctionCalln family expect a C function name as their first
   argument. There are also OidFunctionCalln which take the OID of the
   target function, and some other variants. All of these expect the
   function's arguments to be supplied as Datums, and likewise they return
   Datum. Note that neither arguments nor result are allowed to be NULL
   when using these convenience functions.

   For example, to call the starts_with(text, text) function from C, you
   can search through the catalog and find out that its C implementation
   is the Datum text_starts_with(PG_FUNCTION_ARGS) function. Typically you
   would use DirectFunctionCall2(text_starts_with, ...) to call such a
   function. However, starts_with(text, text) requires collation
   information, so it will fail with “could not determine which collation
   to use for string comparison” if called that way. Instead you must use
   DirectFunctionCall2Coll(text_starts_with, ...) and provide the desired
   collation, which typically is just passed through from
   PG_GET_COLLATION(), as shown in the example below.

   fmgr.h also supplies macros that facilitate conversions between C types
   and Datum. For example to turn Datum into text*, you can use
   DatumGetTextPP(X). While some types have macros named like
   TypeGetDatum(X) for the reverse conversion, text* does not; it's
   sufficient to use the generic macro PointerGetDatum(X) for that. If
   your extension defines additional types, it is usually convenient to
   define similar macros for your types too.

   Here are some examples using the version-1 calling convention:
#include "postgres.h"
#include <string.h>
#include "fmgr.h"
#include "utils/geo_decls.h"
#include "varatt.h"

PG_MODULE_MAGIC;

/* by value */

PG_FUNCTION_INFO_V1(add_one);

Datum
add_one(PG_FUNCTION_ARGS)
{
    int32   arg = PG_GETARG_INT32(0);

    PG_RETURN_INT32(arg + 1);
}

/* by reference, fixed length */

PG_FUNCTION_INFO_V1(add_one_float8);

Datum
add_one_float8(PG_FUNCTION_ARGS)
{
    /* The macros for FLOAT8 hide its pass-by-reference nature. */
    float8   arg = PG_GETARG_FLOAT8(0);

    PG_RETURN_FLOAT8(arg + 1.0);
}

PG_FUNCTION_INFO_V1(makepoint);

Datum
makepoint(PG_FUNCTION_ARGS)
{
    /* Here, the pass-by-reference nature of Point is not hidden. */
    Point     *pointx = PG_GETARG_POINT_P(0);
    Point     *pointy = PG_GETARG_POINT_P(1);
    Point     *new_point = (Point *) palloc(sizeof(Point));

    new_point->x = pointx->x;
    new_point->y = pointy->y;

    PG_RETURN_POINT_P(new_point);
}

/* by reference, variable length */

PG_FUNCTION_INFO_V1(copytext);

Datum
copytext(PG_FUNCTION_ARGS)
{
    text     *t = PG_GETARG_TEXT_PP(0);

    /*
     * VARSIZE_ANY_EXHDR is the size of the struct in bytes, minus the
     * VARHDRSZ or VARHDRSZ_SHORT of its header.  Construct the copy with a
     * full-length header.
     */
    text     *new_t = (text *) palloc(VARSIZE_ANY_EXHDR(t) + VARHDRSZ);
    SET_VARSIZE(new_t, VARSIZE_ANY_EXHDR(t) + VARHDRSZ);

    /*
     * VARDATA is a pointer to the data region of the new struct.  The source
     * could be a short datum, so retrieve its data through VARDATA_ANY.
     */
    memcpy(VARDATA(new_t),          /* destination */
           VARDATA_ANY(t),          /* source */
           VARSIZE_ANY_EXHDR(t));   /* how many bytes */
    PG_RETURN_TEXT_P(new_t);
}

PG_FUNCTION_INFO_V1(concat_text);

Datum
concat_text(PG_FUNCTION_ARGS)
{
    text  *arg1 = PG_GETARG_TEXT_PP(0);
    text  *arg2 = PG_GETARG_TEXT_PP(1);
    int32 arg1_size = VARSIZE_ANY_EXHDR(arg1);
    int32 arg2_size = VARSIZE_ANY_EXHDR(arg2);
    int32 new_text_size = arg1_size + arg2_size + VARHDRSZ;
    text *new_text = (text *) palloc(new_text_size);

    SET_VARSIZE(new_text, new_text_size);
    memcpy(VARDATA(new_text), VARDATA_ANY(arg1), arg1_size);
    memcpy(VARDATA(new_text) + arg1_size, VARDATA_ANY(arg2), arg2_size);
    PG_RETURN_TEXT_P(new_text);
}

/* A wrapper around starts_with(text, text) */

PG_FUNCTION_INFO_V1(t_starts_with);

Datum
t_starts_with(PG_FUNCTION_ARGS)
{
    text       *t1 = PG_GETARG_TEXT_PP(0);
    text       *t2 = PG_GETARG_TEXT_PP(1);
    Oid         collid = PG_GET_COLLATION();
    bool        result;

    result = DatumGetBool(DirectFunctionCall2Coll(text_starts_with,
                                                  collid,
                                                  PointerGetDatum(t1),
                                                  PointerGetDatum(t2)));
    PG_RETURN_BOOL(result);
}


   Supposing that the above code has been prepared in file funcs.c and
   compiled into a shared object, we could define the functions to
   PostgreSQL with commands like this:
CREATE FUNCTION add_one(integer) RETURNS integer
     AS 'DIRECTORY/funcs', 'add_one'
     LANGUAGE C STRICT;

-- note overloading of SQL function name "add_one"
CREATE FUNCTION add_one(double precision) RETURNS double precision
     AS 'DIRECTORY/funcs', 'add_one_float8'
     LANGUAGE C STRICT;

CREATE FUNCTION makepoint(point, point) RETURNS point
     AS 'DIRECTORY/funcs', 'makepoint'
     LANGUAGE C STRICT;

CREATE FUNCTION copytext(text) RETURNS text
     AS 'DIRECTORY/funcs', 'copytext'
     LANGUAGE C STRICT;

CREATE FUNCTION concat_text(text, text) RETURNS text
     AS 'DIRECTORY/funcs', 'concat_text'
     LANGUAGE C STRICT;

CREATE FUNCTION t_starts_with(text, text) RETURNS boolean
     AS 'DIRECTORY/funcs', 't_starts_with'
     LANGUAGE C STRICT;

   Here, DIRECTORY stands for the directory of the shared library file
   (for instance the PostgreSQL tutorial directory, which contains the
   code for the examples used in this section). (Better style would be to
   use just 'funcs' in the AS clause, after having added DIRECTORY to the
   search path. In any case, we can omit the system-specific extension for
   a shared library, commonly .so.)

   Notice that we have specified the functions as “strict”, meaning that
   the system should automatically assume a null result if any input value
   is null. By doing this, we avoid having to check for null inputs in the
   function code. Without this, we'd have to check for null values
   explicitly, using PG_ARGISNULL().

   The macro PG_ARGISNULL(n) allows a function to test whether each input
   is null. (Of course, doing this is only necessary in functions not
   declared “strict”.) As with the PG_GETARG_xxx() macros, the input
   arguments are counted beginning at zero. Note that one should refrain
   from executing PG_GETARG_xxx() until one has verified that the argument
   isn't null. To return a null result, execute PG_RETURN_NULL(); this
   works in both strict and nonstrict functions.

   At first glance, the version-1 coding conventions might appear to be
   just pointless obscurantism, compared to using plain C calling
   conventions. They do however allow us to deal with NULLable
   arguments/return values, and “toasted” (compressed or out-of-line)
   values.

   Other options provided by the version-1 interface are two variants of
   the PG_GETARG_xxx() macros. The first of these, PG_GETARG_xxx_COPY(),
   guarantees to return a copy of the specified argument that is safe for
   writing into. (The normal macros will sometimes return a pointer to a
   value that is physically stored in a table, which must not be written
   to. Using the PG_GETARG_xxx_COPY() macros guarantees a writable
   result.) The second variant consists of the PG_GETARG_xxx_SLICE()
   macros which take three arguments. The first is the number of the
   function argument (as above). The second and third are the offset and
   length of the segment to be returned. Offsets are counted from zero,
   and a negative length requests that the remainder of the value be
   returned. These macros provide more efficient access to parts of large
   values in the case where they have storage type “external”. (The
   storage type of a column can be specified using ALTER TABLE tablename
   ALTER COLUMN colname SET STORAGE storagetype. storagetype is one of
   plain, external, extended, or main.)

   Finally, the version-1 function call conventions make it possible to
   return set results (Section 36.10.9) and implement trigger functions
   (Chapter 37) and procedural-language call handlers (Chapter 57). For
   more details see src/backend/utils/fmgr/README in the source
   distribution.

36.10.4. Writing Code #

   Before we turn to the more advanced topics, we should discuss some
   coding rules for PostgreSQL C-language functions. While it might be
   possible to load functions written in languages other than C into
   PostgreSQL, this is usually difficult (when it is possible at all)
   because other languages, such as C++, FORTRAN, or Pascal often do not
   follow the same calling convention as C. That is, other languages do
   not pass argument and return values between functions in the same way.
   For this reason, we will assume that your C-language functions are
   actually written in C.

   The basic rules for writing and building C functions are as follows:
     * Use pg_config --includedir-server to find out where the PostgreSQL
       server header files are installed on your system (or the system
       that your users will be running on).
     * Compiling and linking your code so that it can be dynamically
       loaded into PostgreSQL always requires special flags. See
       Section 36.10.5 for a detailed explanation of how to do it for your
       particular operating system.
     * Remember to define a “magic block” for your shared library, as
       described in Section 36.10.1.
     * When allocating memory, use the PostgreSQL functions palloc and
       pfree instead of the corresponding C library functions malloc and
       free. The memory allocated by palloc will be freed automatically at
       the end of each transaction, preventing memory leaks.
     * Always zero the bytes of your structures using memset (or allocate
       them with palloc0 in the first place). Even if you assign to each
       field of your structure, there might be alignment padding (holes in
       the structure) that contain garbage values. Without this, it's
       difficult to support hash indexes or hash joins, as you must pick
       out only the significant bits of your data structure to compute a
       hash. The planner also sometimes relies on comparing constants via
       bitwise equality, so you can get undesirable planning results if
       logically-equivalent values aren't bitwise equal.
     * Most of the internal PostgreSQL types are declared in postgres.h,
       while the function manager interfaces (PG_FUNCTION_ARGS, etc.) are
       in fmgr.h, so you will need to include at least these two files.
       For portability reasons it's best to include postgres.h first,
       before any other system or user header files. Including postgres.h
       will also include elog.h and palloc.h for you.
     * Symbol names defined within object files must not conflict with
       each other or with symbols defined in the PostgreSQL server
       executable. You will have to rename your functions or variables if
       you get error messages to this effect.

36.10.5. Compiling and Linking Dynamically-Loaded Functions #

   Before you are able to use your PostgreSQL extension functions written
   in C, they must be compiled and linked in a special way to produce a
   file that can be dynamically loaded by the server. To be precise, a
   shared library needs to be created.

   For information beyond what is contained in this section you should
   read the documentation of your operating system, in particular the
   manual pages for the C compiler, cc, and the link editor, ld. In
   addition, the PostgreSQL source code contains several working examples
   in the contrib directory. If you rely on these examples you will make
   your modules dependent on the availability of the PostgreSQL source
   code, however.

   Creating shared libraries is generally analogous to linking
   executables: first the source files are compiled into object files,
   then the object files are linked together. The object files need to be
   created as position-independent code (PIC), which conceptually means
   that they can be placed at an arbitrary location in memory when they
   are loaded by the executable. (Object files intended for executables
   are usually not compiled that way.) The command to link a shared
   library contains special flags to distinguish it from linking an
   executable (at least in theory — on some systems the practice is much
   uglier).

   In the following examples we assume that your source code is in a file
   foo.c and we will create a shared library foo.so. The intermediate
   object file will be called foo.o unless otherwise noted. A shared
   library can contain more than one object file, but we only use one
   here.

   FreeBSD
          The compiler flag to create PIC is -fPIC. To create shared
          libraries the compiler flag is -shared.

cc -fPIC -c foo.c
cc -shared -o foo.so foo.o

          This is applicable as of version 13.0 of FreeBSD, older versions
          used the gcc compiler.

   Linux
          The compiler flag to create PIC is -fPIC. The compiler flag to
          create a shared library is -shared. A complete example looks
          like this:

cc -fPIC -c foo.c
cc -shared -o foo.so foo.o

   macOS
          Here is an example. It assumes the developer tools are
          installed.

cc -c foo.c
cc -bundle -flat_namespace -undefined suppress -o foo.so foo.o

   NetBSD
          The compiler flag to create PIC is -fPIC. For ELF systems, the
          compiler with the flag -shared is used to link shared libraries.
          On the older non-ELF systems, ld -Bshareable is used.

gcc -fPIC -c foo.c
gcc -shared -o foo.so foo.o

   OpenBSD
          The compiler flag to create PIC is -fPIC. ld -Bshareable is used
          to link shared libraries.

gcc -fPIC -c foo.c
ld -Bshareable -o foo.so foo.o

   Solaris
          The compiler flag to create PIC is -KPIC with the Sun compiler
          and -fPIC with GCC. To link shared libraries, the compiler
          option is -G with either compiler or alternatively -shared with
          GCC.

cc -KPIC -c foo.c
cc -G -o foo.so foo.o

          or

gcc -fPIC -c foo.c
gcc -G -o foo.so foo.o

Tip

   If this is too complicated for you, you should consider using GNU
   Libtool, which hides the platform differences behind a uniform
   interface.

   The resulting shared library file can then be loaded into PostgreSQL.
   When specifying the file name to the CREATE FUNCTION command, one must
   give it the name of the shared library file, not the intermediate
   object file. Note that the system's standard shared-library extension
   (usually .so or .sl) can be omitted from the CREATE FUNCTION command,
   and normally should be omitted for best portability.

   Refer back to Section 36.10.1 about where the server expects to find
   the shared library files.

36.10.6. Server API and ABI Stability Guidance #

   This section contains guidance to authors of extensions and other
   server plugins about API and ABI stability in the PostgreSQL server.

36.10.6.1. General #

   The PostgreSQL server contains several well-demarcated APIs for server
   plugins, such as the function manager (fmgr, described in this
   chapter), SPI (Chapter 45), and various hooks specifically designed for
   extensions. These interfaces are carefully managed for long-term
   stability and compatibility. However, the entire set of global
   functions and variables in the server effectively constitutes the
   publicly usable API, and most of it was not designed with extensibility
   and long-term stability in mind.

   Therefore, while taking advantage of these interfaces is valid, the
   further one strays from the well-trodden path, the likelier it will be
   that one might encounter API or ABI compatibility issues at some point.
   Extension authors are encouraged to provide feedback about their
   requirements, so that over time, as new use patterns arise, certain
   interfaces can be considered more stabilized or new, better-designed
   interfaces can be added.

36.10.6.2. API Compatibility #

   The API, or application programming interface, is the interface used at
   compile time.

36.10.6.2.1. Major Versions #

   There is no promise of API compatibility between PostgreSQL major
   versions. Extension code therefore might require source code changes to
   work with multiple major versions. These can usually be managed with
   preprocessor conditions such as #if PG_VERSION_NUM >= 160000.
   Sophisticated extensions that use interfaces beyond the well-demarcated
   ones usually require a few such changes for each major server version.

36.10.6.2.2. Minor Versions #

   PostgreSQL makes an effort to avoid server API breaks in minor
   releases. In general, extension code that compiles and works with a
   minor release should also compile and work with any other minor release
   of the same major version, past or future.

   When a change is required, it will be carefully managed, taking the
   requirements of extensions into account. Such changes will be
   communicated in the release notes (Appendix E).

36.10.6.3. ABI Compatibility #

   The ABI, or application binary interface, is the interface used at run
   time.

36.10.6.3.1. Major Versions #

   Servers of different major versions have intentionally incompatible
   ABIs. Extensions that use server APIs must therefore be re-compiled for
   each major release. The inclusion of PG_MODULE_MAGIC (see
   Section 36.10.1) ensures that code compiled for one major version will
   be rejected by other major versions.

36.10.6.3.2. Minor Versions #

   PostgreSQL makes an effort to avoid server ABI breaks in minor
   releases. In general, an extension compiled against any minor release
   should work with any other minor release of the same major version,
   past or future.

   When a change is required, PostgreSQL will choose the least invasive
   change possible, for example by squeezing a new field into padding
   space or appending it to the end of a struct. These sorts of changes
   should not impact extensions unless they use very unusual code
   patterns.

   In rare cases, however, even such non-invasive changes may be
   impractical or impossible. In such an event, the change will be
   carefully managed, taking the requirements of extensions into account.
   Such changes will also be documented in the release notes (Appendix E).

   Note, however, that many parts of the server are not designed or
   maintained as publicly-consumable APIs (and that, in most cases, the
   actual boundary is also not well-defined). If urgent needs arise,
   changes in those parts will naturally be made with less consideration
   for extension code than changes in well-defined and widely used
   interfaces.

   Also, in the absence of automated detection of such changes, this is
   not a guarantee, but historically such breaking changes have been
   extremely rare.

36.10.7. Composite-Type Arguments #

   Composite types do not have a fixed layout like C structures. Instances
   of a composite type can contain null fields. In addition, composite
   types that are part of an inheritance hierarchy can have different
   fields than other members of the same inheritance hierarchy. Therefore,
   PostgreSQL provides a function interface for accessing fields of
   composite types from C.

   Suppose we want to write a function to answer the query:
SELECT name, c_overpaid(emp, 1500) AS overpaid
    FROM emp
    WHERE name = 'Bill' OR name = 'Sam';

   Using the version-1 calling conventions, we can define c_overpaid as:
#include "postgres.h"
#include "executor/executor.h"  /* for GetAttributeByName() */

PG_MODULE_MAGIC;

PG_FUNCTION_INFO_V1(c_overpaid);

Datum
c_overpaid(PG_FUNCTION_ARGS)
{
    HeapTupleHeader  t = PG_GETARG_HEAPTUPLEHEADER(0);
    int32            limit = PG_GETARG_INT32(1);
    bool isnull;
    Datum salary;

    salary = GetAttributeByName(t, "salary", &isnull);
    if (isnull)
        PG_RETURN_BOOL(false);
    /* Alternatively, we might prefer to do PG_RETURN_NULL() for null salary. */

    PG_RETURN_BOOL(DatumGetInt32(salary) > limit);
}


   GetAttributeByName is the PostgreSQL system function that returns
   attributes out of the specified row. It has three arguments: the
   argument of type HeapTupleHeader passed into the function, the name of
   the desired attribute, and a return parameter that tells whether the
   attribute is null. GetAttributeByName returns a Datum value that you
   can convert to the proper data type by using the appropriate
   DatumGetXXX() function. Note that the return value is meaningless if
   the null flag is set; always check the null flag before trying to do
   anything with the result.

   There is also GetAttributeByNum, which selects the target attribute by
   column number instead of name.

   The following command declares the function c_overpaid in SQL:
CREATE FUNCTION c_overpaid(emp, integer) RETURNS boolean
    AS 'DIRECTORY/funcs', 'c_overpaid'
    LANGUAGE C STRICT;

   Notice we have used STRICT so that we did not have to check whether the
   input arguments were NULL.

36.10.8. Returning Rows (Composite Types) #

   To return a row or composite-type value from a C-language function, you
   can use a special API that provides macros and functions to hide most
   of the complexity of building composite data types. To use this API,
   the source file must include:
#include "funcapi.h"

   There are two ways you can build a composite data value (henceforth a
   “tuple”): you can build it from an array of Datum values, or from an
   array of C strings that can be passed to the input conversion functions
   of the tuple's column data types. In either case, you first need to
   obtain or construct a TupleDesc descriptor for the tuple structure.
   When working with Datums, you pass the TupleDesc to BlessTupleDesc, and
   then call heap_form_tuple for each row. When working with C strings,
   you pass the TupleDesc to TupleDescGetAttInMetadata, and then call
   BuildTupleFromCStrings for each row. In the case of a function
   returning a set of tuples, the setup steps can all be done once during
   the first call of the function.

   Several helper functions are available for setting up the needed
   TupleDesc. The recommended way to do this in most functions returning
   composite values is to call:
TypeFuncClass get_call_result_type(FunctionCallInfo fcinfo,
                                   Oid *resultTypeId,
                                   TupleDesc *resultTupleDesc)

   passing the same fcinfo struct passed to the calling function itself.
   (This of course requires that you use the version-1 calling
   conventions.) resultTypeId can be specified as NULL or as the address
   of a local variable to receive the function's result type OID.
   resultTupleDesc should be the address of a local TupleDesc variable.
   Check that the result is TYPEFUNC_COMPOSITE; if so, resultTupleDesc has
   been filled with the needed TupleDesc. (If it is not, you can report an
   error along the lines of “function returning record called in context
   that cannot accept type record”.)

Tip

   get_call_result_type can resolve the actual type of a polymorphic
   function result; so it is useful in functions that return scalar
   polymorphic results, not only functions that return composites. The
   resultTypeId output is primarily useful for functions returning
   polymorphic scalars.

Note

   get_call_result_type has a sibling get_expr_result_type, which can be
   used to resolve the expected output type for a function call
   represented by an expression tree. This can be used when trying to
   determine the result type from outside the function itself. There is
   also get_func_result_type, which can be used when only the function's
   OID is available. However these functions are not able to deal with
   functions declared to return record, and get_func_result_type cannot
   resolve polymorphic types, so you should preferentially use
   get_call_result_type.

   Older, now-deprecated functions for obtaining TupleDescs are:
TupleDesc RelationNameGetTupleDesc(const char *relname)

   to get a TupleDesc for the row type of a named relation, and:
TupleDesc TypeGetTupleDesc(Oid typeoid, List *colaliases)

   to get a TupleDesc based on a type OID. This can be used to get a
   TupleDesc for a base or composite type. It will not work for a function
   that returns record, however, and it cannot resolve polymorphic types.

   Once you have a TupleDesc, call:
TupleDesc BlessTupleDesc(TupleDesc tupdesc)

   if you plan to work with Datums, or:
AttInMetadata *TupleDescGetAttInMetadata(TupleDesc tupdesc)

   if you plan to work with C strings. If you are writing a function
   returning set, you can save the results of these functions in the
   FuncCallContext structure — use the tuple_desc or attinmeta field
   respectively.

   When working with Datums, use:
HeapTuple heap_form_tuple(TupleDesc tupdesc, Datum *values, bool *isnull)

   to build a HeapTuple given user data in Datum form.

   When working with C strings, use:
HeapTuple BuildTupleFromCStrings(AttInMetadata *attinmeta, char **values)

   to build a HeapTuple given user data in C string form. values is an
   array of C strings, one for each attribute of the return row. Each C
   string should be in the form expected by the input function of the
   attribute data type. In order to return a null value for one of the
   attributes, the corresponding pointer in the values array should be set
   to NULL. This function will need to be called again for each row you
   return.

   Once you have built a tuple to return from your function, it must be
   converted into a Datum. Use:
HeapTupleGetDatum(HeapTuple tuple)

   to convert a HeapTuple into a valid Datum. This Datum can be returned
   directly if you intend to return just a single row, or it can be used
   as the current return value in a set-returning function.

   An example appears in the next section.

36.10.9. Returning Sets #

   C-language functions have two options for returning sets (multiple
   rows). In one method, called ValuePerCall mode, a set-returning
   function is called repeatedly (passing the same arguments each time)
   and it returns one new row on each call, until it has no more rows to
   return and signals that by returning NULL. The set-returning function
   (SRF) must therefore save enough state across calls to remember what it
   was doing and return the correct next item on each call. In the other
   method, called Materialize mode, an SRF fills and returns a tuplestore
   object containing its entire result; then only one call occurs for the
   whole result, and no inter-call state is needed.

   When using ValuePerCall mode, it is important to remember that the
   query is not guaranteed to be run to completion; that is, due to
   options such as LIMIT, the executor might stop making calls to the
   set-returning function before all rows have been fetched. This means it
   is not safe to perform cleanup activities in the last call, because
   that might not ever happen. It's recommended to use Materialize mode
   for functions that need access to external resources, such as file
   descriptors.

   The remainder of this section documents a set of helper macros that are
   commonly used (though not required to be used) for SRFs using
   ValuePerCall mode. Additional details about Materialize mode can be
   found in src/backend/utils/fmgr/README. Also, the contrib modules in
   the PostgreSQL source distribution contain many examples of SRFs using
   both ValuePerCall and Materialize mode.

   To use the ValuePerCall support macros described here, include
   funcapi.h. These macros work with a structure FuncCallContext that
   contains the state that needs to be saved across calls. Within the
   calling SRF, fcinfo->flinfo->fn_extra is used to hold a pointer to
   FuncCallContext across calls. The macros automatically fill that field
   on first use, and expect to find the same pointer there on subsequent
   uses.
typedef struct FuncCallContext
{
    /*
     * Number of times we've been called before
     *
     * call_cntr is initialized to 0 for you by SRF_FIRSTCALL_INIT(), and
     * incremented for you every time SRF_RETURN_NEXT() is called.
     */
    uint64 call_cntr;

    /*
     * OPTIONAL maximum number of calls
     *
     * max_calls is here for convenience only and setting it is optional.
     * If not set, you must provide alternative means to know when the
     * function is done.
     */
    uint64 max_calls;

    /*
     * OPTIONAL pointer to miscellaneous user-provided context information
     *
     * user_fctx is for use as a pointer to your own data to retain
     * arbitrary context information between calls of your function.
     */
    void *user_fctx;

    /*
     * OPTIONAL pointer to struct containing attribute type input metadata
     *
     * attinmeta is for use when returning tuples (i.e., composite data types)
     * and is not used when returning base data types. It is only needed
     * if you intend to use BuildTupleFromCStrings() to create the return
     * tuple.
     */
    AttInMetadata *attinmeta;

    /*
     * memory context used for structures that must live for multiple calls
     *
     * multi_call_memory_ctx is set by SRF_FIRSTCALL_INIT() for you, and used
     * by SRF_RETURN_DONE() for cleanup. It is the most appropriate memory
     * context for any memory that is to be reused across multiple calls
     * of the SRF.
     */
    MemoryContext multi_call_memory_ctx;

    /*
     * OPTIONAL pointer to struct containing tuple description
     *
     * tuple_desc is for use when returning tuples (i.e., composite data types)
     * and is only needed if you are going to build the tuples with
     * heap_form_tuple() rather than with BuildTupleFromCStrings().  Note that
     * the TupleDesc pointer stored here should usually have been run through
     * BlessTupleDesc() first.
     */
    TupleDesc tuple_desc;

} FuncCallContext;

   The macros to be used by an SRF using this infrastructure are:
SRF_IS_FIRSTCALL()

   Use this to determine if your function is being called for the first or
   a subsequent time. On the first call (only), call:
SRF_FIRSTCALL_INIT()

   to initialize the FuncCallContext. On every function call, including
   the first, call:
SRF_PERCALL_SETUP()

   to set up for using the FuncCallContext.

   If your function has data to return in the current call, use:
SRF_RETURN_NEXT(funcctx, result)

   to return it to the caller. (result must be of type Datum, either a
   single value or a tuple prepared as described above.) Finally, when
   your function is finished returning data, use:
SRF_RETURN_DONE(funcctx)

   to clean up and end the SRF.

   The memory context that is current when the SRF is called is a
   transient context that will be cleared between calls. This means that
   you do not need to call pfree on everything you allocated using palloc;
   it will go away anyway. However, if you want to allocate any data
   structures to live across calls, you need to put them somewhere else.
   The memory context referenced by multi_call_memory_ctx is a suitable
   location for any data that needs to survive until the SRF is finished
   running. In most cases, this means that you should switch into
   multi_call_memory_ctx while doing the first-call setup. Use
   funcctx->user_fctx to hold a pointer to any such cross-call data
   structures. (Data you allocate in multi_call_memory_ctx will go away
   automatically when the query ends, so it is not necessary to free that
   data manually, either.)

Warning

   While the actual arguments to the function remain unchanged between
   calls, if you detoast the argument values (which is normally done
   transparently by the PG_GETARG_xxx macro) in the transient context then
   the detoasted copies will be freed on each cycle. Accordingly, if you
   keep references to such values in your user_fctx, you must either copy
   them into the multi_call_memory_ctx after detoasting, or ensure that
   you detoast the values only in that context.

   A complete pseudo-code example looks like the following:
Datum
my_set_returning_function(PG_FUNCTION_ARGS)
{
    FuncCallContext  *funcctx;
    Datum             result;
    further declarations as needed

    if (SRF_IS_FIRSTCALL())
    {
        MemoryContext oldcontext;

        funcctx = SRF_FIRSTCALL_INIT();
        oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
        /* One-time setup code appears here: */
        user code
        if returning composite
            build TupleDesc, and perhaps AttInMetadata
        endif returning composite
        user code
        MemoryContextSwitchTo(oldcontext);
    }

    /* Each-time setup code appears here: */
    user code
    funcctx = SRF_PERCALL_SETUP();
    user code

    /* this is just one way we might test whether we are done: */
    if (funcctx->call_cntr < funcctx->max_calls)
    {
        /* Here we want to return another item: */
        user code
        obtain result Datum
        SRF_RETURN_NEXT(funcctx, result);
    }
    else
    {
        /* Here we are done returning items, so just report that fact. */
        /* (Resist the temptation to put cleanup code here.) */
        SRF_RETURN_DONE(funcctx);
    }
}

   A complete example of a simple SRF returning a composite type looks
   like:
PG_FUNCTION_INFO_V1(retcomposite);

Datum
retcomposite(PG_FUNCTION_ARGS)
{
    FuncCallContext     *funcctx;
    int                  call_cntr;
    int                  max_calls;
    TupleDesc            tupdesc;
    AttInMetadata       *attinmeta;

    /* stuff done only on the first call of the function */
    if (SRF_IS_FIRSTCALL())
    {
        MemoryContext   oldcontext;

        /* create a function context for cross-call persistence */
        funcctx = SRF_FIRSTCALL_INIT();

        /* switch to memory context appropriate for multiple function calls */
        oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);

        /* total number of tuples to be returned */
        funcctx->max_calls = PG_GETARG_INT32(0);

        /* Build a tuple descriptor for our result type */
        if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("function returning record called in context "
                            "that cannot accept type record")));

        /*
         * generate attribute metadata needed later to produce tuples from raw
         * C strings
         */
        attinmeta = TupleDescGetAttInMetadata(tupdesc);
        funcctx->attinmeta = attinmeta;

        MemoryContextSwitchTo(oldcontext);
    }

    /* stuff done on every call of the function */
    funcctx = SRF_PERCALL_SETUP();

    call_cntr = funcctx->call_cntr;
    max_calls = funcctx->max_calls;
    attinmeta = funcctx->attinmeta;

    if (call_cntr < max_calls)    /* do when there is more left to send */
    {
        char       **values;
        HeapTuple    tuple;
        Datum        result;

        /*
         * Prepare a values array for building the returned tuple.
         * This should be an array of C strings which will
         * be processed later by the type input functions.
         */
        values = (char **) palloc(3 * sizeof(char *));
        values[0] = (char *) palloc(16 * sizeof(char));
        values[1] = (char *) palloc(16 * sizeof(char));
        values[2] = (char *) palloc(16 * sizeof(char));

        snprintf(values[0], 16, "%d", 1 * PG_GETARG_INT32(1));
        snprintf(values[1], 16, "%d", 2 * PG_GETARG_INT32(1));
        snprintf(values[2], 16, "%d", 3 * PG_GETARG_INT32(1));

        /* build a tuple */
        tuple = BuildTupleFromCStrings(attinmeta, values);

        /* make the tuple into a datum */
        result = HeapTupleGetDatum(tuple);

        /* clean up (this is not really necessary) */
        pfree(values[0]);
        pfree(values[1]);
        pfree(values[2]);
        pfree(values);

        SRF_RETURN_NEXT(funcctx, result);
    }
    else    /* do when there is no more left */
    {
        SRF_RETURN_DONE(funcctx);
    }
}


   One way to declare this function in SQL is:
CREATE TYPE __retcomposite AS (f1 integer, f2 integer, f3 integer);

CREATE OR REPLACE FUNCTION retcomposite(integer, integer)
    RETURNS SETOF __retcomposite
    AS 'filename', 'retcomposite'
    LANGUAGE C IMMUTABLE STRICT;

   A different way is to use OUT parameters:
CREATE OR REPLACE FUNCTION retcomposite(IN integer, IN integer,
    OUT f1 integer, OUT f2 integer, OUT f3 integer)
    RETURNS SETOF record
    AS 'filename', 'retcomposite'
    LANGUAGE C IMMUTABLE STRICT;

   Notice that in this method the output type of the function is formally
   an anonymous record type.

36.10.10. Polymorphic Arguments and Return Types #

   C-language functions can be declared to accept and return the
   polymorphic types described in Section 36.2.5. When a function's
   arguments or return types are defined as polymorphic types, the
   function author cannot know in advance what data type it will be called
   with, or need to return. There are two routines provided in fmgr.h to
   allow a version-1 C function to discover the actual data types of its
   arguments and the type it is expected to return. The routines are
   called get_fn_expr_rettype(FmgrInfo *flinfo) and
   get_fn_expr_argtype(FmgrInfo *flinfo, int argnum). They return the
   result or argument type OID, or InvalidOid if the information is not
   available. The structure flinfo is normally accessed as fcinfo->flinfo.
   The parameter argnum is zero based. get_call_result_type can also be
   used as an alternative to get_fn_expr_rettype. There is also
   get_fn_expr_variadic, which can be used to find out whether variadic
   arguments have been merged into an array. This is primarily useful for
   VARIADIC "any" functions, since such merging will always have occurred
   for variadic functions taking ordinary array types.

   For example, suppose we want to write a function to accept a single
   element of any type, and return a one-dimensional array of that type:
PG_FUNCTION_INFO_V1(make_array);
Datum
make_array(PG_FUNCTION_ARGS)
{
    ArrayType  *result;
    Oid         element_type = get_fn_expr_argtype(fcinfo->flinfo, 0);
    Datum       element;
    bool        isnull;
    int16       typlen;
    bool        typbyval;
    char        typalign;
    int         ndims;
    int         dims[MAXDIM];
    int         lbs[MAXDIM];

    if (!OidIsValid(element_type))
        elog(ERROR, "could not determine data type of input");

    /* get the provided element, being careful in case it's NULL */
    isnull = PG_ARGISNULL(0);
    if (isnull)
        element = (Datum) 0;
    else
        element = PG_GETARG_DATUM(0);

    /* we have one dimension */
    ndims = 1;
    /* and one element */
    dims[0] = 1;
    /* and lower bound is 1 */
    lbs[0] = 1;

    /* get required info about the element type */
    get_typlenbyvalalign(element_type, &typlen, &typbyval, &typalign);

    /* now build the array */
    result = construct_md_array(&element, &isnull, ndims, dims, lbs,
                                element_type, typlen, typbyval, typalign);

    PG_RETURN_ARRAYTYPE_P(result);
}

   The following command declares the function make_array in SQL:
CREATE FUNCTION make_array(anyelement) RETURNS anyarray
    AS 'DIRECTORY/funcs', 'make_array'
    LANGUAGE C IMMUTABLE;

   There is a variant of polymorphism that is only available to C-language
   functions: they can be declared to take parameters of type "any". (Note
   that this type name must be double-quoted, since it's also an SQL
   reserved word.) This works like anyelement except that it does not
   constrain different "any" arguments to be the same type, nor do they
   help determine the function's result type. A C-language function can
   also declare its final parameter to be VARIADIC "any". This will match
   one or more actual arguments of any type (not necessarily the same
   type). These arguments will not be gathered into an array as happens
   with normal variadic functions; they will just be passed to the
   function separately. The PG_NARGS() macro and the methods described
   above must be used to determine the number of actual arguments and
   their types when using this feature. Also, users of such a function
   might wish to use the VARIADIC keyword in their function call, with the
   expectation that the function would treat the array elements as
   separate arguments. The function itself must implement that behavior if
   wanted, after using get_fn_expr_variadic to detect that the actual
   argument was marked with VARIADIC.

36.10.11. Shared Memory #

36.10.11.1. Requesting Shared Memory at Startup #

   Add-ins can reserve shared memory on server startup. To do so, the
   add-in's shared library must be preloaded by specifying it in
   shared_preload_libraries. The shared library should also register a
   shmem_request_hook in its _PG_init function. This shmem_request_hook
   can reserve shared memory by calling:
void RequestAddinShmemSpace(Size size)

   Each backend should obtain a pointer to the reserved shared memory by
   calling:
void *ShmemInitStruct(const char *name, Size size, bool *foundPtr)

   If this function sets foundPtr to false, the caller should proceed to
   initialize the contents of the reserved shared memory. If foundPtr is
   set to true, the shared memory was already initialized by another
   backend, and the caller need not initialize further.

   To avoid race conditions, each backend should use the LWLock
   AddinShmemInitLock when initializing its allocation of shared memory,
   as shown here:
static mystruct *ptr = NULL;
bool        found;

LWLockAcquire(AddinShmemInitLock, LW_EXCLUSIVE);
ptr = ShmemInitStruct("my struct name", size, &found);
if (!found)
{
    ... initialize contents of shared memory ...
    ptr->locks = GetNamedLWLockTranche("my tranche name");
}
LWLockRelease(AddinShmemInitLock);

   shmem_startup_hook provides a convenient place for the initialization
   code, but it is not strictly required that all such code be placed in
   this hook. On Windows (and anywhere else where EXEC_BACKEND is
   defined), each backend executes the registered shmem_startup_hook
   shortly after it attaches to shared memory, so add-ins should still
   acquire AddinShmemInitLock within this hook, as shown in the example
   above. On other platforms, only the postmaster process executes the
   shmem_startup_hook, and each backend automatically inherits the
   pointers to shared memory.

   An example of a shmem_request_hook and shmem_startup_hook can be found
   in contrib/pg_stat_statements/pg_stat_statements.c in the PostgreSQL
   source tree.

36.10.11.2. Requesting Shared Memory After Startup #

   There is another, more flexible method of reserving shared memory that
   can be done after server startup and outside a shmem_request_hook. To
   do so, each backend that will use the shared memory should obtain a
   pointer to it by calling:
void *GetNamedDSMSegment(const char *name, size_t size,
                         void (*init_callback) (void *ptr),
                         bool *found)

   If a dynamic shared memory segment with the given name does not yet
   exist, this function will allocate it and initialize it with the
   provided init_callback callback function. If the segment has already
   been allocated and initialized by another backend, this function simply
   attaches the existing dynamic shared memory segment to the current
   backend.

   Unlike shared memory reserved at server startup, there is no need to
   acquire AddinShmemInitLock or otherwise take action to avoid race
   conditions when reserving shared memory with GetNamedDSMSegment. This
   function ensures that only one backend allocates and initializes the
   segment and that all other backends receive a pointer to the fully
   allocated and initialized segment.

   A complete usage example of GetNamedDSMSegment can be found in
   src/test/modules/test_dsm_registry/test_dsm_registry.c in the
   PostgreSQL source tree.

36.10.12. LWLocks #

36.10.12.1. Requesting LWLocks at Startup #

   Add-ins can reserve LWLocks on server startup. As with shared memory
   reserved at server startup, the add-in's shared library must be
   preloaded by specifying it in shared_preload_libraries, and the shared
   library should register a shmem_request_hook in its _PG_init function.
   This shmem_request_hook can reserve LWLocks by calling:
void RequestNamedLWLockTranche(const char *tranche_name, int num_lwlocks)

   This ensures that an array of num_lwlocks LWLocks is available under
   the name tranche_name. A pointer to this array can be obtained by
   calling:
LWLockPadded *GetNamedLWLockTranche(const char *tranche_name)

36.10.12.2. Requesting LWLocks After Startup #

   There is another, more flexible method of obtaining LWLocks that can be
   done after server startup and outside a shmem_request_hook. To do so,
   first allocate a tranche_id by calling:
int LWLockNewTrancheId(void)

   Next, initialize each LWLock, passing the new tranche_id as an
   argument:
void LWLockInitialize(LWLock *lock, int tranche_id)

   Similar to shared memory, each backend should ensure that only one
   process allocates a new tranche_id and initializes each new LWLock. One
   way to do this is to only call these functions in your shared memory
   initialization code with the AddinShmemInitLock held exclusively. If
   using GetNamedDSMSegment, calling these functions in the init_callback
   callback function is sufficient to avoid race conditions.

   Finally, each backend using the tranche_id should associate it with a
   tranche_name by calling:
void LWLockRegisterTranche(int tranche_id, const char *tranche_name)

   A complete usage example of LWLockNewTrancheId, LWLockInitialize, and
   LWLockRegisterTranche can be found in contrib/pg_prewarm/autoprewarm.c
   in the PostgreSQL source tree.

36.10.13. Custom Wait Events #

   Add-ins can define custom wait events under the wait event type
   Extension by calling:
uint32 WaitEventExtensionNew(const char *wait_event_name)

   The wait event is associated to a user-facing custom string. An example
   can be found in src/test/modules/worker_spi in the PostgreSQL source
   tree.

   Custom wait events can be viewed in pg_stat_activity:
=# SELECT wait_event_type, wait_event FROM pg_stat_activity
     WHERE backend_type ~ 'worker_spi';
 wait_event_type |  wait_event
-----------------+---------------
 Extension       | WorkerSpiMain
(1 row)

36.10.14. Injection Points #

   An injection point with a given name is declared using macro:
INJECTION_POINT(name, arg);

   There are a few injection points already declared at strategic points
   within the server code. After adding a new injection point the code
   needs to be compiled in order for that injection point to be available
   in the binary. Add-ins written in C-language can declare injection
   points in their own code using the same macro. The injection point
   names should use lower-case characters, with terms separated by dashes.
   arg is an optional argument value given to the callback at run-time.

   Executing an injection point can require allocating a small amount of
   memory, which can fail. If you need to have an injection point in a
   critical section where dynamic allocations are not allowed, you can use
   a two-step approach with the following macros:
INJECTION_POINT_LOAD(name);
INJECTION_POINT_CACHED(name, arg);

   Before entering the critical section, call INJECTION_POINT_LOAD. It
   checks the shared memory state, and loads the callback into
   backend-private memory if it is active. Inside the critical section,
   use INJECTION_POINT_CACHED to execute the callback.

   Add-ins can attach callbacks to an already-declared injection point by
   calling:
extern void InjectionPointAttach(const char *name,
                                 const char *library,
                                 const char *function,
                                 const void *private_data,
                                 int private_data_size);

   name is the name of the injection point, which when reached during
   execution will execute the function loaded from library. private_data
   is a private area of data of size private_data_size given as argument
   to the callback when executed.

   Here is an example of callback for InjectionPointCallback:
static void
custom_injection_callback(const char *name,
                          const void *private_data,
                          void *arg)
{
    uint32 wait_event_info = WaitEventInjectionPointNew(name);

    pgstat_report_wait_start(wait_event_info);
    elog(NOTICE, "%s: executed custom callback", name);
    pgstat_report_wait_end();
}

   This callback prints a message to server error log with severity
   NOTICE, but callbacks may implement more complex logic.

   An alternative way to define the action to take when an injection point
   is reached is to add the testing code alongside the normal source code.
   This can be useful if the action e.g. depends on local variables that
   are not accessible to loaded modules. The IS_INJECTION_POINT_ATTACHED
   macro can then be used to check if an injection point is attached, for
   example:
#ifdef USE_INJECTION_POINTS
if (IS_INJECTION_POINT_ATTACHED("before-foobar"))
{
    /* change a local variable if injection point is attached */
    local_var = 123;

    /* also execute the callback */
    INJECTION_POINT_CACHED("before-foobar", NULL);
}
#endif

   Note that the callback attached to the injection point will not be
   executed by the IS_INJECTION_POINT_ATTACHED macro. If you want to
   execute the callback, you must also call INJECTION_POINT_CACHED like in
   the above example.

   Optionally, it is possible to detach an injection point by calling:
extern bool InjectionPointDetach(const char *name);

   On success, true is returned, false otherwise.

   A callback attached to an injection point is available across all the
   backends including the backends started after InjectionPointAttach is
   called. It remains attached while the server is running or until the
   injection point is detached using InjectionPointDetach.

   An example can be found in src/test/modules/injection_points in the
   PostgreSQL source tree.

   Enabling injections points requires --enable-injection-points with
   configure or -Dinjection_points=true with Meson.

36.10.15. Custom Cumulative Statistics #

   It is possible for add-ins written in C-language to use custom types of
   cumulative statistics registered in the Cumulative Statistics System.

   First, define a PgStat_KindInfo that includes all the information
   related to the custom type registered. For example:
static const PgStat_KindInfo custom_stats = {
    .name = "custom_stats",
    .fixed_amount = false,
    .shared_size = sizeof(PgStatShared_Custom),
    .shared_data_off = offsetof(PgStatShared_Custom, stats),
    .shared_data_len = sizeof(((PgStatShared_Custom *) 0)->stats),
    .pending_size = sizeof(PgStat_StatCustomEntry),
}

   Then, each backend that needs to use this custom type needs to register
   it with pgstat_register_kind and a unique ID used to store the entries
   related to this type of statistics:
extern PgStat_Kind pgstat_register_kind(PgStat_Kind kind,
                                        const PgStat_KindInfo *kind_info);

   While developing a new extension, use PGSTAT_KIND_EXPERIMENTAL for
   kind. When you are ready to release the extension to users, reserve a
   kind ID at the Custom Cumulative Statistics page.

   The details of the API for PgStat_KindInfo can be found in
   src/include/utils/pgstat_internal.h.

   The type of statistics registered is associated with a name and a
   unique ID shared across the server in shared memory. Each backend using
   a custom type of statistics maintains a local cache storing the
   information of each custom PgStat_KindInfo.

   Place the extension module implementing the custom cumulative
   statistics type in shared_preload_libraries so that it will be loaded
   early during PostgreSQL startup.

   An example describing how to register and use custom statistics can be
   found in src/test/modules/injection_points.

36.10.16. Using C++ for Extensibility #

   Although the PostgreSQL backend is written in C, it is possible to
   write extensions in C++ if these guidelines are followed:
     * All functions accessed by the backend must present a C interface to
       the backend; these C functions can then call C++ functions. For
       example, extern C linkage is required for backend-accessed
       functions. This is also necessary for any functions that are passed
       as pointers between the backend and C++ code.
     * Free memory using the appropriate deallocation method. For example,
       most backend memory is allocated using palloc(), so use pfree() to
       free it. Using C++ delete in such cases will fail.
     * Prevent exceptions from propagating into the C code (use a
       catch-all block at the top level of all extern C functions). This
       is necessary even if the C++ code does not explicitly throw any
       exceptions, because events like out-of-memory can still throw
       exceptions. Any exceptions must be caught and appropriate errors
       passed back to the C interface. If possible, compile C++ with
       -fno-exceptions to eliminate exceptions entirely; in such cases,
       you must check for failures in your C++ code, e.g., check for NULL
       returned by new().
     * If calling backend functions from C++ code, be sure that the C++
       call stack contains only plain old data structures (POD). This is
       necessary because backend errors generate a distant longjmp() that
       does not properly unroll a C++ call stack with non-POD objects.

   In summary, it is best to place C++ code behind a wall of extern C
   functions that interface to the backend, and avoid exception, memory,
   and call stack leakage.