36.10. C-Language Functions
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## 36.10. C-Language Functions #

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 `int`XX`` 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 `DirectFunctionCall`n`(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 `DirectFunctionCall`n`` family expect a C function name as their first argument. There are also `OidFunctionCall`n`` which take the OID of the target function, and some other variants. All of these expect the function's arguments to be supplied as `Datum`s, 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 `NULL`able 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 `DatumGet`XXX`()` 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 `TupleDesc`s 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.

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