38.10. 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.
38.10.1. 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:
If the name is an absolute path, the given file is loaded.
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.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.
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 38.10.5 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
:
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.
38.10.2. 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:
(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 38.2.)
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:
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 38.12.
As an example, we can define the type text
as follows:
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:
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 38.2 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 38.2. Equivalent C Types for Built-in SQL Types
SQL Type | C Type | Defined In |
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Now that we've gone over all of the possible structures for base types, we can show some examples of real functions.
38.10.3. 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:
In addition, the macro call:
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.
Here are some examples using the version-1 calling convention:
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:
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 38.10.8) and implement trigger functions (Chapter 39) and procedural-language call handlers (Chapter 58). For more details see src/backend/utils/fmgr/README
in the source distribution.
38.10.4. 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 38.10.5 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 38.10.1.
When allocating memory, use the PostgreSQL functions
palloc
andpfree
instead of the corresponding C library functionsmalloc
andfree
. The memory allocated bypalloc
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 withpalloc0
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 infmgr.h
, so you will need to include at least these two files. For portability reasons it's best to includepostgres.h
first, before any other system or user header files. Includingpostgres.h
will also includeelog.h
andpalloc.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.
38.10.5. 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
.
This is applicable as of version 3.0 of FreeBSD.
HP-UX
The compiler flag of the system compiler to create PIC is +z
. When using GCC it's -fPIC
. The linker flag for shared libraries is -b
. So:
or:
and then:
HP-UX uses the extension .sl
for shared libraries, unlike most other systems.
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:
macOS
Here is an example. It assumes the developer tools are installed.
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.
OpenBSD
The compiler flag to create PIC is -fPIC
. ld -Bshareable
is used to link shared libraries.
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.
or
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 38.10.1 about where the server expects to find the shared library files.
38.10.6. 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:
Using the version-1 calling conventions, we can define c_overpaid
as:
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
()
macro. 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:
Notice we have used STRICT
so that we did not have to check whether the input arguments were NULL.
38.10.7. 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:
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:
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:
to get a TupleDesc
for the row type of a named relation, and:
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:
if you plan to work with Datums, or:
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:
to build a HeapTuple
given user data in Datum form.
When working with C strings, use:
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:
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.
38.10.8. 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.
The macros to be used by an SRF using this infrastructure are:
Use this to determine if your function is being called for the first or a subsequent time. On the first call (only), call:
to initialize the FuncCallContext
. On every function call, including the first, call:
to set up for using the FuncCallContext
.
If your function has data to return in the current call, use:
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:
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:
A complete example of a simple SRF returning a composite type looks like: