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.)
Two different calling conventions are currently used for C functions.
The newer “version 1” calling convention is indicated by writing
a PG_FUNCTION_INFO_V1()
macro call for the function,
as illustrated below. Lack of such a macro indicates an old-style
(“version 0”) function. The language name specified in CREATE FUNCTION
is C
in either case. Old-style functions are now deprecated
because of portability problems and lack of functionality, but they
are still supported for compatibility reasons.
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.
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 33.9.6, “Compiling and Linking Dynamically-Loaded Functions” 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. A magic block is required as of
PostgreSQL 8.2. To include a magic block,
write this in one (and only one) of the module source files, after having
included the header fmgr.h
:
#ifdef PG_MODULE_MAGIC PG_MODULE_MAGIC; #endif
The #ifdef
test can be omitted if the code doesn't
need to compile against pre-8.2 PostgreSQL
releases.
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, use the LOAD command or begin a fresh session.
Optionally, a dynamically loaded file can contain initialization and
finalization functions. 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. If the file includes a function named
_PG_fini
, that function will be called immediately before
unloading the file. Likewise, the function receives no parameters and
should return void. Note that _PG_fini
will only be called
during an unload of the file, not during process termination.
(Presently, an unload only happens in the context of re-loading
the file due to an explicit LOAD
command.)
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;
On the other hand, fixed-length types of any size may 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 a length field of exactly 4 bytes, and 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.
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 may well point directly into a disk buffer. The sole exception to this rule is explained in Section 33.10, “User-Defined Aggregates”.
As an example, we can define the type text
as
follows:
typedef struct { int4 length; char data[1]; } text;
Obviously, the data field declared here is not long enough to hold all possible strings. Since it's impossible to declare a variable-size structure in C, we rely on the knowledge that the C compiler won't range-check array subscripts. We just allocate the necessary amount of space and then access the array as if it were declared the right length. (This is a common trick, which you can read about in many textbooks about C.)
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); destination->length = VARHDRSZ + 40; memcpy(destination->data, buffer, 40); ...
VARHDRSZ
is the same as sizeof(int4)
, but
it's considered good style to use the macro VARHDRSZ
to refer to the size of the overhead for a variable-length type.
Table 33.1, “Equivalent C Types for Built-In SQL Types” specifies which C type
corresponds to which SQL type when writing a C-language function
that uses a built-in type of PostgreSQL.
The “Defined In” column gives the header file that
needs to be included to get the type definition. (The actual
definition may 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, because
it declares a number of things that you will need anyway.
Table 33.1. Equivalent C Types for Built-In SQL Types
SQL Type | C Type | Defined In |
---|---|---|
abstime |
AbsoluteTime |
utils/nabstime.h |
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 |
smallint (int2 ) |
int2 or int16
|
postgres.h |
int2vector |
int2vector* |
postgres.h |
integer (int4 ) |
int4 or int32
|
postgres.h |
real (float4 ) |
float4* |
postgres.h |
double precision (float8 ) |
float8* |
postgres.h |
interval |
Interval* |
utils/timestamp.h |
lseg |
LSEG* |
utils/geo_decls.h |
name |
Name |
postgres.h |
oid |
Oid |
postgres.h |
oidvector |
oidvector* |
postgres.h |
path |
PATH* |
utils/geo_decls.h |
point |
POINT* |
utils/geo_decls.h |
regproc |
regproc |
postgres.h |
reltime |
RelativeTime |
utils/nabstime.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* |
utils/timestamp.h |
tinterval |
TimeInterval |
utils/nabstime.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.
We present the “old style” calling convention first — although this approach is now deprecated, it's easier to get a handle on initially. In the version-0 method, the arguments and result of the C function are just declared in normal C style, but being careful to use the C representation of each SQL data type as shown above.
Here are some examples:
#include "postgres.h" #include <string.h> /* by value */ int add_one(int arg) { return arg + 1; } /* by reference, fixed length */ float8 * add_one_float8(float8 *arg) { float8 *result = (float8 *) palloc(sizeof(float8)); *result = *arg + 1.0; return result; } Point * makepoint(Point *pointx, Point *pointy) { Point *new_point = (Point *) palloc(sizeof(Point)); new_point->x = pointx->x; new_point->y = pointy->y; return new_point; } /* by reference, variable length */ text * copytext(text *t) { /* * VARSIZE is the total size of the struct in bytes. */ text *new_t = (text *) palloc(VARSIZE(t)); VARATT_SIZEP(new_t) = VARSIZE(t); /* * VARDATA is a pointer to the data region of the struct. */ memcpy((void *) VARDATA(new_t), /* destination */ (void *) VARDATA(t), /* source */ VARSIZE(t) - VARHDRSZ); /* how many bytes */ return new_t; } text * concat_text(text *arg1, text *arg2) { int32 new_text_size = VARSIZE(arg1) + VARSIZE(arg2) - VARHDRSZ; text *new_text = (text *) palloc(new_text_size); VARATT_SIZEP(new_text) = new_text_size; memcpy(VARDATA(new_text), VARDATA(arg1), VARSIZE(arg1) - VARHDRSZ); memcpy(VARDATA(new_text) + (VARSIZE(arg1) - VARHDRSZ), VARDATA(arg2), VARSIZE(arg2) - VARHDRSZ); return new_text; }
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;
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 may omit the system-specific extension for a shared
library, commonly .so
or
.sl
.)
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, by checking for a null pointer for each pass-by-reference argument. (For pass-by-value arguments, we don't even have a way to check!)
Although this calling convention is simple to use,
it is not very portable; on some architectures there are problems
with passing data types that are smaller than int
this way. Also, there is
no simple way to return a null result, nor to cope with null arguments
in any way other than making the function strict. The version-1
convention, presented next, overcomes these objections.
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_
macro that corresponds to the argument's data type, and the
result is returned using a
xxx
()PG_RETURN_
macro for the return type.
xxx
()PG_GETARG_
takes as its argument the number of the function argument to
fetch, where the count starts at 0.
xxx
()PG_RETURN_
takes as its argument the actual value to return.
xxx
()
Here we show the same functions as above, coded in version-1 style:
#include "postgres.h" #include <string.h> #include "fmgr.h" /* 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_P(0); /* * VARSIZE is the total size of the struct in bytes. */ text *new_t = (text *) palloc(VARSIZE(t)); VARATT_SIZEP(new_t) = VARSIZE(t); /* * VARDATA is a pointer to the data region of the struct. */ memcpy((void *) VARDATA(new_t), /* destination */ (void *) VARDATA(t), /* source */ VARSIZE(t) - VARHDRSZ); /* 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_P(0); text *arg2 = PG_GETARG_TEXT_P(1); int32 new_text_size = VARSIZE(arg1) + VARSIZE(arg2) - VARHDRSZ; text *new_text = (text *) palloc(new_text_size); VARATT_SIZEP(new_text) = new_text_size; memcpy(VARDATA(new_text), VARDATA(arg1), VARSIZE(arg1) - VARHDRSZ); memcpy(VARDATA(new_text) + (VARSIZE(arg1) - VARHDRSZ), VARDATA(arg2), VARSIZE(arg2) - VARHDRSZ); PG_RETURN_TEXT_P(new_text); }
The CREATE FUNCTION
commands are the same as
for the version-0 equivalents.
At first glance, the version-1 coding conventions may appear to
be just pointless obscurantism. They do, however, offer a number
of improvements, because the macros can hide unnecessary detail.
An example is that in coding add_one_float8
, we no longer need to
be aware that float8
is a pass-by-reference type. Another
example is that the GETARG
macros for variable-length types allow
for more efficient fetching of “toasted” (compressed or
out-of-line) values.
One big improvement in version-1 functions is better handling of null
inputs and results. The macro PG_ARGISNULL(
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
n
)PG_GETARG_
macros,
the input arguments are counted beginning at zero. Note that one
should refrain from executing
xxx
()PG_GETARG_
until
one has verified that the argument isn't null.
To return a null result, execute xxx
()PG_RETURN_NULL()
;
this works in both strict and nonstrict functions.
Other options provided in the new-style interface are two
variants of the
PG_GETARG_
macros. The first of these,
xxx
()PG_GETARG_
,
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
xxx
_COPY()PG_GETARG_
macros guarantees a writable result.)
The second variant consists of the
xxx
_COPY()PG_GETARG_
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
xxx
_SLICE()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 33.9.10, “Returning Sets”) and
implement trigger functions (Chapter 34, Triggers) and
procedural-language call handlers (Chapter 47, Writing A Procedural Language Handler). Version-1 code is also more
portable than version-0, because it does not break restrictions
on function call protocol in the C standard. For more details
see src/backend/utils/fmgr/README
in the
source distribution.
Before we turn to the more advanced topics, we should discuss some coding rules for PostgreSQL C-language functions. While it may 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 33.9.6, “Compiling and Linking Dynamically-Loaded Functions” 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 33.9.1, “Dynamic Loading”.
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
. 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.
Even if you initialize all fields of your structure, there may be
alignment padding (holes in the structure) that may contain
garbage values.
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.
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.
The compiler flag to create PIC is
-fpic
. The linker flag to create shared
libraries is -shared
.
gcc -fpic -c foo.c ld -shared -o foo.so foo.o
This is applicable as of version 4.0 of BSD/OS.
The compiler flag to create PIC is
-fpic
. To create shared libraries the compiler
flag is -shared
.
gcc -fpic -c foo.c gcc -shared -o foo.so foo.o
This is applicable as of version 3.0 of FreeBSD.
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
cc +z -c foo.c
or
gcc -fpic -c foo.c
and then
ld -b -o foo.sl foo.o
HP-UX uses the extension
.sl
for shared libraries, unlike most other
systems.
PIC is the default, no special compiler
options are necessary. The linker option to produce shared
libraries is -shared
.
cc -c foo.c ld -shared -o foo.so foo.o
The compiler flag to create PIC is
-fpic
. On some platforms in some situations
-fPIC
must be used if -fpic
does not work. Refer to the GCC manual for more information.
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
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
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
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
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
PIC is the default, so the compilation command
is the usual one. ld
with special options is
used to do the linking:
cc -c foo.c ld -shared -expect_unresolved '*' -o foo.so foo.o
The same procedure is used with GCC instead of the system compiler; no special options are required.
The compiler flag to create PIC is -K
PIC
with the SCO compiler and -fpic
with GCC. To link shared libraries,
the compiler option is -G
with the SCO compiler
and -shared
with
GCC.
cc -K PIC -c foo.c cc -G -o foo.so foo.o
or
gcc -fpic -c foo.c gcc -shared -o foo.so foo.o
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 33.9.1, “Dynamic Loading” about where the server expects to find the shared library files.
If you are thinking about distributing your PostgreSQL extension modules, setting up a portable build system for them can be fairly difficult. Therefore the PostgreSQL installation provides a build infrastructure for extensions, called PGXS, so that simple extension modules can be built simply against an already installed server. Note that this infrastructure is not intended to be a universal build system framework that can be used to build all software interfacing to PostgreSQL; it simply automates common build rules for simple server extension modules. For more complicated packages, you need to write your own build system.
To use the infrastructure for your extension, you must write a
simple makefile. In that makefile, you need to set some variables
and finally include the global PGXS makefile.
Here is an example that builds an extension module named
isbn_issn
consisting of a shared library, an
SQL script, and a documentation text file:
MODULES = isbn_issn DATA_built = isbn_issn.sql DOCS = README.isbn_issn PGXS := $(shell pg_config --pgxs) include $(PGXS)
The last two lines should always be the same. Earlier in the file, you assign variables or add custom make rules.
The following variables can be set:
MODULES
list of shared objects to be built from source file with same stem (do not include suffix in this list)
DATA
random files to install into prefix
/share/contrib
DATA_built
random files to install into
,
which need to be built first
prefix
/share/contrib
DOCS
random files to install under
prefix
/doc/contrib
SCRIPTS
script files (not binaries) to install into
prefix
/bin
SCRIPTS_built
script files (not binaries) to install into
,
which need to be built first
prefix
/bin
REGRESS
list of regression test cases (without suffix), see below
or at most one of these two:
PROGRAM
a binary program to build (list objects files in OBJS
)
MODULE_big
a shared object to build (list object files in OBJS
)
The following can also be set:
EXTRA_CLEAN
extra files to remove in make clean
PG_CPPFLAGS
will be added to CPPFLAGS
PG_LIBS
will be added to PROGRAM
link line
SHLIB_LINK
will be added to MODULE_big
link line
Put this makefile as Makefile
in the directory
which holds your extension. Then you can do
make
to compile, and later make
install
to install your module. The extension is
compiled and installed for the
PostgreSQL installation that
corresponds to the first pg_config
command
found in your path.
The scripts listed in the REGRESS
variable are used for
regression testing of your module, just like make
installcheck
is used for the main
PostgreSQL server. For this to work you need
to have a subdirectory named sql/
in your extension's
directory, within which you put one file for each group of tests you want
to run. The files should have extension .sql
, which
should not be included in the REGRESS
list in the
makefile. For each test there should be a file containing the expected
result in a subdirectory named expected/
, with extension
.out
. The tests are run by executing make
installcheck
, and the resulting output will be compared to the
expected files. The differences will be written to the file
regression.diffs
in diff -c
format.
Note that trying to run a test which is missing the expected file will be
reported as “trouble”, so make sure you have all expected
files.
The easiest way of creating the expected files is creating empty files,
then carefully inspecting the result files after a test run (to be found
in the results/
directory), and copying them to
expected/
if they match what you want from the test.
Composite types do not have a fixed layout like C structures. Instances of a composite type may contain null fields. In addition, composite types that are part of an inheritance hierarchy may 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 call conventions version 0, we can define
c_overpaid
as:
#include "postgres.h" #include "executor/executor.h" /* for GetAttributeByName() */ bool c_overpaid(HeapTupleHeader t, /* the current row of emp */ int32 limit) { bool isnull; int32 salary; salary = DatumGetInt32(GetAttributeByName(t, "salary", &isnull)); if (isnull) return false; return salary > limit; }
In version-1 coding, the above would look like this:
#include "postgres.h" #include "executor/executor.h" /* for GetAttributeByName() */ 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
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.
XXX
()
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.
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”.)
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.
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.
There is also a special API that provides support for returning
sets (multiple rows) from a C-language function. A set-returning
function must follow the version-1 calling conventions. Also,
source files must include funcapi.h
, as
above.
A set-returning function (SRF) is called
once for each item it returns. The SRF must
therefore save enough state to remember what it was doing and
return the next item on each call.
The structure FuncCallContext
is provided to help
control this process. Within a function, fcinfo->flinfo->fn_extra
is used to hold a pointer to FuncCallContext
across calls.
typedef struct { /* * 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. */ uint32 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. */ uint32 max_calls; /* * OPTIONAL pointer to result slot * * This is obsolete and only present for backwards compatibility, viz, * user-defined SRFs that use the deprecated TupleDescGetSlot(). */ TupleTableSlot *slot; /* * 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;
An SRF uses several functions and macros that
automatically manipulate the FuncCallContext
structure (and expect to find it via fn_extra
). Use
SRF_IS_FIRSTCALL()
to determine if your function is being called for the first or a subsequent time. On the first call (only) use
SRF_FIRSTCALL_INIT()
to initialize the FuncCallContext
. On every function call,
including the first, use
SRF_PERCALL_SETUP()
to properly set up for using the FuncCallContext
and clearing any previously returned data left over from the
previous pass.
If your function has data to return, 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.
A complete pseudo-code example looks like the following:
Datum my_set_returning_function(PG_FUNCTION_ARGS) { FuncCallContext *funcctx; Datum result; MemoryContext oldcontext;further declarations as needed
if (SRF_IS_FIRSTCALL()) { 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 and just need to clean up: */user code
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_UINT32(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.
The directory contrib/tablefunc
in the source
distribution contains more examples of set-returning functions.
C-language functions may be declared to accept and
return the polymorphic types
anyelement
and anyarray
.
See Section 33.2.5, “Polymorphic Types” for a more detailed explanation
of polymorphic functions. When function 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
.
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;
Add-ins may reserve LWLocks and an allocation of shared memory on server startup. The add-in's shared library must be preloaded by specifying it in shared_preload_libraries. Shared memory is reserved by calling:
void RequestAddinShmemSpace(int size)
from your _PG_init
function.
LWLocks are reserved by calling:
void RequestAddinLWLocks(int n)
from _PG_init
.
To avoid possible race-conditions, each backend should use the LWLock
AddinShmemInitLock
when connecting to and initializing
its allocation of shared memory, as shown here:
static mystruct *ptr = NULL; if (!ptr) { bool found; LWLockAcquire(AddinShmemInitLock, LW_EXCLUSIVE); ptr = ShmemInitStruct("my struct name", size, &found); if (!ptr) elog(ERROR, "out of shared memory"); if (!found) { initialize contents of shmem area; acquire any requested LWLocks using: ptr->mylockid = LWLockAssign(); } LWLockRelease(AddinShmemInitLock); }