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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="XFUNC-C"
>33.7. C-Language Functions</A
></H1
><A
NAME="AEN28910"
></A
><P
> 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 <SPAN
CLASS="QUOTE"
>"C language"</SPAN
> functions
from <SPAN
CLASS="QUOTE"
>"internal"</SPAN
> 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.)
</P
><P
> Two different calling conventions are currently used for C functions.
The newer <SPAN
CLASS="QUOTE"
>"version 1"</SPAN
> calling convention is indicated by writing
a <TT
CLASS="LITERAL"
>PG_FUNCTION_INFO_V1()</TT
> macro call for the function,
as illustrated below. Lack of such a macro indicates an old-style
(<SPAN
CLASS="QUOTE"
>"version 0"</SPAN
>) function. The language name specified in <TT
CLASS="COMMAND"
>CREATE FUNCTION</TT
>
is <TT
CLASS="LITERAL"
>C</TT
> 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.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="XFUNC-C-DYNLOAD"
>33.7.1. Dynamic Loading</A
></H2
><A
NAME="AEN28925"
></A
><P
> 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 <TT
CLASS="COMMAND"
>CREATE FUNCTION</TT
>
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.
</P
><P
> The following algorithm is used to locate the shared object file
based on the name given in the <TT
CLASS="COMMAND"
>CREATE FUNCTION</TT
>
command:
<P
></P
></P><OL
TYPE="1"
><LI
><P
> If the name is an absolute path, the given file is loaded.
</P
></LI
><LI
><P
> If the name starts with the string <TT
CLASS="LITERAL"
>$libdir</TT
>,
that part is replaced by the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> package
library directory
name, which is determined at build time.<A
NAME="AEN28938"
></A
>
</P
></LI
><LI
><P
> If the name does not contain a directory part, the file is
searched for in the path specified by the configuration variable
<VAR
CLASS="VARNAME"
>dynamic_library_path</VAR
>.<A
NAME="AEN28943"
></A
>
</P
></LI
><LI
><P
> 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.)
</P
></LI
></OL
><P>
If this sequence does not work, the platform-specific shared
library file name extension (often <TT
CLASS="FILENAME"
>.so</TT
>) is
appended to the given name and this sequence is tried again. If
that fails as well, the load will fail.
</P
><P
> The user ID the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 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 <SPAN
CLASS="SYSTEMITEM"
>postgres</SPAN
>
user is a common mistake.
</P
><P
> In any case, the file name that is given in the
<TT
CLASS="COMMAND"
>CREATE FUNCTION</TT
> command is recorded literally
in the system catalogs, so if the file needs to be loaded again
the same procedure is applied.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> will not compile a C function
automatically. The object file must be compiled before it is referenced
in a <TT
CLASS="COMMAND"
>CREATE
FUNCTION</TT
> command. See <A
HREF="xfunc-c.html#DFUNC"
>Section 33.7.6</A
> for additional
information.
</P
></BLOCKQUOTE
></DIV
><P
> 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 <TT
CLASS="COMMAND"
>LOAD</TT
>
command or begin a fresh session.
</P
><P
> It is recommended to locate shared libraries either relative to
<TT
CLASS="LITERAL"
>$libdir</TT
> or through the dynamic library path.
This simplifies version upgrades if the new installation is at a
different location. The actual directory that
<TT
CLASS="LITERAL"
>$libdir</TT
> stands for can be found out with the
command <TT
CLASS="LITERAL"
>pg_config --pkglibdir</TT
>.
</P
><P
> Before <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> release 7.2, only
exact absolute paths to object files could be specified in
<TT
CLASS="COMMAND"
>CREATE FUNCTION</TT
>. This approach is now deprecated
since it makes the function definition unnecessarily unportable.
It's best to specify just the shared library name with no path nor
extension, and let the search mechanism provide that information
instead.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="XFUNC-C-BASETYPE"
>33.7.2. Base Types in C-Language Functions</A
></H2
><A
NAME="AEN28969"
></A
><P
> To know how to write C-language functions, you need to know how
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> internally represents base
data types and how they can be passed to and from functions.
Internally, <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> regards a base
type as a <SPAN
CLASS="QUOTE"
>"blob of memory"</SPAN
>. The user-defined
functions that you define over a type in turn define the way that
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> can operate on it. That
is, <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> will only store and
retrieve the data from disk and use your user-defined functions
to input, process, and output the data.
</P
><P
> Base types can have one of three internal formats:
<P
></P
></P><UL
><LI
><P
> pass by value, fixed-length
</P
></LI
><LI
><P
> pass by reference, fixed-length
</P
></LI
><LI
><P
> pass by reference, variable-length
</P
></LI
></UL
><P>
</P
><P
> By-value types can only be 1, 2, or 4 bytes in length
(also 8 bytes, if <TT
CLASS="LITERAL"
>sizeof(Datum)</TT
> 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
<TT
CLASS="LITERAL"
>long</TT
> type is dangerous because it
is 4 bytes on some machines and 8 bytes on others, whereas
<TT
CLASS="TYPE"
>int</TT
> type is 4 bytes on most
Unix machines. A reasonable implementation of
the <TT
CLASS="TYPE"
>int4</TT
> type on Unix
machines might be:
</P><PRE
CLASS="PROGRAMLISTING"
>/* 4-byte integer, passed by value */
typedef int int4;</PRE
><P>
</P
><P
> On the other hand, fixed-length types of any size may
be passed by-reference. For example, here is a sample
implementation of a <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> type:
</P><PRE
CLASS="PROGRAMLISTING"
>/* 16-byte structure, passed by reference */
typedef struct
{
double x, y;
} Point;</PRE
><P>
Only pointers to such types can be used when passing
them in and out of <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> functions.
To return a value of such a type, allocate the right amount of
memory with <TT
CLASS="LITERAL"
>palloc</TT
>, fill in the allocated memory,
and return a pointer to it. (You can also return an input value
that has the same type as the return value directly by returning
the pointer to the input value. <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>Never</I
></SPAN
> modify the
contents of a pass-by-reference input value, however.)
</P
><P
> 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.
</P
><P
> As an example, we can define the type <TT
CLASS="TYPE"
>text</TT
> as
follows:
</P><PRE
CLASS="PROGRAMLISTING"
>typedef struct {
int4 length;
char data[1];
} text;</PRE
><P>
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 <ACRONYM
CLASS="ACRONYM"
>C</ACRONYM
>, we rely on the knowledge that the
<ACRONYM
CLASS="ACRONYM"
>C</ACRONYM
> 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.)
</P
><P
> 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 <TT
CLASS="STRUCTNAME"
>text</TT
>
structure, we might use a code fragment like this:
</P><PRE
CLASS="PROGRAMLISTING"
>#include "postgres.h"
...
char buffer[40]; /* our source data */
...
text *destination = (text *) palloc(VARHDRSZ + 40);
destination->length = VARHDRSZ + 40;
memcpy(destination->data, buffer, 40);
...</PRE
><P>
<TT
CLASS="LITERAL"
>VARHDRSZ</TT
> is the same as <TT
CLASS="LITERAL"
>sizeof(int4)</TT
>, but
it's considered good style to use the macro <TT
CLASS="LITERAL"
>VARHDRSZ</TT
>
to refer to the size of the overhead for a variable-length type.
</P
><P
> <A
HREF="xfunc-c.html#XFUNC-C-TYPE-TABLE"
>Table 33-1</A
> specifies which C type
corresponds to which SQL type when writing a C-language function
that uses a built-in type of <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>.
The <SPAN
CLASS="QUOTE"
>"Defined In"</SPAN
> 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
<TT
CLASS="FILENAME"
>postgres.h</TT
> first in any source file, because
it declares a number of things that you will need anyway.
</P
><DIV
CLASS="TABLE"
><A
NAME="XFUNC-C-TYPE-TABLE"
></A
><P
><B
>Table 33-1. Equivalent C Types for Built-In SQL Types</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><THEAD
><TR
><TH
> SQL Type
</TH
><TH
> C Type
</TH
><TH
> Defined In
</TH
></TR
></THEAD
><TBODY
><TR
><TD
><TT
CLASS="TYPE"
>abstime</TT
></TD
><TD
><TT
CLASS="TYPE"
>AbsoluteTime</TT
></TD
><TD
><TT
CLASS="FILENAME"
>utils/nabstime.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>boolean</TT
></TD
><TD
><TT
CLASS="TYPE"
>bool</TT
></TD
><TD
><TT
CLASS="FILENAME"
>postgres.h</TT
> (maybe compiler built-in)</TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>box</TT
></TD
><TD
><TT
CLASS="TYPE"
>BOX*</TT
></TD
><TD
><TT
CLASS="FILENAME"
>utils/geo_decls.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>bytea</TT
></TD
><TD
><TT
CLASS="TYPE"
>bytea*</TT
></TD
><TD
><TT
CLASS="FILENAME"
>postgres.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>"char"</TT
></TD
><TD
><TT
CLASS="TYPE"
>char</TT
></TD
><TD
>(compiler built-in)</TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>character</TT
></TD
><TD
><TT
CLASS="TYPE"
>BpChar*</TT
></TD
><TD
><TT
CLASS="FILENAME"
>postgres.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>cid</TT
></TD
><TD
><TT
CLASS="TYPE"
>CommandId</TT
></TD
><TD
><TT
CLASS="FILENAME"
>postgres.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>date</TT
></TD
><TD
><TT
CLASS="TYPE"
>DateADT</TT
></TD
><TD
><TT
CLASS="FILENAME"
>utils/date.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>smallint</TT
> (<TT
CLASS="TYPE"
>int2</TT
>)</TD
><TD
><TT
CLASS="TYPE"
>int2</TT
> or <TT
CLASS="TYPE"
>int16</TT
></TD
><TD
><TT
CLASS="FILENAME"
>postgres.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>int2vector</TT
></TD
><TD
><TT
CLASS="TYPE"
>int2vector*</TT
></TD
><TD
><TT
CLASS="FILENAME"
>postgres.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>integer</TT
> (<TT
CLASS="TYPE"
>int4</TT
>)</TD
><TD
><TT
CLASS="TYPE"
>int4</TT
> or <TT
CLASS="TYPE"
>int32</TT
></TD
><TD
><TT
CLASS="FILENAME"
>postgres.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>real</TT
> (<TT
CLASS="TYPE"
>float4</TT
>)</TD
><TD
><TT
CLASS="TYPE"
>float4*</TT
></TD
><TD
><TT
CLASS="FILENAME"
>postgres.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>double precision</TT
> (<TT
CLASS="TYPE"
>float8</TT
>)</TD
><TD
><TT
CLASS="TYPE"
>float8*</TT
></TD
><TD
><TT
CLASS="FILENAME"
>postgres.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>interval</TT
></TD
><TD
><TT
CLASS="TYPE"
>Interval*</TT
></TD
><TD
><TT
CLASS="FILENAME"
>utils/timestamp.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>lseg</TT
></TD
><TD
><TT
CLASS="TYPE"
>LSEG*</TT
></TD
><TD
><TT
CLASS="FILENAME"
>utils/geo_decls.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>name</TT
></TD
><TD
><TT
CLASS="TYPE"
>Name</TT
></TD
><TD
><TT
CLASS="FILENAME"
>postgres.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>oid</TT
></TD
><TD
><TT
CLASS="TYPE"
>Oid</TT
></TD
><TD
><TT
CLASS="FILENAME"
>postgres.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>oidvector</TT
></TD
><TD
><TT
CLASS="TYPE"
>oidvector*</TT
></TD
><TD
><TT
CLASS="FILENAME"
>postgres.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>path</TT
></TD
><TD
><TT
CLASS="TYPE"
>PATH*</TT
></TD
><TD
><TT
CLASS="FILENAME"
>utils/geo_decls.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>point</TT
></TD
><TD
><TT
CLASS="TYPE"
>POINT*</TT
></TD
><TD
><TT
CLASS="FILENAME"
>utils/geo_decls.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>regproc</TT
></TD
><TD
><TT
CLASS="TYPE"
>regproc</TT
></TD
><TD
><TT
CLASS="FILENAME"
>postgres.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>reltime</TT
></TD
><TD
><TT
CLASS="TYPE"
>RelativeTime</TT
></TD
><TD
><TT
CLASS="FILENAME"
>utils/nabstime.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
><TT
CLASS="TYPE"
>text*</TT
></TD
><TD
><TT
CLASS="FILENAME"
>postgres.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>tid</TT
></TD
><TD
><TT
CLASS="TYPE"
>ItemPointer</TT
></TD
><TD
><TT
CLASS="FILENAME"
>storage/itemptr.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>time</TT
></TD
><TD
><TT
CLASS="TYPE"
>TimeADT</TT
></TD
><TD
><TT
CLASS="FILENAME"
>utils/date.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>time with time zone</TT
></TD
><TD
><TT
CLASS="TYPE"
>TimeTzADT</TT
></TD
><TD
><TT
CLASS="FILENAME"
>utils/date.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>timestamp</TT
></TD
><TD
><TT
CLASS="TYPE"
>Timestamp*</TT
></TD
><TD
><TT
CLASS="FILENAME"
>utils/timestamp.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>tinterval</TT
></TD
><TD
><TT
CLASS="TYPE"
>TimeInterval</TT
></TD
><TD
><TT
CLASS="FILENAME"
>utils/nabstime.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>varchar</TT
></TD
><TD
><TT
CLASS="TYPE"
>VarChar*</TT
></TD
><TD
><TT
CLASS="FILENAME"
>postgres.h</TT
></TD
></TR
><TR
><TD
><TT
CLASS="TYPE"
>xid</TT
></TD
><TD
><TT
CLASS="TYPE"
>TransactionId</TT
></TD
><TD
><TT
CLASS="FILENAME"
>postgres.h</TT
></TD
></TR
></TBODY
></TABLE
></DIV
><P
> Now that we've gone over all of the possible structures
for base types, we can show some examples of real functions.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN29240"
>33.7.3. Calling Conventions Version 0 for C-Language Functions</A
></H2
><P
> We present the <SPAN
CLASS="QUOTE"
>"old style"</SPAN
> 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.
</P
><P
> Here are some examples:
</P><PRE
CLASS="PROGRAMLISTING"
>#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;
}</PRE
><P>
</P
><P
> Supposing that the above code has been prepared in file
<TT
CLASS="FILENAME"
>funcs.c</TT
> and compiled into a shared object,
we could define the functions to <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
with commands like this:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION add_one(integer) RETURNS integer
AS '<VAR
CLASS="REPLACEABLE"
>DIRECTORY</VAR
>/funcs', 'add_one'
LANGUAGE C STRICT;
-- note overloading of SQL function name "add_one"
CREATE FUNCTION add_one(double precision) RETURNS double precision
AS '<VAR
CLASS="REPLACEABLE"
>DIRECTORY</VAR
>/funcs', 'add_one_float8'
LANGUAGE C STRICT;
CREATE FUNCTION makepoint(point, point) RETURNS point
AS '<VAR
CLASS="REPLACEABLE"
>DIRECTORY</VAR
>/funcs', 'makepoint'
LANGUAGE C STRICT;
CREATE FUNCTION copytext(text) RETURNS text
AS '<VAR
CLASS="REPLACEABLE"
>DIRECTORY</VAR
>/funcs', 'copytext'
LANGUAGE C STRICT;
CREATE FUNCTION concat_text(text, text) RETURNS text
AS '<VAR
CLASS="REPLACEABLE"
>DIRECTORY</VAR
>/funcs', 'concat_text',
LANGUAGE C STRICT;</PRE
><P>
</P
><P
> Here, <VAR
CLASS="REPLACEABLE"
>DIRECTORY</VAR
> stands for the
directory of the shared library file (for instance the
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> tutorial directory, which
contains the code for the examples used in this section).
(Better style would be to use just <TT
CLASS="LITERAL"
>'funcs'</TT
> in the
<TT
CLASS="LITERAL"
>AS</TT
> clause, after having added
<VAR
CLASS="REPLACEABLE"
>DIRECTORY</VAR
> to the search path. In any
case, we may omit the system-specific extension for a shared
library, commonly <TT
CLASS="LITERAL"
>.so</TT
> or
<TT
CLASS="LITERAL"
>.sl</TT
>.)
</P
><P
> Notice that we have specified the functions as <SPAN
CLASS="QUOTE"
>"strict"</SPAN
>,
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!)
</P
><P
> 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 <TT
CLASS="TYPE"
>int</TT
> 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.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN29267"
>33.7.4. Calling Conventions Version 1 for C-Language Functions</A
></H2
><P
> 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
</P><PRE
CLASS="PROGRAMLISTING"
>Datum funcname(PG_FUNCTION_ARGS)</PRE
><P>
In addition, the macro call
</P><PRE
CLASS="PROGRAMLISTING"
>PG_FUNCTION_INFO_V1(funcname);</PRE
><P>
must appear in the same source file. (Conventionally. it's
written just before the function itself.) This macro call is not
needed for <TT
CLASS="LITERAL"
>internal</TT
>-language functions, since
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> assumes that all internal functions
use the version-1 convention. It is, however, required for
dynamically-loaded functions.
</P
><P
> In a version-1 function, each actual argument is fetched using a
<CODE
CLASS="FUNCTION"
>PG_GETARG_<VAR
CLASS="REPLACEABLE"
>xxx</VAR
>()</CODE
>
macro that corresponds to the argument's data type, and the
result is returned using a
<CODE
CLASS="FUNCTION"
>PG_RETURN_<VAR
CLASS="REPLACEABLE"
>xxx</VAR
>()</CODE
>
macro for the return type.
<CODE
CLASS="FUNCTION"
>PG_GETARG_<VAR
CLASS="REPLACEABLE"
>xxx</VAR
>()</CODE
>
takes as its argument the number of the function argument to
fetch, where the count starts at 0.
<CODE
CLASS="FUNCTION"
>PG_RETURN_<VAR
CLASS="REPLACEABLE"
>xxx</VAR
>()</CODE
>
takes as its argument the actual value to return.
</P
><P
> Here we show the same functions as above, coded in version-1 style:
</P><PRE
CLASS="PROGRAMLISTING"
>#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);
}
/* b 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);
}</PRE
><P>
</P
><P
> The <TT
CLASS="COMMAND"
>CREATE FUNCTION</TT
> commands are the same as
for the version-0 equivalents.
</P
><P
> 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 <CODE
CLASS="FUNCTION"
>add_one_float8</CODE
>, we no longer need to
be aware that <TT
CLASS="TYPE"
>float8</TT
> is a pass-by-reference type. Another
example is that the <TT
CLASS="LITERAL"
>GETARG</TT
> macros for variable-length types allow
for more efficient fetching of <SPAN
CLASS="QUOTE"
>"toasted"</SPAN
> (compressed or
out-of-line) values.
</P
><P
> One big improvement in version-1 functions is better handling of null
inputs and results. The macro <CODE
CLASS="FUNCTION"
>PG_ARGISNULL(<VAR
CLASS="REPLACEABLE"
>n</VAR
>)</CODE
>
allows a function to test whether each input is null. (Of course, doing
this is only necessary in functions not declared <SPAN
CLASS="QUOTE"
>"strict"</SPAN
>.)
As with the
<CODE
CLASS="FUNCTION"
>PG_GETARG_<VAR
CLASS="REPLACEABLE"
>xxx</VAR
>()</CODE
> macros,
the input arguments are counted beginning at zero. Note that one
should refrain from executing
<CODE
CLASS="FUNCTION"
>PG_GETARG_<VAR
CLASS="REPLACEABLE"
>xxx</VAR
>()</CODE
> until
one has verified that the argument isn't null.
To return a null result, execute <CODE
CLASS="FUNCTION"
>PG_RETURN_NULL()</CODE
>;
this works in both strict and nonstrict functions.
</P
><P
> Other options provided in the new-style interface are two
variants of the
<CODE
CLASS="FUNCTION"
>PG_GETARG_<VAR
CLASS="REPLACEABLE"
>xxx</VAR
>()</CODE
>
macros. The first of these,
<CODE
CLASS="FUNCTION"
>PG_GETARG_<VAR
CLASS="REPLACEABLE"
>xxx</VAR
>_COPY()</CODE
>,
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
<CODE
CLASS="FUNCTION"
>PG_GETARG_<VAR
CLASS="REPLACEABLE"
>xxx</VAR
>_COPY()</CODE
>
macros guarantees a writable result.)
The second variant consists of the
<CODE
CLASS="FUNCTION"
>PG_GETARG_<VAR
CLASS="REPLACEABLE"
>xxx</VAR
>_SLICE()</CODE
>
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
<SPAN
CLASS="QUOTE"
>"external"</SPAN
>. (The storage type of a column can be specified using
<TT
CLASS="LITERAL"
>ALTER TABLE <VAR
CLASS="REPLACEABLE"
>tablename</VAR
> ALTER
COLUMN <VAR
CLASS="REPLACEABLE"
>colname</VAR
> SET STORAGE
<VAR
CLASS="REPLACEABLE"
>storagetype</VAR
></TT
>. <VAR
CLASS="REPLACEABLE"
>storagetype</VAR
> is one of
<TT
CLASS="LITERAL"
>plain</TT
>, <TT
CLASS="LITERAL"
>external</TT
>, <TT
CLASS="LITERAL"
>extended</TT
>,
or <TT
CLASS="LITERAL"
>main</TT
>.)
</P
><P
> Finally, the version-1 function call conventions make it possible
to return set results (<A
HREF="xfunc-c.html#XFUNC-C-RETURN-SET"
>Section 33.7.9</A
>) and
implement trigger functions (<A
HREF="triggers.html"
>Chapter 35</A
>) and
procedural-language call handlers (<A
HREF="plhandler.html"
>Chapter 47</A
>). 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 <TT
CLASS="FILENAME"
>src/backend/utils/fmgr/README</TT
> in the
source distribution.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN29325"
>33.7.5. Writing Code</A
></H2
><P
> Before we turn to the more advanced topics, we should discuss
some coding rules for <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
C-language functions. While it may be possible to load functions
written in languages other than C into
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>, 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.
</P
><P
> The basic rules for writing and building C functions are as follows:
<P
></P
></P><UL
><LI
><P
> Use <TT
CLASS="LITERAL"
>pg_config
--includedir-server</TT
><A
NAME="AEN29335"
></A
>
to find out where the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> server header
files are installed on your system (or the system that your
users will be running on). This option is new with
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 7.2. For
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 7.1 you should use the option
<VAR
CLASS="OPTION"
>--includedir</VAR
>. (<TT
CLASS="COMMAND"
>pg_config</TT
>
will exit with a non-zero status if it encounters an unknown
option.) For releases prior to 7.1 you will have to guess,
but since that was before the current calling conventions were
introduced, it is unlikely that you want to support those
releases.
</P
></LI
><LI
><P
> When allocating memory, use the
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> functions
<CODE
CLASS="FUNCTION"
>palloc</CODE
><A
NAME="AEN29347"
></A
> and <CODE
CLASS="FUNCTION"
>pfree</CODE
><A
NAME="AEN29350"
></A
>
instead of the corresponding C library functions
<CODE
CLASS="FUNCTION"
>malloc</CODE
> and <CODE
CLASS="FUNCTION"
>free</CODE
>.
The memory allocated by <CODE
CLASS="FUNCTION"
>palloc</CODE
> will be
freed automatically at the end of each transaction, preventing
memory leaks.
</P
></LI
><LI
><P
> Always zero the bytes of your structures using
<CODE
CLASS="FUNCTION"
>memset</CODE
>. 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.
</P
></LI
><LI
><P
> Most of the internal <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
types are declared in <TT
CLASS="FILENAME"
>postgres.h</TT
>, while
the function manager interfaces
(<TT
CLASS="SYMBOL"
>PG_FUNCTION_ARGS</TT
>, etc.) are in
<TT
CLASS="FILENAME"
>fmgr.h</TT
>, so you will need to include at
least these two files. For portability reasons it's best to
include <TT
CLASS="FILENAME"
>postgres.h</TT
> <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>first</I
></SPAN
>,
before any other system or user header files. Including
<TT
CLASS="FILENAME"
>postgres.h</TT
> will also include
<TT
CLASS="FILENAME"
>elog.h</TT
> and <TT
CLASS="FILENAME"
>palloc.h</TT
>
for you.
</P
></LI
><LI
><P
> Symbol names defined within object files must not conflict
with each other or with symbols defined in the
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> server executable. You
will have to rename your functions or variables if you get
error messages to this effect.
</P
></LI
><LI
><P
> Compiling and linking your code so that it can be dynamically
loaded into <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> always
requires special flags. See <A
HREF="xfunc-c.html#DFUNC"
>Section 33.7.6</A
> for a
detailed explanation of how to do it for your particular
operating system.
</P
></LI
></UL
><P>
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="DFUNC"
>33.7.6. Compiling and Linking Dynamically-Loaded Functions</A
></H2
><P
> Before you are able to use your
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 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
<I
CLASS="FIRSTTERM"
>shared library</I
> needs to be
created.<A
NAME="AEN29381"
></A
>
</P
><P
> 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,
<TT
CLASS="COMMAND"
>cc</TT
>, and the link editor, <TT
CLASS="COMMAND"
>ld</TT
>.
In addition, the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> source code
contains several working examples in the
<TT
CLASS="FILENAME"
>contrib</TT
> directory. If you rely on these
examples you will make your modules dependent on the availability
of the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> source code, however.
</P
><P
> <A
NAME="AEN29390"
></A
> 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
<I
CLASS="FIRSTTERM"
>position-independent code</I
>
(<ACRONYM
CLASS="ACRONYM"
>PIC</ACRONYM
>),<A
NAME="AEN29394"
></A
> 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 this is the
theory. On some systems the practice is much uglier.
</P
><P
> In the following examples we assume that your source code is in a
file <TT
CLASS="FILENAME"
>foo.c</TT
> and we will create a shared library
<TT
CLASS="FILENAME"
>foo.so</TT
>. The intermediate object file will be
called <TT
CLASS="FILENAME"
>foo.o</TT
> unless otherwise noted. A shared
library can contain more than one object file, but we only use one
here.
</P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><SPAN
CLASS="SYSTEMITEM"
>BSD/OS</SPAN
></DT
><DD
><P
> The compiler flag to create <ACRONYM
CLASS="ACRONYM"
>PIC</ACRONYM
> is
<VAR
CLASS="OPTION"
>-fpic</VAR
>. The linker flag to create shared
libraries is <VAR
CLASS="OPTION"
>-shared</VAR
>.
</P><PRE
CLASS="PROGRAMLISTING"
>gcc -fpic -c foo.c
ld -shared -o foo.so foo.o</PRE
><P>
This is applicable as of version 4.0 of
<SPAN
CLASS="SYSTEMITEM"
>BSD/OS</SPAN
>.
</P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>FreeBSD</SPAN
></DT
><DD
><P
> The compiler flag to create <ACRONYM
CLASS="ACRONYM"
>PIC</ACRONYM
> is
<VAR
CLASS="OPTION"
>-fpic</VAR
>. To create shared libraries the compiler
flag is <VAR
CLASS="OPTION"
>-shared</VAR
>.
</P><PRE
CLASS="PROGRAMLISTING"
>gcc -fpic -c foo.c
gcc -shared -o foo.so foo.o</PRE
><P>
This is applicable as of version 3.0 of
<SPAN
CLASS="SYSTEMITEM"
>FreeBSD</SPAN
>.
</P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>HP-UX</SPAN
></DT
><DD
><P
> The compiler flag of the system compiler to create
<ACRONYM
CLASS="ACRONYM"
>PIC</ACRONYM
> is <VAR
CLASS="OPTION"
>+z</VAR
>. When using
<SPAN
CLASS="APPLICATION"
>GCC</SPAN
> it's <VAR
CLASS="OPTION"
>-fpic</VAR
>. The
linker flag for shared libraries is <VAR
CLASS="OPTION"
>-b</VAR
>. So
</P><PRE
CLASS="PROGRAMLISTING"
>cc +z -c foo.c</PRE
><P>
or
</P><PRE
CLASS="PROGRAMLISTING"
>gcc -fpic -c foo.c</PRE
><P>
and then
</P><PRE
CLASS="PROGRAMLISTING"
>ld -b -o foo.sl foo.o</PRE
><P>
<SPAN
CLASS="SYSTEMITEM"
>HP-UX</SPAN
> uses the extension
<TT
CLASS="FILENAME"
>.sl</TT
> for shared libraries, unlike most other
systems.
</P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>IRIX</SPAN
></DT
><DD
><P
> <ACRONYM
CLASS="ACRONYM"
>PIC</ACRONYM
> is the default, no special compiler
options are necessary. The linker option to produce shared
libraries is <VAR
CLASS="OPTION"
>-shared</VAR
>.
</P><PRE
CLASS="PROGRAMLISTING"
>cc -c foo.c
ld -shared -o foo.so foo.o</PRE
><P>
</P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>Linux</SPAN
></DT
><DD
><P
> The compiler flag to create <ACRONYM
CLASS="ACRONYM"
>PIC</ACRONYM
> is
<VAR
CLASS="OPTION"
>-fpic</VAR
>. On some platforms in some situations
<VAR
CLASS="OPTION"
>-fPIC</VAR
> must be used if <VAR
CLASS="OPTION"
>-fpic</VAR
>
does not work. Refer to the GCC manual for more information.
The compiler flag to create a shared library is
<VAR
CLASS="OPTION"
>-shared</VAR
>. A complete example looks like this:
</P><PRE
CLASS="PROGRAMLISTING"
>cc -fpic -c foo.c
cc -shared -o foo.so foo.o</PRE
><P>
</P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>MacOS X</SPAN
></DT
><DD
><P
> Here is an example. It assumes the developer tools are installed.
</P><PRE
CLASS="PROGRAMLISTING"
>cc -c foo.c
cc -bundle -flat_namespace -undefined suppress -o foo.so foo.o</PRE
><P>
</P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>NetBSD</SPAN
></DT
><DD
><P
> The compiler flag to create <ACRONYM
CLASS="ACRONYM"
>PIC</ACRONYM
> is
<VAR
CLASS="OPTION"
>-fpic</VAR
>. For <ACRONYM
CLASS="ACRONYM"
>ELF</ACRONYM
> systems, the
compiler with the flag <VAR
CLASS="OPTION"
>-shared</VAR
> is used to link
shared libraries. On the older non-ELF systems, <TT
CLASS="LITERAL"
>ld
-Bshareable</TT
> is used.
</P><PRE
CLASS="PROGRAMLISTING"
>gcc -fpic -c foo.c
gcc -shared -o foo.so foo.o</PRE
><P>
</P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>OpenBSD</SPAN
></DT
><DD
><P
> The compiler flag to create <ACRONYM
CLASS="ACRONYM"
>PIC</ACRONYM
> is
<VAR
CLASS="OPTION"
>-fpic</VAR
>. <TT
CLASS="LITERAL"
>ld -Bshareable</TT
> is
used to link shared libraries.
</P><PRE
CLASS="PROGRAMLISTING"
>gcc -fpic -c foo.c
ld -Bshareable -o foo.so foo.o</PRE
><P>
</P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>Solaris</SPAN
></DT
><DD
><P
> The compiler flag to create <ACRONYM
CLASS="ACRONYM"
>PIC</ACRONYM
> is
<VAR
CLASS="OPTION"
>-KPIC</VAR
> with the Sun compiler and
<VAR
CLASS="OPTION"
>-fpic</VAR
> with <SPAN
CLASS="APPLICATION"
>GCC</SPAN
>. To
link shared libraries, the compiler option is
<VAR
CLASS="OPTION"
>-G</VAR
> with either compiler or alternatively
<VAR
CLASS="OPTION"
>-shared</VAR
> with <SPAN
CLASS="APPLICATION"
>GCC</SPAN
>.
</P><PRE
CLASS="PROGRAMLISTING"
>cc -KPIC -c foo.c
cc -G -o foo.so foo.o</PRE
><P>
or
</P><PRE
CLASS="PROGRAMLISTING"
>gcc -fpic -c foo.c
gcc -G -o foo.so foo.o</PRE
><P>
</P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>Tru64 UNIX</SPAN
></DT
><DD
><P
> <ACRONYM
CLASS="ACRONYM"
>PIC</ACRONYM
> is the default, so the compilation command
is the usual one. <TT
CLASS="COMMAND"
>ld</TT
> with special options is
used to do the linking:
</P><PRE
CLASS="PROGRAMLISTING"
>cc -c foo.c
ld -shared -expect_unresolved '*' -o foo.so foo.o</PRE
><P>
The same procedure is used with GCC instead of the system
compiler; no special options are required.
</P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>UnixWare</SPAN
></DT
><DD
><P
> The compiler flag to create <ACRONYM
CLASS="ACRONYM"
>PIC</ACRONYM
> is <VAR
CLASS="OPTION"
>-K
PIC</VAR
> with the SCO compiler and <VAR
CLASS="OPTION"
>-fpic</VAR
>
with <SPAN
CLASS="PRODUCTNAME"
>GCC</SPAN
>. To link shared libraries,
the compiler option is <VAR
CLASS="OPTION"
>-G</VAR
> with the SCO compiler
and <VAR
CLASS="OPTION"
>-shared</VAR
> with
<SPAN
CLASS="PRODUCTNAME"
>GCC</SPAN
>.
</P><PRE
CLASS="PROGRAMLISTING"
>cc -K PIC -c foo.c
cc -G -o foo.so foo.o</PRE
><P>
or
</P><PRE
CLASS="PROGRAMLISTING"
>gcc -fpic -c foo.c
gcc -shared -o foo.so foo.o</PRE
><P>
</P
></DD
></DL
></DIV
><DIV
CLASS="TIP"
><BLOCKQUOTE
CLASS="TIP"
><P
><B
>Tip: </B
> If this is too complicated for you, you should consider using
<A
HREF="http://www.gnu.org/software/libtool/"
TARGET="_top"
><SPAN
CLASS="PRODUCTNAME"
>GNU
Libtool</SPAN
></A
>, which hides the platform differences
behind a uniform interface.
</P
></BLOCKQUOTE
></DIV
><P
> The resulting shared library file can then be loaded into
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>. When specifying the file name
to the <TT
CLASS="COMMAND"
>CREATE FUNCTION</TT
> 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
<TT
CLASS="LITERAL"
>.so</TT
> or <TT
CLASS="LITERAL"
>.sl</TT
>) can be omitted from
the <TT
CLASS="COMMAND"
>CREATE FUNCTION</TT
> command, and normally should
be omitted for best portability.
</P
><P
> Refer back to <A
HREF="xfunc-c.html#XFUNC-C-DYNLOAD"
>Section 33.7.1</A
> about where the
server expects to find the shared library files.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN29565"
>33.7.7. Composite-Type Arguments in C-Language Functions</A
></H2
><P
> 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, <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> provides
a function interface for accessing fields of composite types
from C.
</P
><P
> Suppose we want to write a function to answer the query
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT name, c_overpaid(emp, 1500) AS overpaid
FROM emp
WHERE name = 'Bill' OR name = 'Sam';</PRE
><P>
Using call conventions version 0, we can define
<CODE
CLASS="FUNCTION"
>c_overpaid</CODE
> as:
</P><PRE
CLASS="PROGRAMLISTING"
>#include "postgres.h"
#include "executor/executor.h" /* for GetAttributeByName() */
bool
c_overpaid(TupleTableSlot *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;
}</PRE
><P>
In version-1 coding, the above would look like this:
</P><PRE
CLASS="PROGRAMLISTING"
>#include "postgres.h"
#include "executor/executor.h" /* for GetAttributeByName() */
PG_FUNCTION_INFO_V1(c_overpaid);
Datum
c_overpaid(PG_FUNCTION_ARGS)
{
TupleTableSlot *t = (TupleTableSlot *) PG_GETARG_POINTER(0);
int32 limit = PG_GETARG_INT32(1);
bool isnull;
int32 salary;
salary = DatumGetInt32(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(salary > limit);
}</PRE
><P>
</P
><P
> <CODE
CLASS="FUNCTION"
>GetAttributeByName</CODE
> is the
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> system function that
returns attributes out of the specified row. It has
three arguments: the argument of type <TT
CLASS="TYPE"
>TupleTableSlot*</TT
> passed into
the function, the name of the desired attribute, and a
return parameter that tells whether the attribute
is null. <CODE
CLASS="FUNCTION"
>GetAttributeByName</CODE
> returns a <TT
CLASS="TYPE"
>Datum</TT
>
value that you can convert to the proper data type by using the
appropriate <CODE
CLASS="FUNCTION"
>DatumGet<VAR
CLASS="REPLACEABLE"
>XXX</VAR
>()</CODE
> macro.
</P
><P
> The following command declares the function
<CODE
CLASS="FUNCTION"
>c_overpaid</CODE
> in SQL:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION c_overpaid(emp, integer) RETURNS boolean
AS '<VAR
CLASS="REPLACEABLE"
>DIRECTORY</VAR
>/funcs', 'c_overpaid'
LANGUAGE C;</PRE
><P>
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN29586"
>33.7.8. Returning Rows (Composite Types) from C-Language Functions</A
></H2
><P
> 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:
</P><PRE
CLASS="PROGRAMLISTING"
>#include "funcapi.h"</PRE
><P>
</P
><P
> The support for returning composite data types (or rows) starts
with the <TT
CLASS="STRUCTNAME"
>AttInMetadata</TT
> structure. This structure
holds arrays of individual attribute information needed to create
a row from raw C strings. The information contained in the
structure is derived from a <TT
CLASS="STRUCTNAME"
>TupleDesc</TT
> structure,
but it is stored to avoid redundant computations on each call to
a set-returning function (see next section). In the case of a
function returning a set, the <TT
CLASS="STRUCTNAME"
>AttInMetadata</TT
>
structure should be computed once during the first call and saved
for reuse in later calls. <TT
CLASS="STRUCTNAME"
>AttInMetadata</TT
> also
saves a pointer to the original <TT
CLASS="STRUCTNAME"
>TupleDesc</TT
>.
</P><PRE
CLASS="PROGRAMLISTING"
>typedef struct AttInMetadata
{
/* full TupleDesc */
TupleDesc tupdesc;
/* array of attribute type input function finfo */
FmgrInfo *attinfuncs;
/* array of attribute type typelem */
Oid *attelems;
/* array of attribute typmod */
int32 *atttypmods;
} AttInMetadata;</PRE
><P>
</P
><P
> To assist you in populating this structure, several functions and a macro
are available. Use
</P><PRE
CLASS="PROGRAMLISTING"
>TupleDesc RelationNameGetTupleDesc(const char *relname)</PRE
><P>
to get a <TT
CLASS="STRUCTNAME"
>TupleDesc</TT
> for a named relation, or
</P><PRE
CLASS="PROGRAMLISTING"
>TupleDesc TypeGetTupleDesc(Oid typeoid, List *colaliases)</PRE
><P>
to get a <TT
CLASS="STRUCTNAME"
>TupleDesc</TT
> based on a type OID. This can
be used to get a <TT
CLASS="STRUCTNAME"
>TupleDesc</TT
> for a base or
composite type. Then
</P><PRE
CLASS="PROGRAMLISTING"
>AttInMetadata *TupleDescGetAttInMetadata(TupleDesc tupdesc)</PRE
><P>
will return a pointer to an <TT
CLASS="STRUCTNAME"
>AttInMetadata</TT
>,
initialized based on the given
<TT
CLASS="STRUCTNAME"
>TupleDesc</TT
>. <TT
CLASS="STRUCTNAME"
>AttInMetadata</TT
> can be
used in conjunction with C strings to produce a properly formed
row value (internally called tuple).
</P
><P
> To return a tuple you must create a tuple slot based on the
<TT
CLASS="STRUCTNAME"
>TupleDesc</TT
>. You can use
</P><PRE
CLASS="PROGRAMLISTING"
>TupleTableSlot *TupleDescGetSlot(TupleDesc tupdesc)</PRE
><P>
to initialize this tuple slot, or obtain one through other (user provided)
means. The tuple slot is needed to create a <TT
CLASS="TYPE"
>Datum</TT
> for return by the
function. The same slot can (and should) be reused on each call.
</P
><P
> After constructing an <TT
CLASS="STRUCTNAME"
>AttInMetadata</TT
> structure,
</P><PRE
CLASS="PROGRAMLISTING"
>HeapTuple BuildTupleFromCStrings(AttInMetadata *attinmeta, char **values)</PRE
><P>
can be used to build a <TT
CLASS="STRUCTNAME"
>HeapTuple</TT
> given user data
in C string form. <TT
CLASS="LITERAL"
>values</TT
> 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 <VAR
CLASS="PARAMETER"
>values</VAR
> array
should be set to <TT
CLASS="SYMBOL"
>NULL</TT
>. This function will need to
be called again for each row you return.
</P
><P
> Building a tuple via <CODE
CLASS="FUNCTION"
>TupleDescGetAttInMetadata</CODE
> and
<CODE
CLASS="FUNCTION"
>BuildTupleFromCStrings</CODE
> is only convenient if your
function naturally computes the values to be returned as text
strings. If your code naturally computes the values as a set of
<TT
CLASS="TYPE"
>Datum</TT
> values, you should instead use the underlying
function <CODE
CLASS="FUNCTION"
>heap_formtuple</CODE
> to convert the
<TT
CLASS="TYPE"
>Datum</TT
> values directly into a tuple. You will still need
the <TT
CLASS="STRUCTNAME"
>TupleDesc</TT
> and a <TT
CLASS="STRUCTNAME"
>TupleTableSlot</TT
>,
but not <TT
CLASS="STRUCTNAME"
>AttInMetadata</TT
>.
</P
><P
> Once you have built a tuple to return from your function, it
must be converted into a <TT
CLASS="TYPE"
>Datum</TT
>. Use
</P><PRE
CLASS="PROGRAMLISTING"
>TupleGetDatum(TupleTableSlot *slot, HeapTuple tuple)</PRE
><P>
to get a <TT
CLASS="TYPE"
>Datum</TT
> given a tuple and a slot. This
<TT
CLASS="TYPE"
>Datum</TT
> 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.
</P
><P
> An example appears in the next section.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="XFUNC-C-RETURN-SET"
>33.7.9. Returning Sets from C-Language Functions</A
></H2
><P
> 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 <TT
CLASS="FILENAME"
>funcapi.h</TT
>, as
above.
</P
><P
> A set-returning function (<ACRONYM
CLASS="ACRONYM"
>SRF</ACRONYM
>) is called
once for each item it returns. The <ACRONYM
CLASS="ACRONYM"
>SRF</ACRONYM
> must
therefore save enough state to remember what it was doing and
return the next item on each call.
The structure <TT
CLASS="STRUCTNAME"
>FuncCallContext</TT
> is provided to help
control this process. Within a function, <TT
CLASS="LITERAL"
>fcinfo->flinfo->fn_extra</TT
>
is used to hold a pointer to <TT
CLASS="STRUCTNAME"
>FuncCallContext</TT
>
across calls.
</P><PRE
CLASS="PROGRAMLISTING"
>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
*
* slot is for use when returning tuples (i.e., composite data types)
* and is not needed when returning base data types.
*/
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 needed 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;
} FuncCallContext;</PRE
><P>
</P
><P
> An <ACRONYM
CLASS="ACRONYM"
>SRF</ACRONYM
> uses several functions and macros that
automatically manipulate the <TT
CLASS="STRUCTNAME"
>FuncCallContext</TT
>
structure (and expect to find it via <TT
CLASS="LITERAL"
>fn_extra</TT
>). Use
</P><PRE
CLASS="PROGRAMLISTING"
>SRF_IS_FIRSTCALL()</PRE
><P>
to determine if your function is being called for the first or a
subsequent time. On the first call (only) use
</P><PRE
CLASS="PROGRAMLISTING"
>SRF_FIRSTCALL_INIT()</PRE
><P>
to initialize the <TT
CLASS="STRUCTNAME"
>FuncCallContext</TT
>. On every function call,
including the first, use
</P><PRE
CLASS="PROGRAMLISTING"
>SRF_PERCALL_SETUP()</PRE
><P>
to properly set up for using the <TT
CLASS="STRUCTNAME"
>FuncCallContext</TT
>
and clearing any previously returned data left over from the
previous pass.
</P
><P
> If your function has data to return, use
</P><PRE
CLASS="PROGRAMLISTING"
>SRF_RETURN_NEXT(funcctx, result)</PRE
><P>
to return it to the caller. (<TT
CLASS="LITERAL"
>result</TT
> must be of type
<TT
CLASS="TYPE"
>Datum</TT
>, either a single value or a tuple prepared as
described above.) Finally, when your function is finished
returning data, use
</P><PRE
CLASS="PROGRAMLISTING"
>SRF_RETURN_DONE(funcctx)</PRE
><P>
to clean up and end the <ACRONYM
CLASS="ACRONYM"
>SRF</ACRONYM
>.
</P
><P
> The memory context that is current when the <ACRONYM
CLASS="ACRONYM"
>SRF</ACRONYM
> is called is
a transient context that will be cleared between calls. This means
that you do not need to call <CODE
CLASS="FUNCTION"
>pfree</CODE
> on everything
you allocated using <CODE
CLASS="FUNCTION"
>palloc</CODE
>; 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
<TT
CLASS="STRUCTFIELD"
>multi_call_memory_ctx</TT
> is a suitable location for any
data that needs to survive until the <ACRONYM
CLASS="ACRONYM"
>SRF</ACRONYM
> is finished running. In most
cases, this means that you should switch into
<TT
CLASS="STRUCTFIELD"
>multi_call_memory_ctx</TT
> while doing the first-call setup.
</P
><P
> A complete pseudo-code example looks like the following:
</P><PRE
CLASS="PROGRAMLISTING"
>Datum
my_set_returning_function(PG_FUNCTION_ARGS)
{
FuncCallContext *funcctx;
Datum result;
MemoryContext oldcontext;
<VAR
CLASS="REPLACEABLE"
>further declarations as needed</VAR
>
if (SRF_IS_FIRSTCALL())
{
funcctx = SRF_FIRSTCALL_INIT();
oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
/* One-time setup code appears here: */
<VAR
CLASS="REPLACEABLE"
>user code</VAR
>
<VAR
CLASS="REPLACEABLE"
>if returning composite</VAR
>
<VAR
CLASS="REPLACEABLE"
>build TupleDesc, and perhaps AttInMetadata</VAR
>
<VAR
CLASS="REPLACEABLE"
>obtain slot</VAR
>
funcctx->slot = slot;
<VAR
CLASS="REPLACEABLE"
>endif returning composite</VAR
>
<VAR
CLASS="REPLACEABLE"
>user code</VAR
>
MemoryContextSwitchTo(oldcontext);
}
/* Each-time setup code appears here: */
<VAR
CLASS="REPLACEABLE"
>user code</VAR
>
funcctx = SRF_PERCALL_SETUP();
<VAR
CLASS="REPLACEABLE"
>user code</VAR
>
/* 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: */
<VAR
CLASS="REPLACEABLE"
>user code</VAR
>
<VAR
CLASS="REPLACEABLE"
>obtain result Datum</VAR
>
SRF_RETURN_NEXT(funcctx, result);
}
else
{
/* Here we are done returning items and just need to clean up: */
<VAR
CLASS="REPLACEABLE"
>user code</VAR
>
SRF_RETURN_DONE(funcctx);
}
}</PRE
><P>
</P
><P
> A complete example of a simple <ACRONYM
CLASS="ACRONYM"
>SRF</ACRONYM
> returning a composite type looks like:
</P><PRE
CLASS="PROGRAMLISTING"
>PG_FUNCTION_INFO_V1(testpassbyval);
Datum
testpassbyval(PG_FUNCTION_ARGS)
{
FuncCallContext *funcctx;
int call_cntr;
int max_calls;
TupleDesc tupdesc;
TupleTableSlot *slot;
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 description for a __testpassbyval tuple */
tupdesc = RelationNameGetTupleDesc("__testpassbyval");
/* allocate a slot for a tuple with this tupdesc */
slot = TupleDescGetSlot(tupdesc);
/* assign slot to function context */
funcctx->slot = slot;
/*
* 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;
slot = funcctx->slot;
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 storage in our slot.
* 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 = TupleGetDatum(slot, 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);
}
}</PRE
><P>
The SQL code to declare this function is:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TYPE __testpassbyval AS (f1 integer, f2 integer, f3 integer);
CREATE OR REPLACE FUNCTION testpassbyval(integer, integer) RETURNS SETOF __testpassbyval
AS '<VAR
CLASS="REPLACEABLE"
>filename</VAR
>', 'testpassbyval'
LANGUAGE C IMMUTABLE STRICT;</PRE
><P>
</P
><P
> The directory <TT
CLASS="FILENAME"
>contrib/tablefunc</TT
> in the source
distribution contains more examples of set-returning functions.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN29687"
>33.7.10. Polymorphic Arguments and Return Types</A
></H2
><P
> C-language functions may be declared to accept and
return the polymorphic types
<TT
CLASS="TYPE"
>anyelement</TT
> and <TT
CLASS="TYPE"
>anyarray</TT
>.
See <A
HREF="extend-type-system.html#EXTEND-TYPES-POLYMORPHIC"
>Section 33.2.5</A
> 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 <TT
CLASS="FILENAME"
>fmgr.h</TT
>
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 <TT
CLASS="LITERAL"
>get_fn_expr_rettype(FmgrInfo *flinfo)</TT
> and
<TT
CLASS="LITERAL"
>get_fn_expr_argtype(FmgrInfo *flinfo, int argnum)</TT
>.
They return the result or argument type OID, or <TT
CLASS="SYMBOL"
>InvalidOid</TT
> if the
information is not available.
The structure <TT
CLASS="LITERAL"
>flinfo</TT
> is normally accessed as
<TT
CLASS="LITERAL"
>fcinfo->flinfo</TT
>. The parameter <TT
CLASS="LITERAL"
>argnum</TT
>
is zero based.
</P
><P
> 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:
</P><PRE
CLASS="PROGRAMLISTING"
>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;
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 */
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, ndims, dims, lbs,
element_type, typlen, typbyval, typalign);
PG_RETURN_ARRAYTYPE_P(result);
}</PRE
><P>
</P
><P
> The following command declares the function
<CODE
CLASS="FUNCTION"
>make_array</CODE
> in SQL:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION make_array(anyelement) RETURNS anyarray
AS '<VAR
CLASS="REPLACEABLE"
>DIRECTORY</VAR
>/funcs', 'make_array'
LANGUAGE C STRICT;</PRE
><P>
Note the use of <TT
CLASS="LITERAL"
>STRICT</TT
>; this is essential
since the code is not bothering to test for a null input.
</P
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