PostgreSQL Programmer's Guide
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# Compiled (C) Language Functions

Functions written in C can be compiled into dynamically loadable objects, and used to implement user-defined SQL functions. The first time the user defined function is called inside the backend, the dynamic loader loads the function's object code into memory, and links the function with the running Postgres executable. The SQL syntax for CREATE FUNCTION links the SQL function to the C source function in one of two ways. If the SQL function has the same name as the C source function the first form of the statement is used. The string argument in the AS clause is the full pathname of the file that contains the dynamically loadable compiled object. If the name of the C function is different from the desired name of the SQL function, then the second form is used. In this form the AS clause takes two string arguments, the first is the full pathname of the dynamically loadable object file, and the second is the link symbol that the dynamic loader should search for. This link symbol is just the function name in the C source code.

> Note: After it is used for the first time, a dynamically loaded, user function is retained in memory, and future calls to the function only incur the small overhead of a symbol table lookup.

The string which specifies the object file (the string in the AS clause) should be the full path of the object code file for the function, bracketed by quotation marks. If a link symbol is used in the AS clause, the link symbol should also be bracketed by single quotation marks, and should be exactly the same as the name of the function in the C source code. On Unix systems the command nm will print all of the link symbols in a dynamically loadable object. (Postgres will not compile a function automatically; it must be compiled before it is used in a CREATE FUNCTION command. See below for additional information.)

## C Language Functions on Base Types

The following table gives the C type required for parameters in the C functions that will be loaded into Postgres. The "Defined In" column gives the actual header file (in the `.../src/backend/` directory) that the equivalent C type is defined. However, if you include `utils/builtins.h`, these files will automatically be included.

Table 4-1. Equivalent C Types for Built-In Postgres Types

Built-In Type | C Type | Defined In
--- | --- | ---
abstime | AbsoluteTime | utils/nabstime.h
bool | bool | include/c.h
box | (BOX *) | utils/geo-decls.h
bytea | (bytea *) | include/postgres.h
char | char | N/A
cid | CID | include/postgres.h
datetime | (DateTime *) | include/c.h or include/postgres.h
int2 | int2 | include/postgres.h
int2vector | (int2vector *) | include/postgres.h
int4 | int4 | include/postgres.h
float4 | float32 or (float4 *) | include/c.h or include/postgres.h
float8 | float64 or (float8 *) | include/c.h or include/postgres.h
lseg | (LSEG *) | include/geo-decls.h
name | (Name) | include/postgres.h
oid | oid | include/postgres.h
oidvector | (oidvector *) | include/postgres.h
path | (PATH *) | utils/geo-decls.h
point | (POINT *) | utils/geo-decls.h
regproc | regproc or REGPROC | include/postgres.h
reltime | RelativeTime | utils/nabstime.h
text | (text *) | include/postgres.h
tid | ItemPointer | storage/itemptr.h
timespan | (TimeSpan *) | include/c.h or include/postgres.h
tinterval | TimeInterval | utils/nabstime.h
uint2 | uint16 | include/c.h
uint4 | uint32 | include/c.h
xid | (XID *) | include/postgres.h

Internally, Postgres 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 Postgres can operate on it. That is, Postgres 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 (even if your computer supports by-value types of other sizes). Postgres itself only passes integer types by value. 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 (though not on most personal computers). 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 Postgres 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 Postgres functions. 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 is the total length of the structure (i.e., it includes the size of the length field itself). We can define the text type as follows:

    typedef struct {
        int4 length;
        char data[1];
    } text;

Obviously, the data field is not long enough to hold all possible strings; it's impossible to declare such a structure in C. When manipulating variable-length types, we must be careful to allocate the correct amount of memory and initialize the length field. 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;
    memmove(destination->data, buffer, 40);
    ...

Now that we've gone over all of the possible structures for base types, we can show some examples of real functions. Suppose `funcs.c` look like:

    #include <string.h>
    #include "postgres.h"

    /* By Value */

    int
    add_one(int arg)
    {
        return(arg + 1);
    }

    /* By Reference, Fixed Length */

    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));
        memset(new_t, 0, VARSIZE(t));
        VARSIZE(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);

        memset((void *) new_text, 0, new_text_size);
        VARSIZE(new_text) = new_text_size;
        strncpy(VARDATA(new_text), VARDATA(arg1), VARSIZE(arg1)-VARHDRSZ);
        strncat(VARDATA(new_text), VARDATA(arg2), VARSIZE(arg2)-VARHDRSZ);
        return (new_text);
    }

On OSF/1 we would type:

    CREATE FUNCTION add_one(int4) RETURNS int4
         AS 'PGROOT/tutorial/funcs.so' LANGUAGE 'c';

    CREATE FUNCTION makepoint(point, point) RETURNS point
         AS 'PGROOT/tutorial/funcs.so' LANGUAGE 'c';

    CREATE FUNCTION concat_text(text, text) RETURNS text
         AS 'PGROOT/tutorial/funcs.so' LANGUAGE 'c';

    CREATE FUNCTION copytext(text) RETURNS text
         AS 'PGROOT/tutorial/funcs.so' LANGUAGE 'c';

On other systems, we might have to make the filename end in .sl (to indicate that it's a shared library).

## C Language Functions on Composite Types

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, Postgres provides a procedural interface for accessing fields of composite types from C. As Postgres processes a set of instances, each instance will be passed into your function as an opaque structure of type `TUPLE`. 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';

In the query above, we can define c_overpaid as:

    #include "postgres.h"
    #include "executor/executor.h"  /* for GetAttributeByName() */

    bool
    c_overpaid(TupleTableSlot *t, /* the current instance of EMP */
               int4 limit)
    {
        bool isnull = false;
        int4 salary;
        salary = (int4) GetAttributeByName(t, "salary", &isnull);
        if (isnull)
            return (false);
        return(salary > limit);
    }

`GetAttributeByName` is the Postgres system function that returns attributes out of the current instance. It has three arguments: the argument of type TUPLE passed into the function, the name of the desired attribute, and a return parameter that describes whether the attribute is null. `GetAttributeByName` will align data properly so you can cast its return value to the desired type. For example, if you have an attribute name which is of the type name, the `GetAttributeByName` call would look like:

    char *str;
    ...
    str = (char *) GetAttributeByName(t, "name", &isnull)

The following query lets Postgres know about the c_overpaid function:

    * CREATE FUNCTION c_overpaid(EMP, int4) RETURNS bool
         AS 'PGROOT/tutorial/obj/funcs.so' LANGUAGE 'c';

While there are ways to construct new instances or modify existing instances from within a C function, these are far too complex to discuss in this manual.

## Writing Code

We now turn to the more difficult task of writing programming language functions. Be warned: this section of the manual will not make you a programmer. You must have a good understanding of C (including the use of pointers and the malloc memory manager) before trying to write C functions for use with Postgres. While it may be possible to load functions written in languages other than C into Postgres, this is often difficult (when it is possible at all) because other languages, such as FORTRAN and 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 programming language functions are written in C.

C functions with base type arguments can be written in a straightforward fashion. The C equivalents of built-in Postgres types are accessible in a C file if ``PGROOT`/src/backend/utils/builtins.h` is included as a header file. This can be achieved by having

    #include <utils/builtins.h>

at the top of the C source file.

The basic rules for building C functions are as follows:

- Most of the header (include) files for Postgres should already be installed in ``PGROOT`/include` (see Figure 2). You should always include

      -I$PGROOT/include

  on your cc command lines. Sometimes, you may find that you require header files that are in the server source itself (i.e., you need a file we neglected to install in include). In those cases you may need to add one or more of

      -I$PGROOT/src/backend
      -I$PGROOT/src/backend/include
      -I$PGROOT/src/backend/port/<PORTNAME>
      -I$PGROOT/src/backend/obj

  (where <PORTNAME> is the name of the port, e.g., alpha or sparc).

- When allocating memory, use the Postgres routines palloc and pfree instead of the corresponding C library routines malloc and free. The memory allocated by palloc will be freed automatically at the end of each transaction, preventing memory leaks.

- Always zero the bytes of your structures using memset or bzero. Several routines (such as the hash access method, hash join and the sort algorithm) compute functions of the raw bits contained in your structure. Even if you initialize all fields of your structure, there may be several bytes of alignment padding (holes in the structure) that may contain garbage values.

- Most of the internal Postgres types are declared in `postgres.h`, so it's a good idea to always include that file as well. Including postgres.h will also include elog.h and palloc.h for you.

- Compiling and loading your object code so that it can be dynamically loaded into Postgres always requires special flags. See Linking Dynamically-Loaded Functions for a detailed explanation of how to do it for your particular operating system.

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