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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>36.4. Using Host Variables</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="ecpg-commands.html" title="36.3. Running SQL Commands" /><link rel="next" href="ecpg-dynamic.html" title="36.5. Dynamic SQL" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">36.4. Using Host Variables</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="ecpg-commands.html" title="36.3. Running SQL Commands">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="ecpg.html" title="Chapter 36. ECPG — Embedded SQL in C">Up</a></td><th width="60%" align="center">Chapter 36. <span class="application">ECPG</span> — Embedded <acronym class="acronym">SQL</acronym> in C</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.2 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="ecpg-dynamic.html" title="36.5. Dynamic SQL">Next</a></td></tr></table><hr /></div><div class="sect1" id="ECPG-VARIABLES"><div class="titlepage"><div><div><h2 class="title" style="clear: both">36.4. Using Host Variables <a href="#ECPG-VARIABLES" class="id_link">#</a></h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="ecpg-variables.html#ECPG-VARIABLES-OVERVIEW">36.4.1. Overview</a></span></dt><dt><span class="sect2"><a href="ecpg-variables.html#ECPG-DECLARE-SECTIONS">36.4.2. Declare Sections</a></span></dt><dt><span class="sect2"><a href="ecpg-variables.html#ECPG-RETRIEVING">36.4.3. Retrieving Query Results</a></span></dt><dt><span class="sect2"><a href="ecpg-variables.html#ECPG-VARIABLES-TYPE-MAPPING">36.4.4. Type Mapping</a></span></dt><dt><span class="sect2"><a href="ecpg-variables.html#ECPG-VARIABLES-NONPRIMITIVE-SQL">36.4.5. Handling Nonprimitive SQL Data Types</a></span></dt><dt><span class="sect2"><a href="ecpg-variables.html#ECPG-INDICATORS">36.4.6. Indicators</a></span></dt></dl></div><p>
In <a class="xref" href="ecpg-commands.html" title="36.3. Running SQL Commands">Section 36.3</a> you saw how you can execute SQL
statements from an embedded SQL program. Some of those statements
only used fixed values and did not provide a way to insert
user-supplied values into statements or have the program process
the values returned by the query. Those kinds of statements are
not really useful in real applications. This section explains in
detail how you can pass data between your C program and the
embedded SQL statements using a simple mechanism called
<em class="firstterm">host variables</em>. In an embedded SQL program we
consider the SQL statements to be <em class="firstterm">guests</em> in the C
program code which is the <em class="firstterm">host language</em>. Therefore
the variables of the C program are called <em class="firstterm">host
variables</em>.
</p><p>
Another way to exchange values between PostgreSQL backends and ECPG
applications is the use of SQL descriptors, described
in <a class="xref" href="ecpg-descriptors.html" title="36.7. Using Descriptor Areas">Section 36.7</a>.
</p><div class="sect2" id="ECPG-VARIABLES-OVERVIEW"><div class="titlepage"><div><div><h3 class="title">36.4.1. Overview <a href="#ECPG-VARIABLES-OVERVIEW" class="id_link">#</a></h3></div></div></div><p>
Passing data between the C program and the SQL statements is
particularly simple in embedded SQL. Instead of having the
program paste the data into the statement, which entails various
complications, such as properly quoting the value, you can simply
write the name of a C variable into the SQL statement, prefixed by
a colon. For example:
</p><pre class="programlisting">
EXEC SQL INSERT INTO sometable VALUES (:v1, 'foo', :v2);
</pre><p>
This statement refers to two C variables named
<code class="varname">v1</code> and <code class="varname">v2</code> and also uses a
regular SQL string literal, to illustrate that you are not
restricted to use one kind of data or the other.
</p><p>
This style of inserting C variables in SQL statements works
anywhere a value expression is expected in an SQL statement.
</p></div><div class="sect2" id="ECPG-DECLARE-SECTIONS"><div class="titlepage"><div><div><h3 class="title">36.4.2. Declare Sections <a href="#ECPG-DECLARE-SECTIONS" class="id_link">#</a></h3></div></div></div><p>
To pass data from the program to the database, for example as
parameters in a query, or to pass data from the database back to
the program, the C variables that are intended to contain this
data need to be declared in specially marked sections, so the
embedded SQL preprocessor is made aware of them.
</p><p>
This section starts with:
</p><pre class="programlisting">
EXEC SQL BEGIN DECLARE SECTION;
</pre><p>
and ends with:
</p><pre class="programlisting">
EXEC SQL END DECLARE SECTION;
</pre><p>
Between those lines, there must be normal C variable declarations,
such as:
</p><pre class="programlisting">
int x = 4;
char foo[16], bar[16];
</pre><p>
As you can see, you can optionally assign an initial value to the variable.
The variable's scope is determined by the location of its declaring
section within the program.
You can also declare variables with the following syntax which implicitly
creates a declare section:
</p><pre class="programlisting">
EXEC SQL int i = 4;
</pre><p>
You can have as many declare sections in a program as you like.
</p><p>
The declarations are also echoed to the output file as normal C
variables, so there's no need to declare them again. Variables
that are not intended to be used in SQL commands can be declared
normally outside these special sections.
</p><p>
The definition of a structure or union also must be listed inside
a <code class="literal">DECLARE</code> section. Otherwise the preprocessor cannot
handle these types since it does not know the definition.
</p></div><div class="sect2" id="ECPG-RETRIEVING"><div class="titlepage"><div><div><h3 class="title">36.4.3. Retrieving Query Results <a href="#ECPG-RETRIEVING" class="id_link">#</a></h3></div></div></div><p>
Now you should be able to pass data generated by your program into
an SQL command. But how do you retrieve the results of a query?
For that purpose, embedded SQL provides special variants of the
usual commands <code class="command">SELECT</code> and
<code class="command">FETCH</code>. These commands have a special
<code class="literal">INTO</code> clause that specifies which host variables
the retrieved values are to be stored in.
<code class="command">SELECT</code> is used for a query that returns only
single row, and <code class="command">FETCH</code> is used for a query that
returns multiple rows, using a cursor.
</p><p>
Here is an example:
</p><pre class="programlisting">
/*
* assume this table:
* CREATE TABLE test1 (a int, b varchar(50));
*/
EXEC SQL BEGIN DECLARE SECTION;
int v1;
VARCHAR v2;
EXEC SQL END DECLARE SECTION;
...
EXEC SQL SELECT a, b INTO :v1, :v2 FROM test;
</pre><p>
So the <code class="literal">INTO</code> clause appears between the select
list and the <code class="literal">FROM</code> clause. The number of
elements in the select list and the list after
<code class="literal">INTO</code> (also called the target list) must be
equal.
</p><p>
Here is an example using the command <code class="command">FETCH</code>:
</p><pre class="programlisting">
EXEC SQL BEGIN DECLARE SECTION;
int v1;
VARCHAR v2;
EXEC SQL END DECLARE SECTION;
...
EXEC SQL DECLARE foo CURSOR FOR SELECT a, b FROM test;
...
do
{
...
EXEC SQL FETCH NEXT FROM foo INTO :v1, :v2;
...
} while (...);
</pre><p>
Here the <code class="literal">INTO</code> clause appears after all the
normal clauses.
</p></div><div class="sect2" id="ECPG-VARIABLES-TYPE-MAPPING"><div class="titlepage"><div><div><h3 class="title">36.4.4. Type Mapping <a href="#ECPG-VARIABLES-TYPE-MAPPING" class="id_link">#</a></h3></div></div></div><p>
When ECPG applications exchange values between the PostgreSQL
server and the C application, such as when retrieving query
results from the server or executing SQL statements with input
parameters, the values need to be converted between PostgreSQL
data types and host language variable types (C language data
types, concretely). One of the main points of ECPG is that it
takes care of this automatically in most cases.
</p><p>
In this respect, there are two kinds of data types: Some simple
PostgreSQL data types, such as <code class="type">integer</code>
and <code class="type">text</code>, can be read and written by the application
directly. Other PostgreSQL data types, such
as <code class="type">timestamp</code> and <code class="type">numeric</code> can only be
accessed through special library functions; see
<a class="xref" href="ecpg-variables.html#ECPG-SPECIAL-TYPES" title="36.4.4.2. Accessing Special Data Types">Section 36.4.4.2</a>.
</p><p>
<a class="xref" href="ecpg-variables.html#ECPG-DATATYPE-HOSTVARS-TABLE" title="Table 36.1. Mapping Between PostgreSQL Data Types and C Variable Types">Table 36.1</a> shows which PostgreSQL
data types correspond to which C data types. When you wish to
send or receive a value of a given PostgreSQL data type, you
should declare a C variable of the corresponding C data type in
the declare section.
</p><div class="table" id="ECPG-DATATYPE-HOSTVARS-TABLE"><p class="title"><strong>Table 36.1. Mapping Between PostgreSQL Data Types and C Variable Types</strong></p><div class="table-contents"><table class="table" summary="Mapping Between PostgreSQL Data Types and C Variable Types" border="1"><colgroup><col /><col /></colgroup><thead><tr><th>PostgreSQL data type</th><th>Host variable type</th></tr></thead><tbody><tr><td><code class="type">smallint</code></td><td><code class="type">short</code></td></tr><tr><td><code class="type">integer</code></td><td><code class="type">int</code></td></tr><tr><td><code class="type">bigint</code></td><td><code class="type">long long int</code></td></tr><tr><td><code class="type">decimal</code></td><td><code class="type">decimal</code><a href="#ftn.ECPG-DATATYPE-TABLE-FN" class="footnote"><sup class="footnote" id="ECPG-DATATYPE-TABLE-FN">[a]</sup></a></td></tr><tr><td><code class="type">numeric</code></td><td><code class="type">numeric</code><a href="ecpg-variables.html#ftn.ECPG-DATATYPE-TABLE-FN" class="footnoteref"><sup class="footnoteref">[a]</sup></a></td></tr><tr><td><code class="type">real</code></td><td><code class="type">float</code></td></tr><tr><td><code class="type">double precision</code></td><td><code class="type">double</code></td></tr><tr><td><code class="type">smallserial</code></td><td><code class="type">short</code></td></tr><tr><td><code class="type">serial</code></td><td><code class="type">int</code></td></tr><tr><td><code class="type">bigserial</code></td><td><code class="type">long long int</code></td></tr><tr><td><code class="type">oid</code></td><td><code class="type">unsigned int</code></td></tr><tr><td><code class="type">character(<em class="replaceable"><code>n</code></em>)</code>, <code class="type">varchar(<em class="replaceable"><code>n</code></em>)</code>, <code class="type">text</code></td><td><code class="type">char[<em class="replaceable"><code>n</code></em>+1]</code>, <code class="type">VARCHAR[<em class="replaceable"><code>n</code></em>+1]</code></td></tr><tr><td><code class="type">name</code></td><td><code class="type">char[NAMEDATALEN]</code></td></tr><tr><td><code class="type">timestamp</code></td><td><code class="type">timestamp</code><a href="ecpg-variables.html#ftn.ECPG-DATATYPE-TABLE-FN" class="footnoteref"><sup class="footnoteref">[a]</sup></a></td></tr><tr><td><code class="type">interval</code></td><td><code class="type">interval</code><a href="ecpg-variables.html#ftn.ECPG-DATATYPE-TABLE-FN" class="footnoteref"><sup class="footnoteref">[a]</sup></a></td></tr><tr><td><code class="type">date</code></td><td><code class="type">date</code><a href="ecpg-variables.html#ftn.ECPG-DATATYPE-TABLE-FN" class="footnoteref"><sup class="footnoteref">[a]</sup></a></td></tr><tr><td><code class="type">boolean</code></td><td><code class="type">bool</code><a href="#ftn.id-1.7.5.10.7.5.2.2.17.2.2" class="footnote"><sup class="footnote" id="id-1.7.5.10.7.5.2.2.17.2.2">[b]</sup></a></td></tr><tr><td><code class="type">bytea</code></td><td><code class="type">char *</code>, <code class="type">bytea[<em class="replaceable"><code>n</code></em>]</code></td></tr></tbody><tbody class="footnotes"><tr><td colspan="2"><div id="ftn.ECPG-DATATYPE-TABLE-FN" class="footnote"><p><a href="#ECPG-DATATYPE-TABLE-FN" class="para"><sup class="para">[a] </sup></a>This type can only be accessed through special library functions; see <a class="xref" href="ecpg-variables.html#ECPG-SPECIAL-TYPES" title="36.4.4.2. Accessing Special Data Types">Section 36.4.4.2</a>.</p></div><div id="ftn.id-1.7.5.10.7.5.2.2.17.2.2" class="footnote"><p><a href="#id-1.7.5.10.7.5.2.2.17.2.2" class="para"><sup class="para">[b] </sup></a>declared in <code class="filename">ecpglib.h</code> if not native</p></div></td></tr></tbody></table></div></div><br class="table-break" /><div class="sect3" id="ECPG-CHAR"><div class="titlepage"><div><div><h4 class="title">36.4.4.1. Handling Character Strings <a href="#ECPG-CHAR" class="id_link">#</a></h4></div></div></div><p>
To handle SQL character string data types, such
as <code class="type">varchar</code> and <code class="type">text</code>, there are two
possible ways to declare the host variables.
</p><p>
One way is using <code class="type">char[]</code>, an array
of <code class="type">char</code>, which is the most common way to handle
character data in C.
</p><pre class="programlisting">
EXEC SQL BEGIN DECLARE SECTION;
char str[50];
EXEC SQL END DECLARE SECTION;
</pre><p>
Note that you have to take care of the length yourself. If you
use this host variable as the target variable of a query which
returns a string with more than 49 characters, a buffer overflow
occurs.
</p><p>
The other way is using the <code class="type">VARCHAR</code> type, which is a
special type provided by ECPG. The definition on an array of
type <code class="type">VARCHAR</code> is converted into a
named <code class="type">struct</code> for every variable. A declaration like:
</p><pre class="programlisting">
VARCHAR var[180];
</pre><p>
is converted into:
</p><pre class="programlisting">
struct varchar_var { int len; char arr[180]; } var;
</pre><p>
The member <code class="structfield">arr</code> hosts the string
including a terminating zero byte. Thus, to store a string in
a <code class="type">VARCHAR</code> host variable, the host variable has to be
declared with the length including the zero byte terminator. The
member <code class="structfield">len</code> holds the length of the
string stored in the <code class="structfield">arr</code> without the
terminating zero byte. When a host variable is used as input for
a query, if <code class="literal">strlen(arr)</code>
and <code class="structfield">len</code> are different, the shorter one
is used.
</p><p>
<code class="type">VARCHAR</code> can be written in upper or lower case, but
not in mixed case.
</p><p>
<code class="type">char</code> and <code class="type">VARCHAR</code> host variables can
also hold values of other SQL types, which will be stored in
their string forms.
</p></div><div class="sect3" id="ECPG-SPECIAL-TYPES"><div class="titlepage"><div><div><h4 class="title">36.4.4.2. Accessing Special Data Types <a href="#ECPG-SPECIAL-TYPES" class="id_link">#</a></h4></div></div></div><p>
ECPG contains some special types that help you to interact easily
with some special data types from the PostgreSQL server. In
particular, it has implemented support for the
<code class="type">numeric</code>, <code class="type">decimal</code>, <code class="type">date</code>, <code class="type">timestamp</code>,
and <code class="type">interval</code> types. These data types cannot usefully be
mapped to primitive host variable types (such
as <code class="type">int</code>, <code class="type">long long int</code>,
or <code class="type">char[]</code>), because they have a complex internal
structure. Applications deal with these types by declaring host
variables in special types and accessing them using functions in
the pgtypes library. The pgtypes library, described in detail
in <a class="xref" href="ecpg-pgtypes.html" title="36.6. pgtypes Library">Section 36.6</a> contains basic functions to deal
with those types, such that you do not need to send a query to
the SQL server just for adding an interval to a time stamp for
example.
</p><p>
The follow subsections describe these special data types. For
more details about pgtypes library functions,
see <a class="xref" href="ecpg-pgtypes.html" title="36.6. pgtypes Library">Section 36.6</a>.
</p><div class="sect4" id="ECPG-SPECIAL-TYPES-TIMESTAMP-DATE"><div class="titlepage"><div><div><h5 class="title">36.4.4.2.1. timestamp, date <a href="#ECPG-SPECIAL-TYPES-TIMESTAMP-DATE" class="id_link">#</a></h5></div></div></div><p>
Here is a pattern for handling <code class="type">timestamp</code> variables
in the ECPG host application.
</p><p>
First, the program has to include the header file for the
<code class="type">timestamp</code> type:
</p><pre class="programlisting">
#include <pgtypes_timestamp.h>
</pre><p>
</p><p>
Next, declare a host variable as type <code class="type">timestamp</code> in
the declare section:
</p><pre class="programlisting">
EXEC SQL BEGIN DECLARE SECTION;
timestamp ts;
EXEC SQL END DECLARE SECTION;
</pre><p>
</p><p>
And after reading a value into the host variable, process it
using pgtypes library functions. In following example, the
<code class="type">timestamp</code> value is converted into text (ASCII) form
with the <code class="function">PGTYPEStimestamp_to_asc()</code>
function:
</p><pre class="programlisting">
EXEC SQL SELECT now()::timestamp INTO :ts;
printf("ts = %s\n", PGTYPEStimestamp_to_asc(ts));
</pre><p>
This example will show some result like following:
</p><pre class="screen">
ts = 2010-06-27 18:03:56.949343
</pre><p>
</p><p>
In addition, the DATE type can be handled in the same way. The
program has to include <code class="filename">pgtypes_date.h</code>, declare a host variable
as the date type and convert a DATE value into a text form using
<code class="function">PGTYPESdate_to_asc()</code> function. For more details about the
pgtypes library functions, see <a class="xref" href="ecpg-pgtypes.html" title="36.6. pgtypes Library">Section 36.6</a>.
</p></div><div class="sect4" id="ECPG-TYPE-INTERVAL"><div class="titlepage"><div><div><h5 class="title">36.4.4.2.2. interval <a href="#ECPG-TYPE-INTERVAL" class="id_link">#</a></h5></div></div></div><p>
The handling of the <code class="type">interval</code> type is also similar
to the <code class="type">timestamp</code> and <code class="type">date</code> types. It
is required, however, to allocate memory for
an <code class="type">interval</code> type value explicitly. In other words,
the memory space for the variable has to be allocated in the
heap memory, not in the stack memory.
</p><p>
Here is an example program:
</p><pre class="programlisting">
#include <stdio.h>
#include <stdlib.h>
#include <pgtypes_interval.h>
int
main(void)
{
EXEC SQL BEGIN DECLARE SECTION;
interval *in;
EXEC SQL END DECLARE SECTION;
EXEC SQL CONNECT TO testdb;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
in = PGTYPESinterval_new();
EXEC SQL SELECT '1 min'::interval INTO :in;
printf("interval = %s\n", PGTYPESinterval_to_asc(in));
PGTYPESinterval_free(in);
EXEC SQL COMMIT;
EXEC SQL DISCONNECT ALL;
return 0;
}
</pre><p>
</p></div><div class="sect4" id="ECPG-TYPE-NUMERIC-DECIMAL"><div class="titlepage"><div><div><h5 class="title">36.4.4.2.3. numeric, decimal <a href="#ECPG-TYPE-NUMERIC-DECIMAL" class="id_link">#</a></h5></div></div></div><p>
The handling of the <code class="type">numeric</code>
and <code class="type">decimal</code> types is similar to the
<code class="type">interval</code> type: It requires defining a pointer,
allocating some memory space on the heap, and accessing the
variable using the pgtypes library functions. For more details
about the pgtypes library functions,
see <a class="xref" href="ecpg-pgtypes.html" title="36.6. pgtypes Library">Section 36.6</a>.
</p><p>
No functions are provided specifically for
the <code class="type">decimal</code> type. An application has to convert it
to a <code class="type">numeric</code> variable using a pgtypes library
function to do further processing.
</p><p>
Here is an example program handling <code class="type">numeric</code>
and <code class="type">decimal</code> type variables.
</p><pre class="programlisting">
#include <stdio.h>
#include <stdlib.h>
#include <pgtypes_numeric.h>
EXEC SQL WHENEVER SQLERROR STOP;
int
main(void)
{
EXEC SQL BEGIN DECLARE SECTION;
numeric *num;
numeric *num2;
decimal *dec;
EXEC SQL END DECLARE SECTION;
EXEC SQL CONNECT TO testdb;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
num = PGTYPESnumeric_new();
dec = PGTYPESdecimal_new();
EXEC SQL SELECT 12.345::numeric(4,2), 23.456::decimal(4,2) INTO :num, :dec;
printf("numeric = %s\n", PGTYPESnumeric_to_asc(num, 0));
printf("numeric = %s\n", PGTYPESnumeric_to_asc(num, 1));
printf("numeric = %s\n", PGTYPESnumeric_to_asc(num, 2));
/* Convert decimal to numeric to show a decimal value. */
num2 = PGTYPESnumeric_new();
PGTYPESnumeric_from_decimal(dec, num2);
printf("decimal = %s\n", PGTYPESnumeric_to_asc(num2, 0));
printf("decimal = %s\n", PGTYPESnumeric_to_asc(num2, 1));
printf("decimal = %s\n", PGTYPESnumeric_to_asc(num2, 2));
PGTYPESnumeric_free(num2);
PGTYPESdecimal_free(dec);
PGTYPESnumeric_free(num);
EXEC SQL COMMIT;
EXEC SQL DISCONNECT ALL;
return 0;
}
</pre><p>
</p></div><div class="sect4" id="ECPG-SPECIAL-TYPES-BYTEA"><div class="titlepage"><div><div><h5 class="title">36.4.4.2.4. bytea <a href="#ECPG-SPECIAL-TYPES-BYTEA" class="id_link">#</a></h5></div></div></div><p>
The handling of the <code class="type">bytea</code> type is similar to
that of <code class="type">VARCHAR</code>. The definition on an array of type
<code class="type">bytea</code> is converted into a named struct for every
variable. A declaration like:
</p><pre class="programlisting">
bytea var[180];
</pre><p>
is converted into:
</p><pre class="programlisting">
struct bytea_var { int len; char arr[180]; } var;
</pre><p>
The member <code class="structfield">arr</code> hosts binary format
data. It can also handle <code class="literal">'\0'</code> as part of
data, unlike <code class="type">VARCHAR</code>.
The data is converted from/to hex format and sent/received by
ecpglib.
</p><div class="note"><h3 class="title">Note</h3><p>
<code class="type">bytea</code> variable can be used only when
<a class="xref" href="runtime-config-client.html#GUC-BYTEA-OUTPUT">bytea_output</a> is set to <code class="literal">hex</code>.
</p></div></div></div><div class="sect3" id="ECPG-VARIABLES-NONPRIMITIVE-C"><div class="titlepage"><div><div><h4 class="title">36.4.4.3. Host Variables with Nonprimitive Types <a href="#ECPG-VARIABLES-NONPRIMITIVE-C" class="id_link">#</a></h4></div></div></div><p>
As a host variable you can also use arrays, typedefs, structs, and
pointers.
</p><div class="sect4" id="ECPG-VARIABLES-ARRAYS"><div class="titlepage"><div><div><h5 class="title">36.4.4.3.1. Arrays <a href="#ECPG-VARIABLES-ARRAYS" class="id_link">#</a></h5></div></div></div><p>
There are two use cases for arrays as host variables. The first
is a way to store some text string in <code class="type">char[]</code>
or <code class="type">VARCHAR[]</code>, as
explained in <a class="xref" href="ecpg-variables.html#ECPG-CHAR" title="36.4.4.1. Handling Character Strings">Section 36.4.4.1</a>. The second use case is to
retrieve multiple rows from a query result without using a
cursor. Without an array, to process a query result consisting
of multiple rows, it is required to use a cursor and
the <code class="command">FETCH</code> command. But with array host
variables, multiple rows can be received at once. The length of
the array has to be defined to be able to accommodate all rows,
otherwise a buffer overflow will likely occur.
</p><p>
Following example scans the <code class="literal">pg_database</code>
system table and shows all OIDs and names of the available
databases:
</p><pre class="programlisting">
int
main(void)
{
EXEC SQL BEGIN DECLARE SECTION;
int dbid[8];
char dbname[8][16];
int i;
EXEC SQL END DECLARE SECTION;
memset(dbname, 0, sizeof(char)* 16 * 8);
memset(dbid, 0, sizeof(int) * 8);
EXEC SQL CONNECT TO testdb;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
/* Retrieve multiple rows into arrays at once. */
EXEC SQL SELECT oid,datname INTO :dbid, :dbname FROM pg_database;
for (i = 0; i < 8; i++)
printf("oid=%d, dbname=%s\n", dbid[i], dbname[i]);
EXEC SQL COMMIT;
EXEC SQL DISCONNECT ALL;
return 0;
}
</pre><p>
This example shows following result. (The exact values depend on
local circumstances.)
</p><pre class="screen">
oid=1, dbname=template1
oid=11510, dbname=template0
oid=11511, dbname=postgres
oid=313780, dbname=testdb
oid=0, dbname=
oid=0, dbname=
oid=0, dbname=
</pre><p>
</p></div><div class="sect4" id="ECPG-VARIABLES-STRUCT"><div class="titlepage"><div><div><h5 class="title">36.4.4.3.2. Structures <a href="#ECPG-VARIABLES-STRUCT" class="id_link">#</a></h5></div></div></div><p>
A structure whose member names match the column names of a query
result, can be used to retrieve multiple columns at once. The
structure enables handling multiple column values in a single
host variable.
</p><p>
The following example retrieves OIDs, names, and sizes of the
available databases from the <code class="literal">pg_database</code>
system table and using
the <code class="function">pg_database_size()</code> function. In this
example, a structure variable <code class="varname">dbinfo_t</code> with
members whose names match each column in
the <code class="literal">SELECT</code> result is used to retrieve one
result row without putting multiple host variables in
the <code class="literal">FETCH</code> statement.
</p><pre class="programlisting">
EXEC SQL BEGIN DECLARE SECTION;
typedef struct
{
int oid;
char datname[65];
long long int size;
} dbinfo_t;
dbinfo_t dbval;
EXEC SQL END DECLARE SECTION;
memset(&dbval, 0, sizeof(dbinfo_t));
EXEC SQL DECLARE cur1 CURSOR FOR SELECT oid, datname, pg_database_size(oid) AS size FROM pg_database;
EXEC SQL OPEN cur1;
/* when end of result set reached, break out of while loop */
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
/* Fetch multiple columns into one structure. */
EXEC SQL FETCH FROM cur1 INTO :dbval;
/* Print members of the structure. */
printf("oid=%d, datname=%s, size=%lld\n", dbval.oid, dbval.datname, dbval.size);
}
EXEC SQL CLOSE cur1;
</pre><p>
</p><p>
This example shows following result. (The exact values depend on
local circumstances.)
</p><pre class="screen">
oid=1, datname=template1, size=4324580
oid=11510, datname=template0, size=4243460
oid=11511, datname=postgres, size=4324580
oid=313780, datname=testdb, size=8183012
</pre><p>
</p><p>
Structure host variables <span class="quote">“<span class="quote">absorb</span>”</span> as many columns
as the structure as fields. Additional columns can be assigned
to other host variables. For example, the above program could
also be restructured like this, with the <code class="varname">size</code>
variable outside the structure:
</p><pre class="programlisting">
EXEC SQL BEGIN DECLARE SECTION;
typedef struct
{
int oid;
char datname[65];
} dbinfo_t;
dbinfo_t dbval;
long long int size;
EXEC SQL END DECLARE SECTION;
memset(&dbval, 0, sizeof(dbinfo_t));
EXEC SQL DECLARE cur1 CURSOR FOR SELECT oid, datname, pg_database_size(oid) AS size FROM pg_database;
EXEC SQL OPEN cur1;
/* when end of result set reached, break out of while loop */
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
/* Fetch multiple columns into one structure. */
EXEC SQL FETCH FROM cur1 INTO :dbval, :size;
/* Print members of the structure. */
printf("oid=%d, datname=%s, size=%lld\n", dbval.oid, dbval.datname, size);
}
EXEC SQL CLOSE cur1;
</pre><p>
</p></div><div class="sect4" id="ECPG-VARIABLES-NONPRIMITIVE-C-TYPEDEFS"><div class="titlepage"><div><div><h5 class="title">36.4.4.3.3. Typedefs <a href="#ECPG-VARIABLES-NONPRIMITIVE-C-TYPEDEFS" class="id_link">#</a></h5></div></div></div><a id="id-1.7.5.10.7.8.5.2" class="indexterm"></a><p>
Use the <code class="literal">typedef</code> keyword to map new types to already
existing types.
</p><pre class="programlisting">
EXEC SQL BEGIN DECLARE SECTION;
typedef char mychartype[40];
typedef long serial_t;
EXEC SQL END DECLARE SECTION;
</pre><p>
Note that you could also use:
</p><pre class="programlisting">
EXEC SQL TYPE serial_t IS long;
</pre><p>
This declaration does not need to be part of a declare section;
that is, you can also write typedefs as normal C statements.
</p><p>
Any word you declare as a typedef cannot be used as an SQL keyword
in <code class="literal">EXEC SQL</code> commands later in the same program.
For example, this won't work:
</p><pre class="programlisting">
EXEC SQL BEGIN DECLARE SECTION;
typedef int start;
EXEC SQL END DECLARE SECTION;
...
EXEC SQL START TRANSACTION;
</pre><p>
ECPG will report a syntax error for <code class="literal">START
TRANSACTION</code>, because it no longer
recognizes <code class="literal">START</code> as an SQL keyword,
only as a typedef.
(If you have such a conflict, and renaming the typedef
seems impractical, you could write the SQL command
using <a class="link" href="ecpg-dynamic.html" title="36.5. Dynamic SQL">dynamic SQL</a>.)
</p><div class="note"><h3 class="title">Note</h3><p>
In <span class="productname">PostgreSQL</span> releases before v16, use
of SQL keywords as typedef names was likely to result in syntax
errors associated with use of the typedef itself, rather than use
of the name as an SQL keyword. The new behavior is less likely to
cause problems when an existing ECPG application is recompiled in
a new <span class="productname">PostgreSQL</span> release with new
keywords.
</p></div></div><div class="sect4" id="ECPG-VARIABLES-NONPRIMITIVE-C-POINTERS"><div class="titlepage"><div><div><h5 class="title">36.4.4.3.4. Pointers <a href="#ECPG-VARIABLES-NONPRIMITIVE-C-POINTERS" class="id_link">#</a></h5></div></div></div><p>
You can declare pointers to the most common types. Note however
that you cannot use pointers as target variables of queries
without auto-allocation. See <a class="xref" href="ecpg-descriptors.html" title="36.7. Using Descriptor Areas">Section 36.7</a>
for more information on auto-allocation.
</p><p>
</p><pre class="programlisting">
EXEC SQL BEGIN DECLARE SECTION;
int *intp;
char **charp;
EXEC SQL END DECLARE SECTION;
</pre><p>
</p></div></div></div><div class="sect2" id="ECPG-VARIABLES-NONPRIMITIVE-SQL"><div class="titlepage"><div><div><h3 class="title">36.4.5. Handling Nonprimitive SQL Data Types <a href="#ECPG-VARIABLES-NONPRIMITIVE-SQL" class="id_link">#</a></h3></div></div></div><p>
This section contains information on how to handle nonscalar and
user-defined SQL-level data types in ECPG applications. Note that
this is distinct from the handling of host variables of
nonprimitive types, described in the previous section.
</p><div class="sect3" id="ECPG-VARIABLES-NONPRIMITIVE-SQL-ARRAYS"><div class="titlepage"><div><div><h4 class="title">36.4.5.1. Arrays <a href="#ECPG-VARIABLES-NONPRIMITIVE-SQL-ARRAYS" class="id_link">#</a></h4></div></div></div><p>
Multi-dimensional SQL-level arrays are not directly supported in ECPG.
One-dimensional SQL-level arrays can be mapped into C array host
variables and vice-versa. However, when creating a statement ecpg does
not know the types of the columns, so that it cannot check if a C array
is input into a corresponding SQL-level array. When processing the
output of an SQL statement, ecpg has the necessary information and thus
checks if both are arrays.
</p><p>
If a query accesses <span class="emphasis"><em>elements</em></span> of an array
separately, then this avoids the use of arrays in ECPG. Then, a
host variable with a type that can be mapped to the element type
should be used. For example, if a column type is array of
<code class="type">integer</code>, a host variable of type <code class="type">int</code>
can be used. Also if the element type is <code class="type">varchar</code>
or <code class="type">text</code>, a host variable of type <code class="type">char[]</code>
or <code class="type">VARCHAR[]</code> can be used.
</p><p>
Here is an example. Assume the following table:
</p><pre class="programlisting">
CREATE TABLE t3 (
ii integer[]
);
testdb=> SELECT * FROM t3;
ii
-------------
{1,2,3,4,5}
(1 row)
</pre><p>
The following example program retrieves the 4th element of the
array and stores it into a host variable of
type <code class="type">int</code>:
</p><pre class="programlisting">
EXEC SQL BEGIN DECLARE SECTION;
int ii;
EXEC SQL END DECLARE SECTION;
EXEC SQL DECLARE cur1 CURSOR FOR SELECT ii[4] FROM t3;
EXEC SQL OPEN cur1;
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
EXEC SQL FETCH FROM cur1 INTO :ii ;
printf("ii=%d\n", ii);
}
EXEC SQL CLOSE cur1;
</pre><p>
This example shows the following result:
</p><pre class="screen">
ii=4
</pre><p>
</p><p>
To map multiple array elements to the multiple elements in an
array type host variables each element of array column and each
element of the host variable array have to be managed separately,
for example:
</p><pre class="programlisting">
EXEC SQL BEGIN DECLARE SECTION;
int ii_a[8];
EXEC SQL END DECLARE SECTION;
EXEC SQL DECLARE cur1 CURSOR FOR SELECT ii[1], ii[2], ii[3], ii[4] FROM t3;
EXEC SQL OPEN cur1;
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
EXEC SQL FETCH FROM cur1 INTO :ii_a[0], :ii_a[1], :ii_a[2], :ii_a[3];
...
}
</pre><p>
</p><p>
Note again that
</p><pre class="programlisting">
EXEC SQL BEGIN DECLARE SECTION;
int ii_a[8];
EXEC SQL END DECLARE SECTION;
EXEC SQL DECLARE cur1 CURSOR FOR SELECT ii FROM t3;
EXEC SQL OPEN cur1;
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
/* WRONG */
EXEC SQL FETCH FROM cur1 INTO :ii_a;
...
}
</pre><p>
would not work correctly in this case, because you cannot map an
array type column to an array host variable directly.
</p><p>
Another workaround is to store arrays in their external string
representation in host variables of type <code class="type">char[]</code>
or <code class="type">VARCHAR[]</code>. For more details about this
representation, see <a class="xref" href="arrays.html#ARRAYS-INPUT" title="8.15.2. Array Value Input">Section 8.15.2</a>. Note that
this means that the array cannot be accessed naturally as an
array in the host program (without further processing that parses
the text representation).
</p></div><div class="sect3" id="ECPG-VARIABLES-NONPRIMITIVE-SQL-COMPOSITE"><div class="titlepage"><div><div><h4 class="title">36.4.5.2. Composite Types <a href="#ECPG-VARIABLES-NONPRIMITIVE-SQL-COMPOSITE" class="id_link">#</a></h4></div></div></div><p>
Composite types are not directly supported in ECPG, but an easy workaround is possible.
The
available workarounds are similar to the ones described for
arrays above: Either access each attribute separately or use the
external string representation.
</p><p>
For the following examples, assume the following type and table:
</p><pre class="programlisting">
CREATE TYPE comp_t AS (intval integer, textval varchar(32));
CREATE TABLE t4 (compval comp_t);
INSERT INTO t4 VALUES ( (256, 'PostgreSQL') );
</pre><p>
The most obvious solution is to access each attribute separately.
The following program retrieves data from the example table by
selecting each attribute of the type <code class="type">comp_t</code>
separately:
</p><pre class="programlisting">
EXEC SQL BEGIN DECLARE SECTION;
int intval;
varchar textval[33];
EXEC SQL END DECLARE SECTION;
/* Put each element of the composite type column in the SELECT list. */
EXEC SQL DECLARE cur1 CURSOR FOR SELECT (compval).intval, (compval).textval FROM t4;
EXEC SQL OPEN cur1;
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
/* Fetch each element of the composite type column into host variables. */
EXEC SQL FETCH FROM cur1 INTO :intval, :textval;
printf("intval=%d, textval=%s\n", intval, textval.arr);
}
EXEC SQL CLOSE cur1;
</pre><p>
</p><p>
To enhance this example, the host variables to store values in
the <code class="command">FETCH</code> command can be gathered into one
structure. For more details about the host variable in the
structure form, see <a class="xref" href="ecpg-variables.html#ECPG-VARIABLES-STRUCT" title="36.4.4.3.2. Structures">Section 36.4.4.3.2</a>.
To switch to the structure, the example can be modified as below.
The two host variables, <code class="varname">intval</code>
and <code class="varname">textval</code>, become members of
the <code class="structname">comp_t</code> structure, and the structure
is specified on the <code class="command">FETCH</code> command.
</p><pre class="programlisting">
EXEC SQL BEGIN DECLARE SECTION;
typedef struct
{
int intval;
varchar textval[33];
} comp_t;
comp_t compval;
EXEC SQL END DECLARE SECTION;
/* Put each element of the composite type column in the SELECT list. */
EXEC SQL DECLARE cur1 CURSOR FOR SELECT (compval).intval, (compval).textval FROM t4;
EXEC SQL OPEN cur1;
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
/* Put all values in the SELECT list into one structure. */
EXEC SQL FETCH FROM cur1 INTO :compval;
printf("intval=%d, textval=%s\n", compval.intval, compval.textval.arr);
}
EXEC SQL CLOSE cur1;
</pre><p>
Although a structure is used in the <code class="command">FETCH</code>
command, the attribute names in the <code class="command">SELECT</code>
clause are specified one by one. This can be enhanced by using
a <code class="literal">*</code> to ask for all attributes of the composite
type value.
</p><pre class="programlisting">
...
EXEC SQL DECLARE cur1 CURSOR FOR SELECT (compval).* FROM t4;
EXEC SQL OPEN cur1;
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
/* Put all values in the SELECT list into one structure. */
EXEC SQL FETCH FROM cur1 INTO :compval;
printf("intval=%d, textval=%s\n", compval.intval, compval.textval.arr);
}
...
</pre><p>
This way, composite types can be mapped into structures almost
seamlessly, even though ECPG does not understand the composite
type itself.
</p><p>
Finally, it is also possible to store composite type values in
their external string representation in host variables of
type <code class="type">char[]</code> or <code class="type">VARCHAR[]</code>. But that
way, it is not easily possible to access the fields of the value
from the host program.
</p></div><div class="sect3" id="ECPG-VARIABLES-NONPRIMITIVE-SQL-USER-DEFINED-BASE-TYPES"><div class="titlepage"><div><div><h4 class="title">36.4.5.3. User-Defined Base Types <a href="#ECPG-VARIABLES-NONPRIMITIVE-SQL-USER-DEFINED-BASE-TYPES" class="id_link">#</a></h4></div></div></div><p>
New user-defined base types are not directly supported by ECPG.
You can use the external string representation and host variables
of type <code class="type">char[]</code> or <code class="type">VARCHAR[]</code>, and this
solution is indeed appropriate and sufficient for many types.
</p><p>
Here is an example using the data type <code class="type">complex</code> from
the example in <a class="xref" href="xtypes.html" title="38.13. User-Defined Types">Section 38.13</a>. The external string
representation of that type is <code class="literal">(%f,%f)</code>,
which is defined in the
functions <code class="function">complex_in()</code>
and <code class="function">complex_out()</code> functions
in <a class="xref" href="xtypes.html" title="38.13. User-Defined Types">Section 38.13</a>. The following example inserts the
complex type values <code class="literal">(1,1)</code>
and <code class="literal">(3,3)</code> into the
columns <code class="literal">a</code> and <code class="literal">b</code>, and select
them from the table after that.
</p><pre class="programlisting">
EXEC SQL BEGIN DECLARE SECTION;
varchar a[64];
varchar b[64];
EXEC SQL END DECLARE SECTION;
EXEC SQL INSERT INTO test_complex VALUES ('(1,1)', '(3,3)');
EXEC SQL DECLARE cur1 CURSOR FOR SELECT a, b FROM test_complex;
EXEC SQL OPEN cur1;
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
EXEC SQL FETCH FROM cur1 INTO :a, :b;
printf("a=%s, b=%s\n", a.arr, b.arr);
}
EXEC SQL CLOSE cur1;
</pre><p>
This example shows following result:
</p><pre class="screen">
a=(1,1), b=(3,3)
</pre><p>
</p><p>
Another workaround is avoiding the direct use of the user-defined
types in ECPG and instead create a function or cast that converts
between the user-defined type and a primitive type that ECPG can
handle. Note, however, that type casts, especially implicit
ones, should be introduced into the type system very carefully.
</p><p>
For example,
</p><pre class="programlisting">
CREATE FUNCTION create_complex(r double, i double) RETURNS complex
LANGUAGE SQL
IMMUTABLE
AS $$ SELECT $1 * complex '(1,0')' + $2 * complex '(0,1)' $$;
</pre><p>
After this definition, the following
</p><pre class="programlisting">
EXEC SQL BEGIN DECLARE SECTION;
double a, b, c, d;
EXEC SQL END DECLARE SECTION;
a = 1;
b = 2;
c = 3;
d = 4;
EXEC SQL INSERT INTO test_complex VALUES (create_complex(:a, :b), create_complex(:c, :d));
</pre><p>
has the same effect as
</p><pre class="programlisting">
EXEC SQL INSERT INTO test_complex VALUES ('(1,2)', '(3,4)');
</pre><p>
</p></div></div><div class="sect2" id="ECPG-INDICATORS"><div class="titlepage"><div><div><h3 class="title">36.4.6. Indicators <a href="#ECPG-INDICATORS" class="id_link">#</a></h3></div></div></div><p>
The examples above do not handle null values. In fact, the
retrieval examples will raise an error if they fetch a null value
from the database. To be able to pass null values to the database
or retrieve null values from the database, you need to append a
second host variable specification to each host variable that
contains data. This second host variable is called the
<em class="firstterm">indicator</em> and contains a flag that tells
whether the datum is null, in which case the value of the real
host variable is ignored. Here is an example that handles the
retrieval of null values correctly:
</p><pre class="programlisting">
EXEC SQL BEGIN DECLARE SECTION;
VARCHAR val;
int val_ind;
EXEC SQL END DECLARE SECTION:
...
EXEC SQL SELECT b INTO :val :val_ind FROM test1;
</pre><p>
The indicator variable <code class="varname">val_ind</code> will be zero if
the value was not null, and it will be negative if the value was
null. (See <a class="xref" href="ecpg-oracle-compat.html" title="36.16. Oracle Compatibility Mode">Section 36.16</a> to enable
Oracle-specific behavior.)
</p><p>
The indicator has another function: if the indicator value is
positive, it means that the value is not null, but it was
truncated when it was stored in the host variable.
</p><p>
If the argument <code class="literal">-r no_indicator</code> is passed to
the preprocessor <code class="command">ecpg</code>, it works in
<span class="quote">“<span class="quote">no-indicator</span>”</span> mode. In no-indicator mode, if no
indicator variable is specified, null values are signaled (on
input and output) for character string types as empty string and
for integer types as the lowest possible value for type (for
example, <code class="symbol">INT_MIN</code> for <code class="type">int</code>).
</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ecpg-commands.html" title="36.3. Running SQL Commands">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ecpg.html" title="Chapter 36. ECPG — Embedded SQL in C">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ecpg-dynamic.html" title="36.5. Dynamic SQL">Next</a></td></tr><tr><td width="40%" align="left" valign="top">36.3. Running SQL Commands </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.2 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 36.5. Dynamic SQL</td></tr></table></div></body></html>
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