????
Your IP : 18.221.172.197
<?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>36.2. Managing Database Connections</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-concept.html" title="36.1. The Concept" /><link rel="next" href="ecpg-commands.html" title="36.3. Running SQL Commands" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">36.2. Managing Database Connections</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="ecpg-concept.html" title="36.1. The Concept">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 16.3 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="ecpg-commands.html" title="36.3. Running SQL Commands">Next</a></td></tr></table><hr /></div><div class="sect1" id="ECPG-CONNECT"><div class="titlepage"><div><div><h2 class="title" style="clear: both">36.2. Managing Database Connections <a href="#ECPG-CONNECT" class="id_link">#</a></h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="ecpg-connect.html#ECPG-CONNECTING">36.2.1. Connecting to the Database Server</a></span></dt><dt><span class="sect2"><a href="ecpg-connect.html#ECPG-SET-CONNECTION">36.2.2. Choosing a Connection</a></span></dt><dt><span class="sect2"><a href="ecpg-connect.html#ECPG-DISCONNECT">36.2.3. Closing a Connection</a></span></dt></dl></div><p>
This section describes how to open, close, and switch database
connections.
</p><div class="sect2" id="ECPG-CONNECTING"><div class="titlepage"><div><div><h3 class="title">36.2.1. Connecting to the Database Server <a href="#ECPG-CONNECTING" class="id_link">#</a></h3></div></div></div><p>
One connects to a database using the following statement:
</p><pre class="programlisting">
EXEC SQL CONNECT TO <em class="replaceable"><code>target</code></em> [<span class="optional">AS <em class="replaceable"><code>connection-name</code></em></span>] [<span class="optional">USER <em class="replaceable"><code>user-name</code></em></span>];
</pre><p>
The <em class="replaceable"><code>target</code></em> can be specified in the
following ways:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
<code class="literal"><em class="replaceable"><code>dbname</code></em>[<span class="optional">@<em class="replaceable"><code>hostname</code></em></span>][<span class="optional">:<em class="replaceable"><code>port</code></em></span>]</code>
</li><li class="listitem">
<code class="literal">tcp:postgresql://<em class="replaceable"><code>hostname</code></em>[<span class="optional">:<em class="replaceable"><code>port</code></em></span>][<span class="optional">/<em class="replaceable"><code>dbname</code></em></span>][<span class="optional">?<em class="replaceable"><code>options</code></em></span>]</code>
</li><li class="listitem">
<code class="literal">unix:postgresql://localhost[<span class="optional">:<em class="replaceable"><code>port</code></em></span>][<span class="optional">/<em class="replaceable"><code>dbname</code></em></span>][<span class="optional">?<em class="replaceable"><code>options</code></em></span>]</code>
</li><li class="listitem">
an SQL string literal containing one of the above forms
</li><li class="listitem">
a reference to a character variable containing one of the above forms (see examples)
</li><li class="listitem">
<code class="literal">DEFAULT</code>
</li></ul></div><p>
The connection target <code class="literal">DEFAULT</code> initiates a connection
to the default database under the default user name. No separate
user name or connection name can be specified in that case.
</p><p>
If you specify the connection target directly (that is, not as a string
literal or variable reference), then the components of the target are
passed through normal SQL parsing; this means that, for example,
the <em class="replaceable"><code>hostname</code></em> must look like one or more SQL
identifiers separated by dots, and those identifiers will be
case-folded unless double-quoted. Values of
any <em class="replaceable"><code>options</code></em> must be SQL identifiers,
integers, or variable references. Of course, you can put nearly
anything into an SQL identifier by double-quoting it.
In practice, it is probably less error-prone to use a (single-quoted)
string literal or a variable reference than to write the connection
target directly.
</p><p>
There are also different ways to specify the user name:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
<code class="literal"><em class="replaceable"><code>username</code></em></code>
</li><li class="listitem">
<code class="literal"><em class="replaceable"><code>username</code></em>/<em class="replaceable"><code>password</code></em></code>
</li><li class="listitem">
<code class="literal"><em class="replaceable"><code>username</code></em> IDENTIFIED BY <em class="replaceable"><code>password</code></em></code>
</li><li class="listitem">
<code class="literal"><em class="replaceable"><code>username</code></em> USING <em class="replaceable"><code>password</code></em></code>
</li></ul></div><p>
As above, the parameters <em class="replaceable"><code>username</code></em> and
<em class="replaceable"><code>password</code></em> can be an SQL identifier, an
SQL string literal, or a reference to a character variable.
</p><p>
If the connection target includes any <em class="replaceable"><code>options</code></em>,
those consist of
<code class="literal"><em class="replaceable"><code>keyword</code></em>=<em class="replaceable"><code>value</code></em></code>
specifications separated by ampersands (<code class="literal">&</code>).
The allowed key words are the same ones recognized
by <span class="application">libpq</span> (see
<a class="xref" href="libpq-connect.html#LIBPQ-PARAMKEYWORDS" title="34.1.2. Parameter Key Words">Section 34.1.2</a>). Spaces are ignored before
any <em class="replaceable"><code>keyword</code></em> or <em class="replaceable"><code>value</code></em>,
though not within or after one. Note that there is no way to
write <code class="literal">&</code> within a <em class="replaceable"><code>value</code></em>.
</p><p>
Notice that when specifying a socket connection
(with the <code class="literal">unix:</code> prefix), the host name must be
exactly <code class="literal">localhost</code>. To select a non-default
socket directory, write the directory's pathname as the value of
a <code class="varname">host</code> option in
the <em class="replaceable"><code>options</code></em> part of the target.
</p><p>
The <em class="replaceable"><code>connection-name</code></em> is used to handle
multiple connections in one program. It can be omitted if a
program uses only one connection. The most recently opened
connection becomes the current connection, which is used by default
when an SQL statement is to be executed (see later in this
chapter).
</p><p>
Here are some examples of <code class="command">CONNECT</code> statements:
</p><pre class="programlisting">
EXEC SQL CONNECT TO mydb@sql.mydomain.com;
EXEC SQL CONNECT TO tcp:postgresql://sql.mydomain.com/mydb AS myconnection USER john;
EXEC SQL BEGIN DECLARE SECTION;
const char *target = "mydb@sql.mydomain.com";
const char *user = "john";
const char *passwd = "secret";
EXEC SQL END DECLARE SECTION;
...
EXEC SQL CONNECT TO :target USER :user USING :passwd;
/* or EXEC SQL CONNECT TO :target USER :user/:passwd; */
</pre><p>
The last example makes use of the feature referred to above as
character variable references. You will see in later sections how C
variables can be used in SQL statements when you prefix them with a
colon.
</p><p>
Be advised that the format of the connection target is not
specified in the SQL standard. So if you want to develop portable
applications, you might want to use something based on the last
example above to encapsulate the connection target string
somewhere.
</p><p>
If untrusted users have access to a database that has not adopted a
<a class="link" href="ddl-schemas.html#DDL-SCHEMAS-PATTERNS" title="5.9.6. Usage Patterns">secure schema usage pattern</a>,
begin each session by removing publicly-writable schemas
from <code class="varname">search_path</code>. For example,
add <code class="literal">options=-c search_path=</code>
to <code class="literal"><em class="replaceable"><code>options</code></em></code>, or
issue <code class="literal">EXEC SQL SELECT pg_catalog.set_config('search_path', '',
false);</code> after connecting. This consideration is not specific to
ECPG; it applies to every interface for executing arbitrary SQL commands.
</p></div><div class="sect2" id="ECPG-SET-CONNECTION"><div class="titlepage"><div><div><h3 class="title">36.2.2. Choosing a Connection <a href="#ECPG-SET-CONNECTION" class="id_link">#</a></h3></div></div></div><p>
SQL statements in embedded SQL programs are by default executed on
the current connection, that is, the most recently opened one. If
an application needs to manage multiple connections, then there are
three ways to handle this.
</p><p>
The first option is to explicitly choose a connection for each SQL
statement, for example:
</p><pre class="programlisting">
EXEC SQL AT <em class="replaceable"><code>connection-name</code></em> SELECT ...;
</pre><p>
This option is particularly suitable if the application needs to
use several connections in mixed order.
</p><p>
If your application uses multiple threads of execution, they cannot share a
connection concurrently. You must either explicitly control access to the connection
(using mutexes) or use a connection for each thread.
</p><p>
The second option is to execute a statement to switch the current
connection. That statement is:
</p><pre class="programlisting">
EXEC SQL SET CONNECTION <em class="replaceable"><code>connection-name</code></em>;
</pre><p>
This option is particularly convenient if many statements are to be
executed on the same connection.
</p><p>
Here is an example program managing multiple database connections:
</p><pre class="programlisting">
#include <stdio.h>
EXEC SQL BEGIN DECLARE SECTION;
char dbname[1024];
EXEC SQL END DECLARE SECTION;
int
main()
{
EXEC SQL CONNECT TO testdb1 AS con1 USER testuser;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
EXEC SQL CONNECT TO testdb2 AS con2 USER testuser;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
EXEC SQL CONNECT TO testdb3 AS con3 USER testuser;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
/* This query would be executed in the last opened database "testdb3". */
EXEC SQL SELECT current_database() INTO :dbname;
printf("current=%s (should be testdb3)\n", dbname);
/* Using "AT" to run a query in "testdb2" */
EXEC SQL AT con2 SELECT current_database() INTO :dbname;
printf("current=%s (should be testdb2)\n", dbname);
/* Switch the current connection to "testdb1". */
EXEC SQL SET CONNECTION con1;
EXEC SQL SELECT current_database() INTO :dbname;
printf("current=%s (should be testdb1)\n", dbname);
EXEC SQL DISCONNECT ALL;
return 0;
}
</pre><p>
This example would produce this output:
</p><pre class="screen">
current=testdb3 (should be testdb3)
current=testdb2 (should be testdb2)
current=testdb1 (should be testdb1)
</pre><p>
</p><p>
The third option is to declare an SQL identifier linked to
the connection, for example:
</p><pre class="programlisting">
EXEC SQL AT <em class="replaceable"><code>connection-name</code></em> DECLARE <em class="replaceable"><code>statement-name</code></em> STATEMENT;
EXEC SQL PREPARE <em class="replaceable"><code>statement-name</code></em> FROM :<em class="replaceable"><code>dyn-string</code></em>;
</pre><p>
Once you link an SQL identifier to a connection, you execute dynamic SQL
without an AT clause. Note that this option behaves like preprocessor
directives, therefore the link is enabled only in the file.
</p><p>
Here is an example program using this option:
</p><pre class="programlisting">
#include <stdio.h>
EXEC SQL BEGIN DECLARE SECTION;
char dbname[128];
char *dyn_sql = "SELECT current_database()";
EXEC SQL END DECLARE SECTION;
int main(){
EXEC SQL CONNECT TO postgres AS con1;
EXEC SQL CONNECT TO testdb AS con2;
EXEC SQL AT con1 DECLARE stmt STATEMENT;
EXEC SQL PREPARE stmt FROM :dyn_sql;
EXEC SQL EXECUTE stmt INTO :dbname;
printf("%s\n", dbname);
EXEC SQL DISCONNECT ALL;
return 0;
}
</pre><p>
This example would produce this output, even if the default connection is testdb:
</p><pre class="screen">
postgres
</pre><p>
</p></div><div class="sect2" id="ECPG-DISCONNECT"><div class="titlepage"><div><div><h3 class="title">36.2.3. Closing a Connection <a href="#ECPG-DISCONNECT" class="id_link">#</a></h3></div></div></div><p>
To close a connection, use the following statement:
</p><pre class="programlisting">
EXEC SQL DISCONNECT [<span class="optional"><em class="replaceable"><code>connection</code></em></span>];
</pre><p>
The <em class="replaceable"><code>connection</code></em> can be specified
in the following ways:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
<code class="literal"><em class="replaceable"><code>connection-name</code></em></code>
</li><li class="listitem">
<code class="literal">CURRENT</code>
</li><li class="listitem">
<code class="literal">ALL</code>
</li></ul></div><p>
If no connection name is specified, the current connection is
closed.
</p><p>
It is good style that an application always explicitly disconnect
from every connection it opened.
</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ecpg-concept.html" title="36.1. The Concept">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-commands.html" title="36.3. Running SQL Commands">Next</a></td></tr><tr><td width="40%" align="left" valign="top">36.1. The Concept </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 36.3. Running SQL Commands</td></tr></table></div></body></html>