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<!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>34.2. Connection Status Functions</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="libpq-connect.html" title="34.1. Database Connection Control Functions" /><link rel="next" href="libpq-exec.html" title="34.3. Command Execution Functions" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">34.2. Connection Status Functions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="libpq-connect.html" title="34.1. Database Connection Control Functions">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="libpq.html" title="Chapter 34. libpq - C Library">Up</a></td><th width="60%" align="center">Chapter 34. <span xmlns="http://www.w3.org/1999/xhtml" class="application">libpq</span> - C Library</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="libpq-exec.html" title="34.3. Command Execution Functions">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="LIBPQ-STATUS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">34.2. Connection Status Functions</h2></div></div></div><p>
These functions can be used to interrogate the status
of an existing database connection object.
</p><div class="tip"><h3 class="title">Tip</h3><p>
<a id="id-1.7.3.9.3.1.1" class="indexterm"></a>
<a id="id-1.7.3.9.3.1.2" class="indexterm"></a>
<span class="application">libpq</span> application programmers should be careful to
maintain the <code class="structname">PGconn</code> abstraction. Use the accessor
functions described below to get at the contents of <code class="structname">PGconn</code>.
Reference to internal <code class="structname">PGconn</code> fields using
<code class="filename">libpq-int.h</code> is not recommended because they are subject to change
in the future.
</p></div><p>
The following functions return parameter values established at connection.
These values are fixed for the life of the connection. If a multi-host
connection string is used, the values of <code class="function">PQhost</code>,
<code class="function">PQport</code>, and <code class="function">PQpass</code> can change if a new connection
is established using the same <code class="structname">PGconn</code> object. Other values
are fixed for the lifetime of the <code class="structname">PGconn</code> object.
</p><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PQDB"><span class="term">
<code class="function">PQdb</code>
<a id="id-1.7.3.9.4.6.1.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the database name of the connection.
</p><pre class="synopsis">
char *PQdb(const PGconn *conn);
</pre><p>
</p></dd><dt id="LIBPQ-PQUSER"><span class="term">
<code class="function">PQuser</code>
<a id="id-1.7.3.9.4.6.2.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the user name of the connection.
</p><pre class="synopsis">
char *PQuser(const PGconn *conn);
</pre><p>
</p></dd><dt id="LIBPQ-PQPASS"><span class="term">
<code class="function">PQpass</code>
<a id="id-1.7.3.9.4.6.3.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the password of the connection.
</p><pre class="synopsis">
char *PQpass(const PGconn *conn);
</pre><p>
</p><p>
<code class="function">PQpass</code> will return either the password specified
in the connection parameters, or if there was none and the password
was obtained from the <a class="link" href="libpq-pgpass.html" title="34.15. The Password File">password
file</a>, it will return that. In the latter case,
if multiple hosts were specified in the connection parameters, it is
not possible to rely on the result of <code class="function">PQpass</code> until
the connection is established. The status of the connection can be
checked using the function <code class="function">PQstatus</code>.
</p></dd><dt id="LIBPQ-PQHOST"><span class="term">
<code class="function">PQhost</code>
<a id="id-1.7.3.9.4.6.4.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the server host name of the active connection.
This can be a host name, an IP address, or a directory path if the
connection is via Unix socket. (The path case can be distinguished
because it will always be an absolute path, beginning
with <code class="literal">/</code>.)
</p><pre class="synopsis">
char *PQhost(const PGconn *conn);
</pre><p>
</p><p>
If the connection parameters specified both <code class="literal">host</code> and
<code class="literal">hostaddr</code>, then <code class="function">PQhost</code> will
return the <code class="literal">host</code> information. If only
<code class="literal">hostaddr</code> was specified, then that is returned.
If multiple hosts were specified in the connection parameters,
<code class="function">PQhost</code> returns the host actually connected to.
</p><p>
<code class="function">PQhost</code> returns <code class="symbol">NULL</code> if the
<em class="parameter"><code>conn</code></em> argument is <code class="symbol">NULL</code>.
Otherwise, if there is an error producing the host information (perhaps
if the connection has not been fully established or there was an
error), it returns an empty string.
</p><p>
If multiple hosts were specified in the connection parameters, it is
not possible to rely on the result of <code class="function">PQhost</code> until
the connection is established. The status of the connection can be
checked using the function <code class="function">PQstatus</code>.
</p></dd><dt id="LIBPQ-PQPORT"><span class="term">
<code class="function">PQport</code>
<a id="id-1.7.3.9.4.6.5.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the port of the active connection.
</p><pre class="synopsis">
char *PQport(const PGconn *conn);
</pre><p>
</p><p>
If multiple ports were specified in the connection parameters,
<code class="function">PQport</code> returns the port actually connected to.
</p><p>
<code class="function">PQport</code> returns <code class="symbol">NULL</code> if the
<em class="parameter"><code>conn</code></em> argument is <code class="symbol">NULL</code>.
Otherwise, if there is an error producing the port information (perhaps
if the connection has not been fully established or there was an
error), it returns an empty string.
</p><p>
If multiple ports were specified in the connection parameters, it is
not possible to rely on the result of <code class="function">PQport</code> until
the connection is established. The status of the connection can be
checked using the function <code class="function">PQstatus</code>.
</p></dd><dt id="LIBPQ-PQTTY"><span class="term">
<code class="function">PQtty</code>
<a id="id-1.7.3.9.4.6.6.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the debug <acronym class="acronym">TTY</acronym> of the connection.
(This is obsolete, since the server no longer pays attention
to the <acronym class="acronym">TTY</acronym> setting, but the function remains
for backward compatibility.)
</p><pre class="synopsis">
char *PQtty(const PGconn *conn);
</pre><p>
</p></dd><dt id="LIBPQ-PQOPTIONS"><span class="term">
<code class="function">PQoptions</code>
<a id="id-1.7.3.9.4.6.7.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the command-line options passed in the connection request.
</p><pre class="synopsis">
char *PQoptions(const PGconn *conn);
</pre><p>
</p></dd></dl></div><p>
</p><p>
The following functions return status data that can change as operations
are executed on the <code class="structname">PGconn</code> object.
</p><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PQSTATUS"><span class="term">
<code class="function">PQstatus</code>
<a id="id-1.7.3.9.5.2.1.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the status of the connection.
</p><pre class="synopsis">
ConnStatusType PQstatus(const PGconn *conn);
</pre><p>
</p><p>
The status can be one of a number of values. However, only two of
these are seen outside of an asynchronous connection procedure:
<code class="literal">CONNECTION_OK</code> and
<code class="literal">CONNECTION_BAD</code>. A good connection to the database
has the status <code class="literal">CONNECTION_OK</code>. A failed
connection attempt is signaled by status
<code class="literal">CONNECTION_BAD</code>. Ordinarily, an OK status will
remain so until <code class="function">PQfinish</code>, but a communications
failure might result in the status changing to
<code class="literal">CONNECTION_BAD</code> prematurely. In that case the
application could try to recover by calling
<code class="function">PQreset</code>.
</p><p>
See the entry for <code class="function">PQconnectStartParams</code>, <code class="function">PQconnectStart</code>
and <code class="function">PQconnectPoll</code> with regards to other status codes that
might be returned.
</p></dd><dt id="LIBPQ-PQTRANSACTIONSTATUS"><span class="term">
<code class="function">PQtransactionStatus</code>
<a id="id-1.7.3.9.5.2.2.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the current in-transaction status of the server.
</p><pre class="synopsis">
PGTransactionStatusType PQtransactionStatus(const PGconn *conn);
</pre><p>
The status can be <code class="literal">PQTRANS_IDLE</code> (currently idle),
<code class="literal">PQTRANS_ACTIVE</code> (a command is in progress),
<code class="literal">PQTRANS_INTRANS</code> (idle, in a valid transaction block),
or <code class="literal">PQTRANS_INERROR</code> (idle, in a failed transaction block).
<code class="literal">PQTRANS_UNKNOWN</code> is reported if the connection is bad.
<code class="literal">PQTRANS_ACTIVE</code> is reported only when a query
has been sent to the server and not yet completed.
</p></dd><dt id="LIBPQ-PQPARAMETERSTATUS"><span class="term">
<code class="function">PQparameterStatus</code>
<a id="id-1.7.3.9.5.2.3.1.2" class="indexterm"></a>
</span></dt><dd><p>
Looks up a current parameter setting of the server.
</p><pre class="synopsis">
const char *PQparameterStatus(const PGconn *conn, const char *paramName);
</pre><p>
Certain parameter values are reported by the server automatically at
connection startup or whenever their values change.
<code class="function">PQparameterStatus</code> can be used to interrogate these settings.
It returns the current value of a parameter if known, or <code class="symbol">NULL</code>
if the parameter is not known.
</p><p>
Parameters reported as of the current release include
<code class="varname">server_version</code>,
<code class="varname">server_encoding</code>,
<code class="varname">client_encoding</code>,
<code class="varname">application_name</code>,
<code class="varname">is_superuser</code>,
<code class="varname">session_authorization</code>,
<code class="varname">DateStyle</code>,
<code class="varname">IntervalStyle</code>,
<code class="varname">TimeZone</code>,
<code class="varname">integer_datetimes</code>, and
<code class="varname">standard_conforming_strings</code>.
(<code class="varname">server_encoding</code>, <code class="varname">TimeZone</code>, and
<code class="varname">integer_datetimes</code> were not reported by releases before 8.0;
<code class="varname">standard_conforming_strings</code> was not reported by releases
before 8.1;
<code class="varname">IntervalStyle</code> was not reported by releases before 8.4;
<code class="varname">application_name</code> was not reported by releases before 9.0.)
Note that
<code class="varname">server_version</code>,
<code class="varname">server_encoding</code> and
<code class="varname">integer_datetimes</code>
cannot change after startup.
</p><p>
Pre-3.0-protocol servers do not report parameter settings, but
<span class="application">libpq</span> includes logic to obtain values for
<code class="varname">server_version</code> and <code class="varname">client_encoding</code> anyway.
Applications are encouraged to use <code class="function">PQparameterStatus</code>
rather than <span class="foreignphrase"><em class="foreignphrase">ad hoc</em></span> code to determine these values.
(Beware however that on a pre-3.0 connection, changing
<code class="varname">client_encoding</code> via <code class="command">SET</code> after connection
startup will not be reflected by <code class="function">PQparameterStatus</code>.)
For <code class="varname">server_version</code>, see also
<code class="function">PQserverVersion</code>, which returns the information in a
numeric form that is much easier to compare against.
</p><p>
If no value for <code class="varname">standard_conforming_strings</code> is reported,
applications can assume it is <code class="literal">off</code>, that is, backslashes
are treated as escapes in string literals. Also, the presence of
this parameter can be taken as an indication that the escape string
syntax (<code class="literal">E'...'</code>) is accepted.
</p><p>
Although the returned pointer is declared <code class="literal">const</code>, it in fact
points to mutable storage associated with the <code class="literal">PGconn</code> structure.
It is unwise to assume the pointer will remain valid across queries.
</p></dd><dt id="LIBPQ-PQPROTOCOLVERSION"><span class="term">
<code class="function">PQprotocolVersion</code>
<a id="id-1.7.3.9.5.2.4.1.2" class="indexterm"></a>
</span></dt><dd><p>
Interrogates the frontend/backend protocol being used.
</p><pre class="synopsis">
int PQprotocolVersion(const PGconn *conn);
</pre><p>
Applications might wish to use this function to determine whether certain
features are supported. Currently, the possible values are 2 (2.0
protocol), 3 (3.0 protocol), or zero (connection bad). The
protocol version will
not change after connection startup is complete, but it could
theoretically change during a connection reset. The 3.0 protocol
will normally be used when communicating with
<span class="productname">PostgreSQL</span> 7.4 or later servers; pre-7.4 servers
support only protocol 2.0. (Protocol 1.0 is obsolete and not
supported by <span class="application">libpq</span>.)
</p></dd><dt id="LIBPQ-PQSERVERVERSION"><span class="term">
<code class="function">PQserverVersion</code>
<a id="id-1.7.3.9.5.2.5.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns an integer representing the server version.
</p><pre class="synopsis">
int PQserverVersion(const PGconn *conn);
</pre><p>
</p><p>
Applications might use this function to determine the version of the
database server they are connected to. The result is formed by
multiplying the server's major version number by 10000 and adding
the minor version number. For example, version 10.1 will be
returned as 100001, and version 11.0 will be returned as 110000.
Zero is returned if the connection is bad.
</p><p>
Prior to major version 10, <span class="productname">PostgreSQL</span> used
three-part version numbers in which the first two parts together
represented the major version. For those
versions, <code class="function">PQserverVersion</code> uses two digits for each
part; for example version 9.1.5 will be returned as 90105, and
version 9.2.0 will be returned as 90200.
</p><p>
Therefore, for purposes of determining feature compatibility,
applications should divide the result of <code class="function">PQserverVersion</code>
by 100 not 10000 to determine a logical major version number.
In all release series, only the last two digits differ between
minor releases (bug-fix releases).
</p></dd><dt id="LIBPQ-PQERRORMESSAGE"><span class="term">
<code class="function">PQerrorMessage</code>
<a id="id-1.7.3.9.5.2.6.1.2" class="indexterm"></a>
</span></dt><dd><p>
<a id="id-1.7.3.9.5.2.6.2.1.1" class="indexterm"></a> Returns the error message
most recently generated by an operation on the connection.
</p><pre class="synopsis">
char *PQerrorMessage(const PGconn *conn);
</pre><p>
</p><p>
Nearly all <span class="application">libpq</span> functions will set a message for
<code class="function">PQerrorMessage</code> if they fail. Note that by
<span class="application">libpq</span> convention, a nonempty
<code class="function">PQerrorMessage</code> result can consist of multiple lines,
and will include a trailing newline. The caller should not free
the result directly. It will be freed when the associated
<code class="structname">PGconn</code> handle is passed to
<code class="function">PQfinish</code>. The result string should not be
expected to remain the same across operations on the
<code class="literal">PGconn</code> structure.
</p></dd><dt id="LIBPQ-PQSOCKET"><span class="term"><code class="function">PQsocket</code><a id="id-1.7.3.9.5.2.7.1.2" class="indexterm"></a></span></dt><dd><p>
Obtains the file descriptor number of the connection socket to
the server. A valid descriptor will be greater than or equal
to 0; a result of -1 indicates that no server connection is
currently open. (This will not change during normal operation,
but could change during connection setup or reset.)
</p><pre class="synopsis">
int PQsocket(const PGconn *conn);
</pre><p>
</p></dd><dt id="LIBPQ-PQBACKENDPID"><span class="term"><code class="function">PQbackendPID</code><a id="id-1.7.3.9.5.2.8.1.2" class="indexterm"></a></span></dt><dd><p>
Returns the process <acronym class="acronym">ID</acronym> (PID)<a id="id-1.7.3.9.5.2.8.2.1.2" class="indexterm"></a>
of the backend process handling this connection.
</p><pre class="synopsis">
int PQbackendPID(const PGconn *conn);
</pre><p>
</p><p>
The backend <acronym class="acronym">PID</acronym> is useful for debugging
purposes and for comparison to <code class="command">NOTIFY</code>
messages (which include the <acronym class="acronym">PID</acronym> of the
notifying backend process). Note that the
<acronym class="acronym">PID</acronym> belongs to a process executing on the
database server host, not the local host!
</p></dd><dt id="LIBPQ-PQCONNECTIONNEEDSPASSWORD"><span class="term"><code class="function">PQconnectionNeedsPassword</code><a id="id-1.7.3.9.5.2.9.1.2" class="indexterm"></a></span></dt><dd><p>
Returns true (1) if the connection authentication method
required a password, but none was available.
Returns false (0) if not.
</p><pre class="synopsis">
int PQconnectionNeedsPassword(const PGconn *conn);
</pre><p>
</p><p>
This function can be applied after a failed connection attempt
to decide whether to prompt the user for a password.
</p></dd><dt id="LIBPQ-PQCONNECTIONUSEDPASSWORD"><span class="term"><code class="function">PQconnectionUsedPassword</code><a id="id-1.7.3.9.5.2.10.1.2" class="indexterm"></a></span></dt><dd><p>
Returns true (1) if the connection authentication method
used a password. Returns false (0) if not.
</p><pre class="synopsis">
int PQconnectionUsedPassword(const PGconn *conn);
</pre><p>
</p><p>
This function can be applied after either a failed or successful
connection attempt to detect whether the server demanded a password.
</p></dd></dl></div><p>
</p><p>
The following functions return information related to SSL. This information
usually doesn't change after a connection is established.
</p><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PQSSLINUSE"><span class="term"><code class="function">PQsslInUse</code><a id="id-1.7.3.9.6.1.1.1.2" class="indexterm"></a></span></dt><dd><p>
Returns true (1) if the connection uses SSL, false (0) if not.
</p><pre class="synopsis">
int PQsslInUse(const PGconn *conn);
</pre><p>
</p></dd><dt id="LIBPQ-PQSSLATTRIBUTE"><span class="term"><code class="function">PQsslAttribute</code><a id="id-1.7.3.9.6.1.2.1.2" class="indexterm"></a></span></dt><dd><p>
Returns SSL-related information about the connection.
</p><pre class="synopsis">
const char *PQsslAttribute(const PGconn *conn, const char *attribute_name);
</pre><p>
</p><p>
The list of available attributes varies depending on the SSL library
being used, and the type of connection. If an attribute is not
available, returns NULL.
</p><p>
The following attributes are commonly available:
</p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">library</code></span></dt><dd><p>
Name of the SSL implementation in use. (Currently, only
<code class="literal">"OpenSSL"</code> is implemented)
</p></dd><dt><span class="term"><code class="literal">protocol</code></span></dt><dd><p>
SSL/TLS version in use. Common values
are <code class="literal">"TLSv1"</code>, <code class="literal">"TLSv1.1"</code>
and <code class="literal">"TLSv1.2"</code>, but an implementation may
return other strings if some other protocol is used.
</p></dd><dt><span class="term"><code class="literal">key_bits</code></span></dt><dd><p>
Number of key bits used by the encryption algorithm.
</p></dd><dt><span class="term"><code class="literal">cipher</code></span></dt><dd><p>
A short name of the ciphersuite used, e.g.
<code class="literal">"DHE-RSA-DES-CBC3-SHA"</code>. The names are specific
to each SSL implementation.
</p></dd><dt><span class="term"><code class="literal">compression</code></span></dt><dd><p>
If SSL compression is in use, returns the name of the compression
algorithm, or "on" if compression is used but the algorithm is
not known. If compression is not in use, returns "off".
</p></dd></dl></div><p>
</p></dd><dt id="LIBPQ-PQSSLATTRIBUTENAMES"><span class="term"><code class="function">PQsslAttributeNames</code><a id="id-1.7.3.9.6.1.3.1.2" class="indexterm"></a></span></dt><dd><p>
Return an array of SSL attribute names available. The array is terminated by a NULL pointer.
</p><pre class="synopsis">
const char * const * PQsslAttributeNames(const PGconn *conn);
</pre><p>
</p></dd><dt id="LIBPQ-PQSSLSTRUCT"><span class="term"><code class="function">PQsslStruct</code><a id="id-1.7.3.9.6.1.4.1.2" class="indexterm"></a></span></dt><dd><p>
Return a pointer to an SSL-implementation-specific object describing
the connection.
</p><pre class="synopsis">
void *PQsslStruct(const PGconn *conn, const char *struct_name);
</pre><p>
</p><p>
The struct(s) available depend on the SSL implementation in use.
For OpenSSL, there is one struct, available under the name "OpenSSL",
and it returns a pointer to the OpenSSL <code class="literal">SSL</code> struct.
To use this function, code along the following lines could be used:
</p><pre class="programlisting">
#include <libpq-fe.h>
#include <openssl/ssl.h>
...
SSL *ssl;
dbconn = PQconnectdb(...);
...
ssl = PQsslStruct(dbconn, "OpenSSL");
if (ssl)
{
/* use OpenSSL functions to access ssl */
}
</pre><p>
</p><p>
This structure can be used to verify encryption levels, check server
certificates, and more. Refer to the <span class="productname">OpenSSL</span>
documentation for information about this structure.
</p></dd><dt id="LIBPQ-PQGETSSL"><span class="term"><code class="function">PQgetssl</code><a id="id-1.7.3.9.6.1.5.1.2" class="indexterm"></a></span></dt><dd><p>
<a id="id-1.7.3.9.6.1.5.2.1.1" class="indexterm"></a>
Returns the SSL structure used in the connection, or null
if SSL is not in use.
</p><pre class="synopsis">
void *PQgetssl(const PGconn *conn);
</pre><p>
</p><p>
This function is equivalent to <code class="literal">PQsslStruct(conn, "OpenSSL")</code>. It should
not be used in new applications, because the returned struct is
specific to OpenSSL and will not be available if another SSL
implementation is used. To check if a connection uses SSL, call
<code class="function">PQsslInUse</code> instead, and for more details about the
connection, use <code class="function">PQsslAttribute</code>.
</p></dd></dl></div><p>
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