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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>34.3. Command Execution 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-status.html" title="34.2. Connection Status Functions" /><link rel="next" href="libpq-async.html" title="34.4. Asynchronous Command Processing" /></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">34.3. Command Execution Functions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="libpq-status.html" title="34.2. Connection Status 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-async.html" title="34.4. Asynchronous Command Processing">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="LIBPQ-EXEC"><div class="titlepage"><div><div><h2 class="title" style="clear: both">34.3. Command Execution Functions</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="libpq-exec.html#LIBPQ-EXEC-MAIN">34.3.1. Main Functions</a></span></dt><dt><span class="sect2"><a href="libpq-exec.html#LIBPQ-EXEC-SELECT-INFO">34.3.2. Retrieving Query Result Information</a></span></dt><dt><span class="sect2"><a href="libpq-exec.html#LIBPQ-EXEC-NONSELECT">34.3.3. Retrieving Other Result Information</a></span></dt><dt><span class="sect2"><a href="libpq-exec.html#LIBPQ-EXEC-ESCAPE-STRING">34.3.4. Escaping Strings for Inclusion in SQL Commands</a></span></dt></dl></div><p>
Once a connection to a database server has been successfully
established, the functions described here are used to perform
SQL queries and commands.
</p><div class="sect2" id="LIBPQ-EXEC-MAIN"><div class="titlepage"><div><div><h3 class="title">34.3.1. Main Functions</h3></div></div></div><p>
</p><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PQEXEC"><span class="term">
<code class="function">PQexec</code>
<a id="id-1.7.3.10.3.2.1.1.1.2" class="indexterm"></a>
</span></dt><dd><p>
Submits a command to the server and waits for the result.
</p><pre class="synopsis">
PGresult *PQexec(PGconn *conn, const char *command);
</pre><p>
</p><p>
Returns a <code class="structname">PGresult</code> pointer or possibly a null
pointer. A non-null pointer will generally be returned except in
out-of-memory conditions or serious errors such as inability to send
the command to the server. The <code class="function">PQresultStatus</code> function
should be called to check the return value for any errors (including
the value of a null pointer, in which case it will return
<code class="symbol">PGRES_FATAL_ERROR</code>). Use
<code class="function">PQerrorMessage</code> to get more information about such
errors.
</p></dd></dl></div><p>
The command string can include multiple SQL commands
(separated by semicolons). Multiple queries sent in a single
<code class="function">PQexec</code> call are processed in a single transaction, unless
there are explicit <code class="command">BEGIN</code>/<code class="command">COMMIT</code>
commands included in the query string to divide it into multiple
transactions. (See <a class="xref" href="protocol-flow.html#PROTOCOL-FLOW-MULTI-STATEMENT" title="53.2.2.1. Multiple Statements in a Simple Query">Section 53.2.2.1</a>
for more details about how the server handles multi-query strings.)
Note however that the returned
<code class="structname">PGresult</code> structure describes only the result
of the last command executed from the string. Should one of the
commands fail, processing of the string stops with it and the returned
<code class="structname">PGresult</code> describes the error condition.
</p><p>
</p><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PQEXECPARAMS"><span class="term">
<code class="function">PQexecParams</code>
<a id="id-1.7.3.10.3.3.1.1.1.2" class="indexterm"></a>
</span></dt><dd><p>
Submits a command to the server and waits for the result,
with the ability to pass parameters separately from the SQL
command text.
</p><pre class="synopsis">
PGresult *PQexecParams(PGconn *conn,
const char *command,
int nParams,
const Oid *paramTypes,
const char * const *paramValues,
const int *paramLengths,
const int *paramFormats,
int resultFormat);
</pre><p>
</p><p>
<code class="function">PQexecParams</code> is like <code class="function">PQexec</code>, but offers additional
functionality: parameter values can be specified separately from the command
string proper, and query results can be requested in either text or binary
format. <code class="function">PQexecParams</code> is supported only in protocol 3.0 and later
connections; it will fail when using protocol 2.0.
</p><p>
The function arguments are:
</p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><em class="parameter"><code>conn</code></em></span></dt><dd><p>
The connection object to send the command through.
</p></dd><dt><span class="term"><em class="parameter"><code>command</code></em></span></dt><dd><p>
The SQL command string to be executed. If parameters are used,
they are referred to in the command string as <code class="literal">$1</code>,
<code class="literal">$2</code>, etc.
</p></dd><dt><span class="term"><em class="parameter"><code>nParams</code></em></span></dt><dd><p>
The number of parameters supplied; it is the length of the arrays
<em class="parameter"><code>paramTypes[]</code></em>, <em class="parameter"><code>paramValues[]</code></em>,
<em class="parameter"><code>paramLengths[]</code></em>, and <em class="parameter"><code>paramFormats[]</code></em>. (The
array pointers can be <code class="symbol">NULL</code> when <em class="parameter"><code>nParams</code></em>
is zero.)
</p></dd><dt><span class="term"><em class="parameter"><code>paramTypes[]</code></em></span></dt><dd><p>
Specifies, by OID, the data types to be assigned to the
parameter symbols. If <em class="parameter"><code>paramTypes</code></em> is
<code class="symbol">NULL</code>, or any particular element in the array
is zero, the server infers a data type for the parameter symbol
in the same way it would do for an untyped literal string.
</p></dd><dt><span class="term"><em class="parameter"><code>paramValues[]</code></em></span></dt><dd><p>
Specifies the actual values of the parameters. A null pointer
in this array means the corresponding parameter is null;
otherwise the pointer points to a zero-terminated text string
(for text format) or binary data in the format expected by the
server (for binary format).
</p></dd><dt><span class="term"><em class="parameter"><code>paramLengths[]</code></em></span></dt><dd><p>
Specifies the actual data lengths of binary-format parameters.
It is ignored for null parameters and text-format parameters.
The array pointer can be null when there are no binary parameters.
</p></dd><dt><span class="term"><em class="parameter"><code>paramFormats[]</code></em></span></dt><dd><p>
Specifies whether parameters are text (put a zero in the
array entry for the corresponding parameter) or binary (put
a one in the array entry for the corresponding parameter).
If the array pointer is null then all parameters are presumed
to be text strings.
</p><p>
Values passed in binary format require knowledge of
the internal representation expected by the backend.
For example, integers must be passed in network byte
order. Passing <code class="type">numeric</code> values requires
knowledge of the server storage format, as implemented
in
<code class="filename">src/backend/utils/adt/numeric.c::numeric_send()</code> and
<code class="filename">src/backend/utils/adt/numeric.c::numeric_recv()</code>.
</p></dd><dt><span class="term"><em class="parameter"><code>resultFormat</code></em></span></dt><dd><p>
Specify zero to obtain results in text format, or one to obtain
results in binary format. (There is not currently a provision
to obtain different result columns in different formats,
although that is possible in the underlying protocol.)
</p></dd></dl></div><p>
</p></dd></dl></div><p>
</p><p>
The primary advantage of <code class="function">PQexecParams</code> over
<code class="function">PQexec</code> is that parameter values can be separated from the
command string, thus avoiding the need for tedious and error-prone
quoting and escaping.
</p><p>
Unlike <code class="function">PQexec</code>, <code class="function">PQexecParams</code> allows at most
one SQL command in the given string. (There can be semicolons in it,
but not more than one nonempty command.) This is a limitation of the
underlying protocol, but has some usefulness as an extra defense against
SQL-injection attacks.
</p><div class="tip"><h3 class="title">Tip</h3><p>
Specifying parameter types via OIDs is tedious, particularly if you prefer
not to hard-wire particular OID values into your program. However, you can
avoid doing so even in cases where the server by itself cannot determine the
type of the parameter, or chooses a different type than you want. In the
SQL command text, attach an explicit cast to the parameter symbol to show what
data type you will send. For example:
</p><pre class="programlisting">
SELECT * FROM mytable WHERE x = $1::bigint;
</pre><p>
This forces parameter <code class="literal">$1</code> to be treated as <code class="type">bigint</code>, whereas
by default it would be assigned the same type as <code class="literal">x</code>. Forcing the
parameter type decision, either this way or by specifying a numeric type OID,
is strongly recommended when sending parameter values in binary format, because
binary format has less redundancy than text format and so there is less chance
that the server will detect a type mismatch mistake for you.
</p></div><p>
</p><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PQPREPARE"><span class="term"><code class="function">PQprepare</code>
<a id="id-1.7.3.10.3.7.1.1.1.2" class="indexterm"></a>
</span></dt><dd><p>
Submits a request to create a prepared statement with the
given parameters, and waits for completion.
</p><pre class="synopsis">
PGresult *PQprepare(PGconn *conn,
const char *stmtName,
const char *query,
int nParams,
const Oid *paramTypes);
</pre><p>
</p><p>
<code class="function">PQprepare</code> creates a prepared statement for later
execution with <code class="function">PQexecPrepared</code>. This feature allows
commands to be executed repeatedly without being parsed and
planned each time; see <a class="xref" href="sql-prepare.html" title="PREPARE"><span class="refentrytitle">PREPARE</span></a> for details.
<code class="function">PQprepare</code> is supported only in protocol 3.0 and later
connections; it will fail when using protocol 2.0.
</p><p>
The function creates a prepared statement named
<em class="parameter"><code>stmtName</code></em> from the <em class="parameter"><code>query</code></em> string, which
must contain a single SQL command. <em class="parameter"><code>stmtName</code></em> can be
<code class="literal">""</code> to create an unnamed statement, in which case any
pre-existing unnamed statement is automatically replaced; otherwise
it is an error if the statement name is already defined in the
current session. If any parameters are used, they are referred
to in the query as <code class="literal">$1</code>, <code class="literal">$2</code>, etc.
<em class="parameter"><code>nParams</code></em> is the number of parameters for which types
are pre-specified in the array <em class="parameter"><code>paramTypes[]</code></em>. (The
array pointer can be <code class="symbol">NULL</code> when
<em class="parameter"><code>nParams</code></em> is zero.) <em class="parameter"><code>paramTypes[]</code></em>
specifies, by OID, the data types to be assigned to the parameter
symbols. If <em class="parameter"><code>paramTypes</code></em> is <code class="symbol">NULL</code>,
or any particular element in the array is zero, the server assigns
a data type to the parameter symbol in the same way it would do
for an untyped literal string. Also, the query can use parameter
symbols with numbers higher than <em class="parameter"><code>nParams</code></em>; data types
will be inferred for these symbols as well. (See
<code class="function">PQdescribePrepared</code> for a means to find out
what data types were inferred.)
</p><p>
As with <code class="function">PQexec</code>, the result is normally a
<code class="structname">PGresult</code> object whose contents indicate
server-side success or failure. A null result indicates
out-of-memory or inability to send the command at all. Use
<code class="function">PQerrorMessage</code> to get more information about
such errors.
</p></dd></dl></div><p>
Prepared statements for use with <code class="function">PQexecPrepared</code> can also
be created by executing SQL <a class="xref" href="sql-prepare.html" title="PREPARE"><span class="refentrytitle">PREPARE</span></a>
statements. Also, although there is no <span class="application">libpq</span>
function for deleting a prepared statement, the SQL <a class="xref" href="sql-deallocate.html" title="DEALLOCATE"><span class="refentrytitle">DEALLOCATE</span></a> statement
can be used for that purpose.
</p><p>
</p><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PQEXECPREPARED"><span class="term">
<code class="function">PQexecPrepared</code>
<a id="id-1.7.3.10.3.8.1.1.1.2" class="indexterm"></a>
</span></dt><dd><p>
Sends a request to execute a prepared statement with given
parameters, and waits for the result.
</p><pre class="synopsis">
PGresult *PQexecPrepared(PGconn *conn,
const char *stmtName,
int nParams,
const char * const *paramValues,
const int *paramLengths,
const int *paramFormats,
int resultFormat);
</pre><p>
</p><p>
<code class="function">PQexecPrepared</code> is like <code class="function">PQexecParams</code>,
but the command to be executed is specified by naming a
previously-prepared statement, instead of giving a query string.
This feature allows commands that will be used repeatedly to be
parsed and planned just once, rather than each time they are
executed. The statement must have been prepared previously in
the current session. <code class="function">PQexecPrepared</code> is supported
only in protocol 3.0 and later connections; it will fail when
using protocol 2.0.
</p><p>
The parameters are identical to <code class="function">PQexecParams</code>, except that the
name of a prepared statement is given instead of a query string, and the
<em class="parameter"><code>paramTypes[]</code></em> parameter is not present (it is not needed since
the prepared statement's parameter types were determined when it was created).
</p></dd><dt id="LIBPQ-PQDESCRIBEPREPARED"><span class="term">
<code class="function">PQdescribePrepared</code>
<a id="id-1.7.3.10.3.8.1.2.1.2" class="indexterm"></a>
</span></dt><dd><p>
Submits a request to obtain information about the specified
prepared statement, and waits for completion.
</p><pre class="synopsis">
PGresult *PQdescribePrepared(PGconn *conn, const char *stmtName);
</pre><p>
</p><p>
<code class="function">PQdescribePrepared</code> allows an application to obtain
information about a previously prepared statement.
<code class="function">PQdescribePrepared</code> is supported only in protocol 3.0
and later connections; it will fail when using protocol 2.0.
</p><p>
<em class="parameter"><code>stmtName</code></em> can be <code class="literal">""</code> or <code class="symbol">NULL</code> to reference
the unnamed statement, otherwise it must be the name of an existing
prepared statement. On success, a <code class="structname">PGresult</code> with
status <code class="literal">PGRES_COMMAND_OK</code> is returned. The
functions <code class="function">PQnparams</code> and
<code class="function">PQparamtype</code> can be applied to this
<code class="structname">PGresult</code> to obtain information about the parameters
of the prepared statement, and the functions
<code class="function">PQnfields</code>, <code class="function">PQfname</code>,
<code class="function">PQftype</code>, etc provide information about the
result columns (if any) of the statement.
</p></dd><dt id="LIBPQ-PQDESCRIBEPORTAL"><span class="term">
<code class="function">PQdescribePortal</code>
<a id="id-1.7.3.10.3.8.1.3.1.2" class="indexterm"></a>
</span></dt><dd><p>
Submits a request to obtain information about the specified
portal, and waits for completion.
</p><pre class="synopsis">
PGresult *PQdescribePortal(PGconn *conn, const char *portalName);
</pre><p>
</p><p>
<code class="function">PQdescribePortal</code> allows an application to obtain
information about a previously created portal.
(<span class="application">libpq</span> does not provide any direct access to
portals, but you can use this function to inspect the properties
of a cursor created with a <code class="command">DECLARE CURSOR</code> SQL command.)
<code class="function">PQdescribePortal</code> is supported only in protocol 3.0
and later connections; it will fail when using protocol 2.0.
</p><p>
<em class="parameter"><code>portalName</code></em> can be <code class="literal">""</code> or <code class="symbol">NULL</code> to reference
the unnamed portal, otherwise it must be the name of an existing
portal. On success, a <code class="structname">PGresult</code> with status
<code class="literal">PGRES_COMMAND_OK</code> is returned. The functions
<code class="function">PQnfields</code>, <code class="function">PQfname</code>,
<code class="function">PQftype</code>, etc can be applied to the
<code class="structname">PGresult</code> to obtain information about the result
columns (if any) of the portal.
</p></dd></dl></div><p>
</p><p>
The <code class="structname">PGresult</code><a id="id-1.7.3.10.3.9.2" class="indexterm"></a>
structure encapsulates the result returned by the server.
<span class="application">libpq</span> application programmers should be
careful to maintain the <code class="structname">PGresult</code> abstraction.
Use the accessor functions below to get at the contents of
<code class="structname">PGresult</code>. Avoid directly referencing the
fields of the <code class="structname">PGresult</code> structure because they
are subject to change in the future.
</p><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PQRESULTSTATUS"><span class="term">
<code class="function">PQresultStatus</code>
<a id="id-1.7.3.10.3.9.7.1.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the result status of the command.
</p><pre class="synopsis">
ExecStatusType PQresultStatus(const PGresult *res);
</pre><p>
</p><p>
<code class="function">PQresultStatus</code> can return one of the following values:
</p><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PGRES-EMPTY-QUERY"><span class="term"><code class="literal">PGRES_EMPTY_QUERY</code></span></dt><dd><p>
The string sent to the server was empty.
</p></dd><dt id="LIBPQ-PGRES-COMMAND-OK"><span class="term"><code class="literal">PGRES_COMMAND_OK</code></span></dt><dd><p>
Successful completion of a command returning no data.
</p></dd><dt id="LIBPQ-PGRES-TUPLES-OK"><span class="term"><code class="literal">PGRES_TUPLES_OK</code></span></dt><dd><p>
Successful completion of a command returning data (such as
a <code class="command">SELECT</code> or <code class="command">SHOW</code>).
</p></dd><dt id="LIBPQ-PGRES-COPY-OUT"><span class="term"><code class="literal">PGRES_COPY_OUT</code></span></dt><dd><p>
Copy Out (from server) data transfer started.
</p></dd><dt id="LIBPQ-PGRES-COPY-IN"><span class="term"><code class="literal">PGRES_COPY_IN</code></span></dt><dd><p>
Copy In (to server) data transfer started.
</p></dd><dt id="LIBPQ-PGRES-BAD-RESPONSE"><span class="term"><code class="literal">PGRES_BAD_RESPONSE</code></span></dt><dd><p>
The server's response was not understood.
</p></dd><dt id="LIBPQ-PGRES-NONFATAL-ERROR"><span class="term"><code class="literal">PGRES_NONFATAL_ERROR</code></span></dt><dd><p>
A nonfatal error (a notice or warning) occurred.
</p></dd><dt id="LIBPQ-PGRES-FATAL-ERROR"><span class="term"><code class="literal">PGRES_FATAL_ERROR</code></span></dt><dd><p>
A fatal error occurred.
</p></dd><dt id="LIBPQ-PGRES-COPY-BOTH"><span class="term"><code class="literal">PGRES_COPY_BOTH</code></span></dt><dd><p>
Copy In/Out (to and from server) data transfer started. This
feature is currently used only for streaming replication,
so this status should not occur in ordinary applications.
</p></dd><dt id="LIBPQ-PGRES-SINGLE-TUPLE"><span class="term"><code class="literal">PGRES_SINGLE_TUPLE</code></span></dt><dd><p>
The <code class="structname">PGresult</code> contains a single result tuple
from the current command. This status occurs only when
single-row mode has been selected for the query
(see <a class="xref" href="libpq-single-row-mode.html" title="34.5. Retrieving Query Results Row-By-Row">Section 34.5</a>).
</p></dd></dl></div><p>
If the result status is <code class="literal">PGRES_TUPLES_OK</code> or
<code class="literal">PGRES_SINGLE_TUPLE</code>, then
the functions described below can be used to retrieve the rows
returned by the query. Note that a <code class="command">SELECT</code>
command that happens to retrieve zero rows still shows
<code class="literal">PGRES_TUPLES_OK</code>.
<code class="literal">PGRES_COMMAND_OK</code> is for commands that can never
return rows (<code class="command">INSERT</code> or <code class="command">UPDATE</code>
without a <code class="literal">RETURNING</code> clause,
etc.). A response of <code class="literal">PGRES_EMPTY_QUERY</code> might
indicate a bug in the client software.
</p><p>
A result of status <code class="symbol">PGRES_NONFATAL_ERROR</code> will
never be returned directly by <code class="function">PQexec</code> or other
query execution functions; results of this kind are instead passed
to the notice processor (see <a class="xref" href="libpq-notice-processing.html" title="34.12. Notice Processing">Section 34.12</a>).
</p></dd><dt id="LIBPQ-PQRESSTATUS"><span class="term">
<code class="function">PQresStatus</code>
<a id="id-1.7.3.10.3.9.7.2.1.2" class="indexterm"></a>
</span></dt><dd><p>
Converts the enumerated type returned by
<code class="function">PQresultStatus</code> into a string constant describing the
status code. The caller should not free the result.
</p><pre class="synopsis">
char *PQresStatus(ExecStatusType status);
</pre><p>
</p></dd><dt id="LIBPQ-PQRESULTERRORMESSAGE"><span class="term">
<code class="function">PQresultErrorMessage</code>
<a id="id-1.7.3.10.3.9.7.3.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the error message associated with the command, or an empty string
if there was no error.
</p><pre class="synopsis">
char *PQresultErrorMessage(const PGresult *res);
</pre><p>
If there was an error, the returned string will include a trailing
newline. The caller should not free the result directly. It will
be freed when the associated <code class="structname">PGresult</code> handle is
passed to <code class="function">PQclear</code>.
</p><p>
Immediately following a <code class="function">PQexec</code> or
<code class="function">PQgetResult</code> call,
<code class="function">PQerrorMessage</code> (on the connection) will return
the same string as <code class="function">PQresultErrorMessage</code> (on
the result). However, a <code class="structname">PGresult</code> will
retain its error message until destroyed, whereas the connection's
error message will change when subsequent operations are done.
Use <code class="function">PQresultErrorMessage</code> when you want to
know the status associated with a particular
<code class="structname">PGresult</code>; use
<code class="function">PQerrorMessage</code> when you want to know the
status from the latest operation on the connection.
</p></dd><dt id="LIBPQ-PQRESULTVERBOSEERRORMESSAGE"><span class="term">
<code class="function">PQresultVerboseErrorMessage</code>
<a id="id-1.7.3.10.3.9.7.4.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns a reformatted version of the error message associated with
a <code class="structname">PGresult</code> object.
</p><pre class="synopsis">
char *PQresultVerboseErrorMessage(const PGresult *res,
PGVerbosity verbosity,
PGContextVisibility show_context);
</pre><p>
In some situations a client might wish to obtain a more detailed
version of a previously-reported error.
<code class="function">PQresultVerboseErrorMessage</code> addresses this need
by computing the message that would have been produced
by <code class="function">PQresultErrorMessage</code> if the specified
verbosity settings had been in effect for the connection when the
given <code class="structname">PGresult</code> was generated. If
the <code class="structname">PGresult</code> is not an error result,
<span class="quote">“<span class="quote">PGresult is not an error result</span>”</span> is reported instead.
The returned string includes a trailing newline.
</p><p>
Unlike most other functions for extracting data from
a <code class="structname">PGresult</code>, the result of this function is a freshly
allocated string. The caller must free it
using <code class="function">PQfreemem()</code> when the string is no longer needed.
</p><p>
A NULL return is possible if there is insufficient memory.
</p></dd><dt id="LIBPQ-PQRESULTERRORFIELD"><span class="term"><code class="function">PQresultErrorField</code><a id="id-1.7.3.10.3.9.7.5.1.2" class="indexterm"></a></span></dt><dd><p>
Returns an individual field of an error report.
</p><pre class="synopsis">
char *PQresultErrorField(const PGresult *res, int fieldcode);
</pre><p>
<em class="parameter"><code>fieldcode</code></em> is an error field identifier; see the symbols
listed below. <code class="symbol">NULL</code> is returned if the
<code class="structname">PGresult</code> is not an error or warning result,
or does not include the specified field. Field values will normally
not include a trailing newline. The caller should not free the
result directly. It will be freed when the
associated <code class="structname">PGresult</code> handle is passed to
<code class="function">PQclear</code>.
</p><p>
The following field codes are available:
</p><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PG-DIAG-SEVERITY"><span class="term"><code class="symbol">PG_DIAG_SEVERITY</code></span></dt><dd><p>
The severity; the field contents are <code class="literal">ERROR</code>,
<code class="literal">FATAL</code>, or <code class="literal">PANIC</code> (in an error message),
or <code class="literal">WARNING</code>, <code class="literal">NOTICE</code>, <code class="literal">DEBUG</code>,
<code class="literal">INFO</code>, or <code class="literal">LOG</code> (in a notice message), or
a localized translation of one of these. Always present.
</p></dd><dt id="LIBPQ-PG-DIAG-SEVERITY-NONLOCALIZED"><span class="term"><code class="symbol">PG_DIAG_SEVERITY_NONLOCALIZED</code></span></dt><dd><p>
The severity; the field contents are <code class="literal">ERROR</code>,
<code class="literal">FATAL</code>, or <code class="literal">PANIC</code> (in an error message),
or <code class="literal">WARNING</code>, <code class="literal">NOTICE</code>, <code class="literal">DEBUG</code>,
<code class="literal">INFO</code>, or <code class="literal">LOG</code> (in a notice message).
This is identical to the <code class="symbol">PG_DIAG_SEVERITY</code> field except
that the contents are never localized. This is present only in
reports generated by <span class="productname">PostgreSQL</span> versions 9.6
and later.
</p></dd><dt id="LIBPQ-PG-DIAG-SQLSTATE"><span class="term">
<code class="symbol">PG_DIAG_SQLSTATE</code>
<a id="id-1.7.3.10.3.9.7.5.2.2.1.3.1.2" class="indexterm"></a>
</span></dt><dd><p>
The SQLSTATE code for the error. The SQLSTATE code identifies
the type of error that has occurred; it can be used by
front-end applications to perform specific operations (such
as error handling) in response to a particular database error.
For a list of the possible SQLSTATE codes, see <a class="xref" href="errcodes-appendix.html" title="Appendix A. PostgreSQL Error Codes">Appendix A</a>. This field is not localizable,
and is always present.
</p></dd><dt id="LIBPQ-PG-DIAG-MESSAGE-PRIMARY"><span class="term"><code class="symbol">PG_DIAG_MESSAGE_PRIMARY</code></span></dt><dd><p>
The primary human-readable error message (typically one line).
Always present.
</p></dd><dt id="LIBPQ-PG-DIAG-MESSAGE-DETAIL"><span class="term"><code class="symbol">PG_DIAG_MESSAGE_DETAIL</code></span></dt><dd><p>
Detail: an optional secondary error message carrying more
detail about the problem. Might run to multiple lines.
</p></dd><dt id="LIBPQ-PG-DIAG-MESSAGE-HINT"><span class="term"><code class="symbol">PG_DIAG_MESSAGE_HINT</code></span></dt><dd><p>
Hint: an optional suggestion what to do about the problem.
This is intended to differ from detail in that it offers advice
(potentially inappropriate) rather than hard facts. Might
run to multiple lines.
</p></dd><dt id="LIBPQ-PG-DIAG-STATEMENT-POSITION"><span class="term"><code class="symbol">PG_DIAG_STATEMENT_POSITION</code></span></dt><dd><p>
A string containing a decimal integer indicating an error cursor
position as an index into the original statement string. The
first character has index 1, and positions are measured in
characters not bytes.
</p></dd><dt id="LIBPQ-PG-DIAG-INTERNAL-POSITION"><span class="term"><code class="symbol">PG_DIAG_INTERNAL_POSITION</code></span></dt><dd><p>
This is defined the same as the
<code class="symbol">PG_DIAG_STATEMENT_POSITION</code> field, but it is used
when the cursor position refers to an internally generated
command rather than the one submitted by the client. The
<code class="symbol">PG_DIAG_INTERNAL_QUERY</code> field will always appear when
this field appears.
</p></dd><dt id="LIBPQ-PG-DIAG-INTERNAL-QUERY"><span class="term"><code class="symbol">PG_DIAG_INTERNAL_QUERY</code></span></dt><dd><p>
The text of a failed internally-generated command. This could
be, for example, a SQL query issued by a PL/pgSQL function.
</p></dd><dt id="LIBPQ-PG-DIAG-CONTEXT"><span class="term"><code class="symbol">PG_DIAG_CONTEXT</code></span></dt><dd><p>
An indication of the context in which the error occurred.
Presently this includes a call stack traceback of active
procedural language functions and internally-generated queries.
The trace is one entry per line, most recent first.
</p></dd><dt id="LIBPQ-PG-DIAG-SCHEMA-NAME"><span class="term"><code class="symbol">PG_DIAG_SCHEMA_NAME</code></span></dt><dd><p>
If the error was associated with a specific database object,
the name of the schema containing that object, if any.
</p></dd><dt id="LIBPQ-PG-DIAG-TABLE-NAME"><span class="term"><code class="symbol">PG_DIAG_TABLE_NAME</code></span></dt><dd><p>
If the error was associated with a specific table, the name of the
table. (Refer to the schema name field for the name of the
table's schema.)
</p></dd><dt id="LIBPQ-PG-DIAG-COLUMN-NAME"><span class="term"><code class="symbol">PG_DIAG_COLUMN_NAME</code></span></dt><dd><p>
If the error was associated with a specific table column, the name
of the column. (Refer to the schema and table name fields to
identify the table.)
</p></dd><dt id="LIBPQ-PG-DIAG-DATATYPE-NAME"><span class="term"><code class="symbol">PG_DIAG_DATATYPE_NAME</code></span></dt><dd><p>
If the error was associated with a specific data type, the name of
the data type. (Refer to the schema name field for the name of
the data type's schema.)
</p></dd><dt id="LIBPQ-PG-DIAG-CONSTRAINT-NAME"><span class="term"><code class="symbol">PG_DIAG_CONSTRAINT_NAME</code></span></dt><dd><p>
If the error was associated with a specific constraint, the name
of the constraint. Refer to fields listed above for the
associated table or domain. (For this purpose, indexes are
treated as constraints, even if they weren't created with
constraint syntax.)
</p></dd><dt id="LIBPQ-PG-DIAG-SOURCE-FILE"><span class="term"><code class="symbol">PG_DIAG_SOURCE_FILE</code></span></dt><dd><p>
The file name of the source-code location where the error was
reported.
</p></dd><dt id="LIBPQ-PG-DIAG-SOURCE-LINE"><span class="term"><code class="symbol">PG_DIAG_SOURCE_LINE</code></span></dt><dd><p>
The line number of the source-code location where the error
was reported.
</p></dd><dt id="LIBPQ-PG-DIAG-SOURCE-FUNCTION"><span class="term"><code class="symbol">PG_DIAG_SOURCE_FUNCTION</code></span></dt><dd><p>
The name of the source-code function reporting the error.
</p></dd></dl></div><p>
</p><div class="note"><h3 class="title">Note</h3><p>
The fields for schema name, table name, column name, data type name,
and constraint name are supplied only for a limited number of error
types; see <a class="xref" href="errcodes-appendix.html" title="Appendix A. PostgreSQL Error Codes">Appendix A</a>. Do not assume that
the presence of any of these fields guarantees the presence of
another field. Core error sources observe the interrelationships
noted above, but user-defined functions may use these fields in other
ways. In the same vein, do not assume that these fields denote
contemporary objects in the current database.
</p></div><p>
The client is responsible for formatting displayed information to meet
its needs; in particular it should break long lines as needed.
Newline characters appearing in the error message fields should be
treated as paragraph breaks, not line breaks.
</p><p>
Errors generated internally by <span class="application">libpq</span> will
have severity and primary message, but typically no other fields.
Errors returned by a pre-3.0-protocol server will include severity and
primary message, and sometimes a detail message, but no other fields.
</p><p>
Note that error fields are only available from
<code class="structname">PGresult</code> objects, not
<code class="structname">PGconn</code> objects; there is no
<code class="function">PQerrorField</code> function.
</p></dd><dt id="LIBPQ-PQCLEAR"><span class="term"><code class="function">PQclear</code><a id="id-1.7.3.10.3.9.7.6.1.2" class="indexterm"></a></span></dt><dd><p>
Frees the storage associated with a
<code class="structname">PGresult</code>. Every command result should be
freed via <code class="function">PQclear</code> when it is no longer
needed.
</p><pre class="synopsis">
void PQclear(PGresult *res);
</pre><p>
</p><p>
You can keep a <code class="structname">PGresult</code> object around for
as long as you need it; it does not go away when you issue a new
command, nor even if you close the connection. To get rid of it,
you must call <code class="function">PQclear</code>. Failure to do this
will result in memory leaks in your application.
</p></dd></dl></div><p>
</p></div><div class="sect2" id="LIBPQ-EXEC-SELECT-INFO"><div class="titlepage"><div><div><h3 class="title">34.3.2. Retrieving Query Result Information</h3></div></div></div><p>
These functions are used to extract information from a
<code class="structname">PGresult</code> object that represents a successful
query result (that is, one that has status
<code class="literal">PGRES_TUPLES_OK</code> or <code class="literal">PGRES_SINGLE_TUPLE</code>).
They can also be used to extract
information from a successful Describe operation: a Describe's result
has all the same column information that actual execution of the query
would provide, but it has zero rows. For objects with other status values,
these functions will act as though the result has zero rows and zero columns.
</p><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PQNTUPLES"><span class="term">
<code class="function">PQntuples</code>
<a id="id-1.7.3.10.4.3.1.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the number of rows (tuples) in the query result.
(Note that <code class="structname">PGresult</code> objects are limited to no more
than <code class="literal">INT_MAX</code> rows, so an <code class="type">int</code> result is
sufficient.)
</p><pre class="synopsis">
int PQntuples(const PGresult *res);
</pre><p>
</p></dd><dt id="LIBPQ-PQNFIELDS"><span class="term">
<code class="function">PQnfields</code>
<a id="id-1.7.3.10.4.3.2.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the number of columns (fields) in each row of the query
result.
</p><pre class="synopsis">
int PQnfields(const PGresult *res);
</pre><p>
</p></dd><dt id="LIBPQ-PQFNAME"><span class="term">
<code class="function">PQfname</code>
<a id="id-1.7.3.10.4.3.3.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the column name associated with the given column number.
Column numbers start at 0. The caller should not free the result
directly. It will be freed when the associated
<code class="structname">PGresult</code> handle is passed to
<code class="function">PQclear</code>.
</p><pre class="synopsis">
char *PQfname(const PGresult *res,
int column_number);
</pre><p>
</p><p>
<code class="symbol">NULL</code> is returned if the column number is out of range.
</p></dd><dt id="LIBPQ-PQFNUMBER"><span class="term">
<code class="function">PQfnumber</code>
<a id="id-1.7.3.10.4.3.4.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the column number associated with the given column name.
</p><pre class="synopsis">
int PQfnumber(const PGresult *res,
const char *column_name);
</pre><p>
</p><p>
-1 is returned if the given name does not match any column.
</p><p>
The given name is treated like an identifier in an SQL command,
that is, it is downcased unless double-quoted. For example, given
a query result generated from the SQL command:
</p><pre class="programlisting">
SELECT 1 AS FOO, 2 AS "BAR";
</pre><p>
we would have the results:
</p><pre class="programlisting">
PQfname(res, 0) <em class="lineannotation"><span class="lineannotation">foo</span></em>
PQfname(res, 1) <em class="lineannotation"><span class="lineannotation">BAR</span></em>
PQfnumber(res, "FOO") <em class="lineannotation"><span class="lineannotation">0</span></em>
PQfnumber(res, "foo") <em class="lineannotation"><span class="lineannotation">0</span></em>
PQfnumber(res, "BAR") <em class="lineannotation"><span class="lineannotation">-1</span></em>
PQfnumber(res, "\"BAR\"") <em class="lineannotation"><span class="lineannotation">1</span></em>
</pre><p>
</p></dd><dt id="LIBPQ-PQFTABLE"><span class="term">
<code class="function">PQftable</code>
<a id="id-1.7.3.10.4.3.5.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the OID of the table from which the given column was
fetched. Column numbers start at 0.
</p><pre class="synopsis">
Oid PQftable(const PGresult *res,
int column_number);
</pre><p>
</p><p>
<code class="literal">InvalidOid</code> is returned if the column number is out of range,
or if the specified column is not a simple reference to a table column,
or when using pre-3.0 protocol.
You can query the system table <code class="literal">pg_class</code> to determine
exactly which table is referenced.
</p><p>
The type <code class="type">Oid</code> and the constant
<code class="literal">InvalidOid</code> will be defined when you include
the <span class="application">libpq</span> header file. They will both
be some integer type.
</p></dd><dt id="LIBPQ-PQFTABLECOL"><span class="term">
<code class="function">PQftablecol</code>
<a id="id-1.7.3.10.4.3.6.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the column number (within its table) of the column making
up the specified query result column. Query-result column numbers
start at 0, but table columns have nonzero numbers.
</p><pre class="synopsis">
int PQftablecol(const PGresult *res,
int column_number);
</pre><p>
</p><p>
Zero is returned if the column number is out of range, or if the
specified column is not a simple reference to a table column, or
when using pre-3.0 protocol.
</p></dd><dt id="LIBPQ-PQFFORMAT"><span class="term">
<code class="function">PQfformat</code>
<a id="id-1.7.3.10.4.3.7.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the format code indicating the format of the given
column. Column numbers start at 0.
</p><pre class="synopsis">
int PQfformat(const PGresult *res,
int column_number);
</pre><p>
</p><p>
Format code zero indicates textual data representation, while format
code one indicates binary representation. (Other codes are reserved
for future definition.)
</p></dd><dt id="LIBPQ-PQFTYPE"><span class="term">
<code class="function">PQftype</code>
<a id="id-1.7.3.10.4.3.8.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the data type associated with the given column number.
The integer returned is the internal OID number of the type.
Column numbers start at 0.
</p><pre class="synopsis">
Oid PQftype(const PGresult *res,
int column_number);
</pre><p>
</p><p>
You can query the system table <code class="literal">pg_type</code> to
obtain the names and properties of the various data types. The
<acronym class="acronym">OID</acronym>s of the built-in data types are defined
in the file <code class="filename">src/include/catalog/pg_type_d.h</code>
in the source tree.
</p></dd><dt id="LIBPQ-PQFMOD"><span class="term">
<code class="function">PQfmod</code>
<a id="id-1.7.3.10.4.3.9.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the type modifier of the column associated with the
given column number. Column numbers start at 0.
</p><pre class="synopsis">
int PQfmod(const PGresult *res,
int column_number);
</pre><p>
</p><p>
The interpretation of modifier values is type-specific; they
typically indicate precision or size limits. The value -1 is
used to indicate <span class="quote">“<span class="quote">no information available</span>”</span>. Most data
types do not use modifiers, in which case the value is always
-1.
</p></dd><dt id="LIBPQ-PQFSIZE"><span class="term">
<code class="function">PQfsize</code>
<a id="id-1.7.3.10.4.3.10.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the size in bytes of the column associated with the
given column number. Column numbers start at 0.
</p><pre class="synopsis">
int PQfsize(const PGresult *res,
int column_number);
</pre><p>
</p><p>
<code class="function">PQfsize</code> returns the space allocated for this column
in a database row, in other words the size of the server's
internal representation of the data type. (Accordingly, it is
not really very useful to clients.) A negative value indicates
the data type is variable-length.
</p></dd><dt id="LIBPQ-PQBINARYTUPLES"><span class="term">
<code class="function">PQbinaryTuples</code>
<a id="id-1.7.3.10.4.3.11.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns 1 if the <code class="structname">PGresult</code> contains binary data
and 0 if it contains text data.
</p><pre class="synopsis">
int PQbinaryTuples(const PGresult *res);
</pre><p>
</p><p>
This function is deprecated (except for its use in connection with
<code class="command">COPY</code>), because it is possible for a single
<code class="structname">PGresult</code> to contain text data in some columns and
binary data in others. <code class="function">PQfformat</code> is preferred.
<code class="function">PQbinaryTuples</code> returns 1 only if all columns of the
result are binary (format 1).
</p></dd><dt id="LIBPQ-PQGETVALUE"><span class="term">
<code class="function">PQgetvalue</code>
<a id="id-1.7.3.10.4.3.12.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns a single field value of one row of a
<code class="structname">PGresult</code>. Row and column numbers start
at 0. The caller should not free the result directly. It will
be freed when the associated <code class="structname">PGresult</code> handle is
passed to <code class="function">PQclear</code>.
</p><pre class="synopsis">
char *PQgetvalue(const PGresult *res,
int row_number,
int column_number);
</pre><p>
</p><p>
For data in text format, the value returned by
<code class="function">PQgetvalue</code> is a null-terminated character
string representation of the field value. For data in binary
format, the value is in the binary representation determined by
the data type's <code class="function">typsend</code> and <code class="function">typreceive</code>
functions. (The value is actually followed by a zero byte in
this case too, but that is not ordinarily useful, since the
value is likely to contain embedded nulls.)
</p><p>
An empty string is returned if the field value is null. See
<code class="function">PQgetisnull</code> to distinguish null values from
empty-string values.
</p><p>
The pointer returned by <code class="function">PQgetvalue</code> points
to storage that is part of the <code class="structname">PGresult</code>
structure. One should not modify the data it points to, and one
must explicitly copy the data into other storage if it is to be
used past the lifetime of the <code class="structname">PGresult</code>
structure itself.
</p></dd><dt id="LIBPQ-PQGETISNULL"><span class="term">
<code class="function">PQgetisnull</code>
<a id="id-1.7.3.10.4.3.13.1.2" class="indexterm"></a>
<a id="id-1.7.3.10.4.3.13.1.3" class="indexterm"></a>
</span></dt><dd><p>
Tests a field for a null value. Row and column numbers start
at 0.
</p><pre class="synopsis">
int PQgetisnull(const PGresult *res,
int row_number,
int column_number);
</pre><p>
</p><p>
This function returns 1 if the field is null and 0 if it
contains a non-null value. (Note that
<code class="function">PQgetvalue</code> will return an empty string,
not a null pointer, for a null field.)
</p></dd><dt id="LIBPQ-PQGETLENGTH"><span class="term">
<code class="function">PQgetlength</code>
<a id="id-1.7.3.10.4.3.14.1.2" class="indexterm"></a></span></dt><dd><p>
Returns the actual length of a field value in bytes. Row and
column numbers start at 0.
</p><pre class="synopsis">
int PQgetlength(const PGresult *res,
int row_number,
int column_number);
</pre><p>
</p><p>
This is the actual data length for the particular data value,
that is, the size of the object pointed to by
<code class="function">PQgetvalue</code>. For text data format this is
the same as <code class="function">strlen()</code>. For binary format this is
essential information. Note that one should <span class="emphasis"><em>not</em></span>
rely on <code class="function">PQfsize</code> to obtain the actual data
length.
</p></dd><dt id="LIBPQ-PQNPARAMS"><span class="term">
<code class="function">PQnparams</code>
<a id="id-1.7.3.10.4.3.15.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the number of parameters of a prepared statement.
</p><pre class="synopsis">
int PQnparams(const PGresult *res);
</pre><p>
</p><p>
This function is only useful when inspecting the result of
<code class="function">PQdescribePrepared</code>. For other types of queries it
will return zero.
</p></dd><dt id="LIBPQ-PQPARAMTYPE"><span class="term">
<code class="function">PQparamtype</code>
<a id="id-1.7.3.10.4.3.16.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the data type of the indicated statement parameter.
Parameter numbers start at 0.
</p><pre class="synopsis">
Oid PQparamtype(const PGresult *res, int param_number);
</pre><p>
</p><p>
This function is only useful when inspecting the result of
<code class="function">PQdescribePrepared</code>. For other types of queries it
will return zero.
</p></dd><dt id="LIBPQ-PQPRINT"><span class="term">
<code class="function">PQprint</code>
<a id="id-1.7.3.10.4.3.17.1.2" class="indexterm"></a>
</span></dt><dd><p>
Prints out all the rows and, optionally, the column names to
the specified output stream.
</p><pre class="synopsis">
void PQprint(FILE *fout, /* output stream */
const PGresult *res,
const PQprintOpt *po);
typedef struct
{
pqbool header; /* print output field headings and row count */
pqbool align; /* fill align the fields */
pqbool standard; /* old brain dead format */
pqbool html3; /* output HTML tables */
pqbool expanded; /* expand tables */
pqbool pager; /* use pager for output if needed */
char *fieldSep; /* field separator */
char *tableOpt; /* attributes for HTML table element */
char *caption; /* HTML table caption */
char **fieldName; /* null-terminated array of replacement field names */
} PQprintOpt;
</pre><p>
</p><p>
This function was formerly used by <span class="application">psql</span>
to print query results, but this is no longer the case. Note
that it assumes all the data is in text format.
</p></dd></dl></div></div><div class="sect2" id="LIBPQ-EXEC-NONSELECT"><div class="titlepage"><div><div><h3 class="title">34.3.3. Retrieving Other Result Information</h3></div></div></div><p>
These functions are used to extract other information from
<code class="structname">PGresult</code> objects.
</p><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PQCMDSTATUS"><span class="term">
<code class="function">PQcmdStatus</code>
<a id="id-1.7.3.10.5.3.1.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the command status tag from the SQL command that generated
the <code class="structname">PGresult</code>.
</p><pre class="synopsis">
char *PQcmdStatus(PGresult *res);
</pre><p>
</p><p>
Commonly this is just the name of the command, but it might include
additional data such as the number of rows processed. The caller
should not free the result directly. It will be freed when the
associated <code class="structname">PGresult</code> handle is passed to
<code class="function">PQclear</code>.
</p></dd><dt id="LIBPQ-PQCMDTUPLES"><span class="term">
<code class="function">PQcmdTuples</code>
<a id="id-1.7.3.10.5.3.2.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the number of rows affected by the SQL command.
</p><pre class="synopsis">
char *PQcmdTuples(PGresult *res);
</pre><p>
</p><p>
This function returns a string containing the number of rows
affected by the <acronym class="acronym">SQL</acronym> statement that generated the
<code class="structname">PGresult</code>. This function can only be used following
the execution of a <code class="command">SELECT</code>, <code class="command">CREATE TABLE AS</code>,
<code class="command">INSERT</code>, <code class="command">UPDATE</code>, <code class="command">DELETE</code>,
<code class="command">MOVE</code>, <code class="command">FETCH</code>, or <code class="command">COPY</code> statement,
or an <code class="command">EXECUTE</code> of a prepared query that contains an
<code class="command">INSERT</code>, <code class="command">UPDATE</code>, or <code class="command">DELETE</code> statement.
If the command that generated the <code class="structname">PGresult</code> was anything
else, <code class="function">PQcmdTuples</code> returns an empty string. The caller
should not free the return value directly. It will be freed when
the associated <code class="structname">PGresult</code> handle is passed to
<code class="function">PQclear</code>.
</p></dd><dt id="LIBPQ-PQOIDVALUE"><span class="term">
<code class="function">PQoidValue</code>
<a id="id-1.7.3.10.5.3.3.1.2" class="indexterm"></a>
</span></dt><dd><p>
Returns the OID<a id="id-1.7.3.10.5.3.3.2.1.1" class="indexterm"></a>
of the inserted row, if the <acronym class="acronym">SQL</acronym> command was an
<code class="command">INSERT</code> that inserted exactly one row into a table that
has OIDs, or a <code class="command">EXECUTE</code> of a prepared query containing
a suitable <code class="command">INSERT</code> statement. Otherwise, this function
returns <code class="literal">InvalidOid</code>. This function will also
return <code class="literal">InvalidOid</code> if the table affected by the
<code class="command">INSERT</code> statement does not contain OIDs.
</p><pre class="synopsis">
Oid PQoidValue(const PGresult *res);
</pre><p>
</p></dd><dt id="LIBPQ-PQOIDSTATUS"><span class="term">
<code class="function">PQoidStatus</code>
<a id="id-1.7.3.10.5.3.4.1.2" class="indexterm"></a>
</span></dt><dd><p>
This function is deprecated in favor of
<code class="function">PQoidValue</code> and is not thread-safe.
It returns a string with the OID of the inserted row, while
<code class="function">PQoidValue</code> returns the OID value.
</p><pre class="synopsis">
char *PQoidStatus(const PGresult *res);
</pre><p>
</p></dd></dl></div></div><div class="sect2" id="LIBPQ-EXEC-ESCAPE-STRING"><div class="titlepage"><div><div><h3 class="title">34.3.4. Escaping Strings for Inclusion in SQL Commands</h3></div></div></div><a id="id-1.7.3.10.6.2" class="indexterm"></a><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PQESCAPELITERAL"><span class="term">
<code class="function">PQescapeLiteral</code>
<a id="id-1.7.3.10.6.3.1.1.2" class="indexterm"></a>
</span></dt><dd><p>
</p><pre class="synopsis">
char *PQescapeLiteral(PGconn *conn, const char *str, size_t length);
</pre><p>
</p><p>
<code class="function">PQescapeLiteral</code> escapes a string for
use within an SQL command. This is useful when inserting data
values as literal constants in SQL commands. Certain characters
(such as quotes and backslashes) must be escaped to prevent them
from being interpreted specially by the SQL parser.
<code class="function">PQescapeLiteral</code> performs this operation.
</p><p>
<code class="function">PQescapeLiteral</code> returns an escaped version of the
<em class="parameter"><code>str</code></em> parameter in memory allocated with
<code class="function">malloc()</code>. This memory should be freed using
<code class="function">PQfreemem()</code> when the result is no longer needed.
A terminating zero byte is not required, and should not be
counted in <em class="parameter"><code>length</code></em>. (If a terminating zero byte is found
before <em class="parameter"><code>length</code></em> bytes are processed,
<code class="function">PQescapeLiteral</code> stops at the zero; the behavior is
thus rather like <code class="function">strncpy</code>.) The
return string has all special characters replaced so that they can
be properly processed by the <span class="productname">PostgreSQL</span>
string literal parser. A terminating zero byte is also added. The
single quotes that must surround <span class="productname">PostgreSQL</span>
string literals are included in the result string.
</p><p>
On error, <code class="function">PQescapeLiteral</code> returns <code class="symbol">NULL</code> and a suitable
message is stored in the <em class="parameter"><code>conn</code></em> object.
</p><div class="tip"><h3 class="title">Tip</h3><p>
It is especially important to do proper escaping when handling
strings that were received from an untrustworthy source.
Otherwise there is a security risk: you are vulnerable to
<span class="quote">“<span class="quote">SQL injection</span>”</span> attacks wherein unwanted SQL commands are
fed to your database.
</p></div><p>
Note that it is neither necessary nor correct to do escaping when a data
value is passed as a separate parameter in <code class="function">PQexecParams</code> or
its sibling routines.
</p></dd><dt id="LIBPQ-PQESCAPEIDENTIFIER"><span class="term">
<code class="function">PQescapeIdentifier</code>
<a id="id-1.7.3.10.6.3.2.1.2" class="indexterm"></a>
</span></dt><dd><p>
</p><pre class="synopsis">
char *PQescapeIdentifier(PGconn *conn, const char *str, size_t length);
</pre><p>
</p><p>
<code class="function">PQescapeIdentifier</code> escapes a string for
use as an SQL identifier, such as a table, column, or function name.
This is useful when a user-supplied identifier might contain
special characters that would otherwise not be interpreted as part
of the identifier by the SQL parser, or when the identifier might
contain upper case characters whose case should be preserved.
</p><p>
<code class="function">PQescapeIdentifier</code> returns a version of the
<em class="parameter"><code>str</code></em> parameter escaped as an SQL identifier
in memory allocated with <code class="function">malloc()</code>. This memory must be
freed using <code class="function">PQfreemem()</code> when the result is no longer
needed. A terminating zero byte is not required, and should not be
counted in <em class="parameter"><code>length</code></em>. (If a terminating zero byte is found
before <em class="parameter"><code>length</code></em> bytes are processed,
<code class="function">PQescapeIdentifier</code> stops at the zero; the behavior is
thus rather like <code class="function">strncpy</code>.) The
return string has all special characters replaced so that it
will be properly processed as an SQL identifier. A terminating zero byte
is also added. The return string will also be surrounded by double
quotes.
</p><p>
On error, <code class="function">PQescapeIdentifier</code> returns <code class="symbol">NULL</code> and a suitable
message is stored in the <em class="parameter"><code>conn</code></em> object.
</p><div class="tip"><h3 class="title">Tip</h3><p>
As with string literals, to prevent SQL injection attacks,
SQL identifiers must be escaped when they are received from an
untrustworthy source.
</p></div></dd><dt id="LIBPQ-PQESCAPESTRINGCONN"><span class="term">
<code class="function">PQescapeStringConn</code>
<a id="id-1.7.3.10.6.3.3.1.2" class="indexterm"></a>
</span></dt><dd><p>
</p><pre class="synopsis">
size_t PQescapeStringConn(PGconn *conn,
char *to, const char *from, size_t length,
int *error);
</pre><p>
</p><p>
<code class="function">PQescapeStringConn</code> escapes string literals, much like
<code class="function">PQescapeLiteral</code>. Unlike <code class="function">PQescapeLiteral</code>,
the caller is responsible for providing an appropriately sized buffer.
Furthermore, <code class="function">PQescapeStringConn</code> does not generate the
single quotes that must surround <span class="productname">PostgreSQL</span> string
literals; they should be provided in the SQL command that the
result is inserted into. The parameter <em class="parameter"><code>from</code></em> points to
the first character of the string that is to be escaped, and the
<em class="parameter"><code>length</code></em> parameter gives the number of bytes in this
string. A terminating zero byte is not required, and should not be
counted in <em class="parameter"><code>length</code></em>. (If a terminating zero byte is found
before <em class="parameter"><code>length</code></em> bytes are processed,
<code class="function">PQescapeStringConn</code> stops at the zero; the behavior is
thus rather like <code class="function">strncpy</code>.) <em class="parameter"><code>to</code></em> shall point
to a buffer that is able to hold at least one more byte than twice
the value of <em class="parameter"><code>length</code></em>, otherwise the behavior is undefined.
Behavior is likewise undefined if the <em class="parameter"><code>to</code></em> and
<em class="parameter"><code>from</code></em> strings overlap.
</p><p>
If the <em class="parameter"><code>error</code></em> parameter is not <code class="symbol">NULL</code>, then
<code class="literal">*error</code> is set to zero on success, nonzero on error.
Presently the only possible error conditions involve invalid multibyte
encoding in the source string. The output string is still generated
on error, but it can be expected that the server will reject it as
malformed. On error, a suitable message is stored in the
<em class="parameter"><code>conn</code></em> object, whether or not <em class="parameter"><code>error</code></em> is <code class="symbol">NULL</code>.
</p><p>
<code class="function">PQescapeStringConn</code> returns the number of bytes written
to <em class="parameter"><code>to</code></em>, not including the terminating zero byte.
</p></dd><dt id="LIBPQ-PQESCAPESTRING"><span class="term">
<code class="function">PQescapeString</code>
<a id="id-1.7.3.10.6.3.4.1.2" class="indexterm"></a>
</span></dt><dd><p>
<code class="function">PQescapeString</code> is an older, deprecated version of
<code class="function">PQescapeStringConn</code>.
</p><pre class="synopsis">
size_t PQescapeString (char *to, const char *from, size_t length);
</pre><p>
</p><p>
The only difference from <code class="function">PQescapeStringConn</code> is that
<code class="function">PQescapeString</code> does not take <code class="structname">PGconn</code>
or <em class="parameter"><code>error</code></em> parameters.
Because of this, it cannot adjust its behavior depending on the
connection properties (such as character encoding) and therefore
<span class="emphasis"><em>it might give the wrong results</em></span>. Also, it has no way
to report error conditions.
</p><p>
<code class="function">PQescapeString</code> can be used safely in
client programs that work with only one <span class="productname">PostgreSQL</span>
connection at a time (in this case it can find out what it needs to
know <span class="quote">“<span class="quote">behind the scenes</span>”</span>). In other contexts it is a security
hazard and should be avoided in favor of
<code class="function">PQescapeStringConn</code>.
</p></dd><dt id="LIBPQ-PQESCAPEBYTEACONN"><span class="term">
<code class="function">PQescapeByteaConn</code>
<a id="id-1.7.3.10.6.3.5.1.2" class="indexterm"></a>
</span></dt><dd><p>
Escapes binary data for use within an SQL command with the type
<code class="type">bytea</code>. As with <code class="function">PQescapeStringConn</code>,
this is only used when inserting data directly into an SQL command string.
</p><pre class="synopsis">
unsigned char *PQescapeByteaConn(PGconn *conn,
const unsigned char *from,
size_t from_length,
size_t *to_length);
</pre><p>
</p><p>
Certain byte values must be escaped when used as part of a
<code class="type">bytea</code> literal in an <acronym class="acronym">SQL</acronym> statement.
<code class="function">PQescapeByteaConn</code> escapes bytes using
either hex encoding or backslash escaping. See <a class="xref" href="datatype-binary.html" title="8.4. Binary Data Types">Section 8.4</a> for more information.
</p><p>
The <em class="parameter"><code>from</code></em> parameter points to the first
byte of the string that is to be escaped, and the
<em class="parameter"><code>from_length</code></em> parameter gives the number of
bytes in this binary string. (A terminating zero byte is
neither necessary nor counted.) The <em class="parameter"><code>to_length</code></em>
parameter points to a variable that will hold the resultant
escaped string length. This result string length includes the terminating
zero byte of the result.
</p><p>
<code class="function">PQescapeByteaConn</code> returns an escaped version of the
<em class="parameter"><code>from</code></em> parameter binary string in memory
allocated with <code class="function">malloc()</code>. This memory should be freed using
<code class="function">PQfreemem()</code> when the result is no longer needed. The
return string has all special characters replaced so that they can
be properly processed by the <span class="productname">PostgreSQL</span>
string literal parser, and the <code class="type">bytea</code> input function. A
terminating zero byte is also added. The single quotes that must
surround <span class="productname">PostgreSQL</span> string literals are
not part of the result string.
</p><p>
On error, a null pointer is returned, and a suitable error message
is stored in the <em class="parameter"><code>conn</code></em> object. Currently, the only
possible error is insufficient memory for the result string.
</p></dd><dt id="LIBPQ-PQESCAPEBYTEA"><span class="term">
<code class="function">PQescapeBytea</code>
<a id="id-1.7.3.10.6.3.6.1.2" class="indexterm"></a>
</span></dt><dd><p>
<code class="function">PQescapeBytea</code> is an older, deprecated version of
<code class="function">PQescapeByteaConn</code>.
</p><pre class="synopsis">
unsigned char *PQescapeBytea(const unsigned char *from,
size_t from_length,
size_t *to_length);
</pre><p>
</p><p>
The only difference from <code class="function">PQescapeByteaConn</code> is that
<code class="function">PQescapeBytea</code> does not take a <code class="structname">PGconn</code>
parameter. Because of this, <code class="function">PQescapeBytea</code> can
only be used safely in client programs that use a single
<span class="productname">PostgreSQL</span> connection at a time (in this case
it can find out what it needs to know <span class="quote">“<span class="quote">behind the
scenes</span>”</span>). It <span class="emphasis"><em>might give the wrong results</em></span> if
used in programs that use multiple database connections (use
<code class="function">PQescapeByteaConn</code> in such cases).
</p></dd><dt id="LIBPQ-PQUNESCAPEBYTEA"><span class="term">
<code class="function">PQunescapeBytea</code>
<a id="id-1.7.3.10.6.3.7.1.2" class="indexterm"></a>
</span></dt><dd><p>
Converts a string representation of binary data into binary data
— the reverse of <code class="function">PQescapeBytea</code>. This
is needed when retrieving <code class="type">bytea</code> data in text format,
but not when retrieving it in binary format.
</p><pre class="synopsis">
unsigned char *PQunescapeBytea(const unsigned char *from, size_t *to_length);
</pre><p>
</p><p>
The <em class="parameter"><code>from</code></em> parameter points to a string
such as might be returned by <code class="function">PQgetvalue</code> when applied
to a <code class="type">bytea</code> column. <code class="function">PQunescapeBytea</code>
converts this string representation into its binary representation.
It returns a pointer to a buffer allocated with
<code class="function">malloc()</code>, or <code class="symbol">NULL</code> on error, and puts the size of
the buffer in <em class="parameter"><code>to_length</code></em>. The result must be
freed using <code class="function">PQfreemem</code> when it is no longer needed.
</p><p>
This conversion is not exactly the inverse of
<code class="function">PQescapeBytea</code>, because the string is not expected
to be <span class="quote">“<span class="quote">escaped</span>”</span> when received from <code class="function">PQgetvalue</code>.
In particular this means there is no need for string quoting considerations,
and so no need for a <code class="structname">PGconn</code> parameter.
</p></dd></dl></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="libpq-status.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="libpq.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="libpq-async.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">34.2. Connection Status Functions </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 34.4. Asynchronous Command Processing</td></tr></table></div></body></html>
