codekingpro/portable-devtools
114k
1<?xml version="1.0" encoding="UTF-8" standalone="no"?>2<!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>34.4. Asynchronous Command Processing</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-exec.html" title="34.3. Command Execution Functions" /><link rel="next" href="libpq-pipeline-mode.html" title="34.5. Pipeline Mode" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">34.4. Asynchronous Command Processing</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="libpq-exec.html" title="34.3. Command Execution 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 class="application">libpq</span> — C Library</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="libpq-pipeline-mode.html" title="34.5. Pipeline Mode">Next</a></td></tr></table><hr /></div><div class="sect1" id="LIBPQ-ASYNC"><div class="titlepage"><div><div><h2 class="title" style="clear: both">34.4. Asynchronous Command Processing <a href="#LIBPQ-ASYNC" class="id_link">#</a></h2></div></div></div><a id="id-1.7.3.11.2" class="indexterm"></a><p>3 The <a class="xref" href="libpq-exec.html#LIBPQ-PQEXEC"><code class="function">PQexec</code></a> function is adequate for submitting4 commands in normal, synchronous applications. It has a few5 deficiencies, however, that can be of importance to some users:6 7 </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>8 <a class="xref" href="libpq-exec.html#LIBPQ-PQEXEC"><code class="function">PQexec</code></a> waits for the command to be completed.9 The application might have other work to do (such as maintaining a10 user interface), in which case it won't want to block waiting for11 the response.12 </p></li><li class="listitem"><p>13 Since the execution of the client application is suspended while it14 waits for the result, it is hard for the application to decide that15 it would like to try to cancel the ongoing command. (It can be done16 from a signal handler, but not otherwise.)17 </p></li><li class="listitem"><p>18 <a class="xref" href="libpq-exec.html#LIBPQ-PQEXEC"><code class="function">PQexec</code></a> can return only one19 <code class="structname">PGresult</code> structure. If the submitted command20 string contains multiple <acronym class="acronym">SQL</acronym> commands, all but21 the last <code class="structname">PGresult</code> are discarded by22 <a class="xref" href="libpq-exec.html#LIBPQ-PQEXEC"><code class="function">PQexec</code></a>.23 </p></li><li class="listitem"><p>24 <a class="xref" href="libpq-exec.html#LIBPQ-PQEXEC"><code class="function">PQexec</code></a> always collects the command's entire result,25 buffering it in a single <code class="structname">PGresult</code>. While26 this simplifies error-handling logic for the application, it can be27 impractical for results containing many rows.28 </p></li></ul></div><p>29 </p><p>30 Applications that do not like these limitations can instead use the31 underlying functions that <a class="xref" href="libpq-exec.html#LIBPQ-PQEXEC"><code class="function">PQexec</code></a> is built from:32 <a class="xref" href="libpq-async.html#LIBPQ-PQSENDQUERY"><code class="function">PQsendQuery</code></a> and <a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a>.33 There are also34 <a class="xref" href="libpq-async.html#LIBPQ-PQSENDQUERYPARAMS"><code class="function">PQsendQueryParams</code></a>,35 <a class="xref" href="libpq-async.html#LIBPQ-PQSENDPREPARE"><code class="function">PQsendPrepare</code></a>,36 <a class="xref" href="libpq-async.html#LIBPQ-PQSENDQUERYPREPARED"><code class="function">PQsendQueryPrepared</code></a>,37 <a class="xref" href="libpq-async.html#LIBPQ-PQSENDDESCRIBEPREPARED"><code class="function">PQsendDescribePrepared</code></a>, and38 <a class="xref" href="libpq-async.html#LIBPQ-PQSENDDESCRIBEPORTAL"><code class="function">PQsendDescribePortal</code></a>,39 which can be used with <a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a> to duplicate40 the functionality of41 <a class="xref" href="libpq-exec.html#LIBPQ-PQEXECPARAMS"><code class="function">PQexecParams</code></a>,42 <a class="xref" href="libpq-exec.html#LIBPQ-PQPREPARE"><code class="function">PQprepare</code></a>,43 <a class="xref" href="libpq-exec.html#LIBPQ-PQEXECPREPARED"><code class="function">PQexecPrepared</code></a>,44 <a class="xref" href="libpq-exec.html#LIBPQ-PQDESCRIBEPREPARED"><code class="function">PQdescribePrepared</code></a>, and45 <a class="xref" href="libpq-exec.html#LIBPQ-PQDESCRIBEPORTAL"><code class="function">PQdescribePortal</code></a>46 respectively.47 48 </p><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PQSENDQUERY"><span class="term"><code class="function">PQsendQuery</code><a id="id-1.7.3.11.4.15.1.1.2" class="indexterm"></a></span> <a href="#LIBPQ-PQSENDQUERY" class="id_link">#</a></dt><dd><p>49 Submits a command to the server without waiting for the result(s).50 1 is returned if the command was successfully dispatched and 0 if51 not (in which case, use <a class="xref" href="libpq-status.html#LIBPQ-PQERRORMESSAGE"><code class="function">PQerrorMessage</code></a> to get more52 information about the failure).53</p><pre class="synopsis">54int PQsendQuery(PGconn *conn, const char *command);55</pre><p>56 57 After successfully calling <a class="xref" href="libpq-async.html#LIBPQ-PQSENDQUERY"><code class="function">PQsendQuery</code></a>, call58 <a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a> one or more times to obtain the59 results. <a class="xref" href="libpq-async.html#LIBPQ-PQSENDQUERY"><code class="function">PQsendQuery</code></a> cannot be called again60 (on the same connection) until <a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a>61 has returned a null pointer, indicating that the command is done.62 </p><p>63 In pipeline mode, this function is disallowed.64 </p></dd><dt id="LIBPQ-PQSENDQUERYPARAMS"><span class="term"><code class="function">PQsendQueryParams</code><a id="id-1.7.3.11.4.15.2.1.2" class="indexterm"></a></span> <a href="#LIBPQ-PQSENDQUERYPARAMS" class="id_link">#</a></dt><dd><p>65 Submits a command and separate parameters to the server without66 waiting for the result(s).67</p><pre class="synopsis">68int PQsendQueryParams(PGconn *conn,69 const char *command,70 int nParams,71 const Oid *paramTypes,72 const char * const *paramValues,73 const int *paramLengths,74 const int *paramFormats,75 int resultFormat);76</pre><p>77 78 This is equivalent to <a class="xref" href="libpq-async.html#LIBPQ-PQSENDQUERY"><code class="function">PQsendQuery</code></a> except that79 query parameters can be specified separately from the query string.80 The function's parameters are handled identically to81 <a class="xref" href="libpq-exec.html#LIBPQ-PQEXECPARAMS"><code class="function">PQexecParams</code></a>. Like82 <a class="xref" href="libpq-exec.html#LIBPQ-PQEXECPARAMS"><code class="function">PQexecParams</code></a>, it allows only one command in the83 query string.84 </p></dd><dt id="LIBPQ-PQSENDPREPARE"><span class="term"><code class="function">PQsendPrepare</code><a id="id-1.7.3.11.4.15.3.1.2" class="indexterm"></a></span> <a href="#LIBPQ-PQSENDPREPARE" class="id_link">#</a></dt><dd><p>85 Sends a request to create a prepared statement with the given86 parameters, without waiting for completion.87</p><pre class="synopsis">88int PQsendPrepare(PGconn *conn,89 const char *stmtName,90 const char *query,91 int nParams,92 const Oid *paramTypes);93</pre><p>94 95 This is an asynchronous version of <a class="xref" href="libpq-exec.html#LIBPQ-PQPREPARE"><code class="function">PQprepare</code></a>: it96 returns 1 if it was able to dispatch the request, and 0 if not.97 After a successful call, call <a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a> to98 determine whether the server successfully created the prepared99 statement. The function's parameters are handled identically to100 <a class="xref" href="libpq-exec.html#LIBPQ-PQPREPARE"><code class="function">PQprepare</code></a>.101 </p></dd><dt id="LIBPQ-PQSENDQUERYPREPARED"><span class="term"><code class="function">PQsendQueryPrepared</code><a id="id-1.7.3.11.4.15.4.1.2" class="indexterm"></a></span> <a href="#LIBPQ-PQSENDQUERYPREPARED" class="id_link">#</a></dt><dd><p>102 Sends a request to execute a prepared statement with given103 parameters, without waiting for the result(s).104</p><pre class="synopsis">105int PQsendQueryPrepared(PGconn *conn,106 const char *stmtName,107 int nParams,108 const char * const *paramValues,109 const int *paramLengths,110 const int *paramFormats,111 int resultFormat);112</pre><p>113 114 This is similar to <a class="xref" href="libpq-async.html#LIBPQ-PQSENDQUERYPARAMS"><code class="function">PQsendQueryParams</code></a>, but115 the command to be executed is specified by naming a116 previously-prepared statement, instead of giving a query string.117 The function's parameters are handled identically to118 <a class="xref" href="libpq-exec.html#LIBPQ-PQEXECPREPARED"><code class="function">PQexecPrepared</code></a>.119 </p></dd><dt id="LIBPQ-PQSENDDESCRIBEPREPARED"><span class="term"><code class="function">PQsendDescribePrepared</code><a id="id-1.7.3.11.4.15.5.1.2" class="indexterm"></a></span> <a href="#LIBPQ-PQSENDDESCRIBEPREPARED" class="id_link">#</a></dt><dd><p>120 Submits a request to obtain information about the specified121 prepared statement, without waiting for completion.122</p><pre class="synopsis">123int PQsendDescribePrepared(PGconn *conn, const char *stmtName);124</pre><p>125 126 This is an asynchronous version of <a class="xref" href="libpq-exec.html#LIBPQ-PQDESCRIBEPREPARED"><code class="function">PQdescribePrepared</code></a>:127 it returns 1 if it was able to dispatch the request, and 0 if not.128 After a successful call, call <a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a> to129 obtain the results. The function's parameters are handled130 identically to <a class="xref" href="libpq-exec.html#LIBPQ-PQDESCRIBEPREPARED"><code class="function">PQdescribePrepared</code></a>.131 </p></dd><dt id="LIBPQ-PQSENDDESCRIBEPORTAL"><span class="term"><code class="function">PQsendDescribePortal</code><a id="id-1.7.3.11.4.15.6.1.2" class="indexterm"></a></span> <a href="#LIBPQ-PQSENDDESCRIBEPORTAL" class="id_link">#</a></dt><dd><p>132 Submits a request to obtain information about the specified133 portal, without waiting for completion.134</p><pre class="synopsis">135int PQsendDescribePortal(PGconn *conn, const char *portalName);136</pre><p>137 138 This is an asynchronous version of <a class="xref" href="libpq-exec.html#LIBPQ-PQDESCRIBEPORTAL"><code class="function">PQdescribePortal</code></a>:139 it returns 1 if it was able to dispatch the request, and 0 if not.140 After a successful call, call <a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a> to141 obtain the results. The function's parameters are handled142 identically to <a class="xref" href="libpq-exec.html#LIBPQ-PQDESCRIBEPORTAL"><code class="function">PQdescribePortal</code></a>.143 </p></dd><dt id="LIBPQ-PQGETRESULT"><span class="term"><code class="function">PQgetResult</code><a id="id-1.7.3.11.4.15.7.1.2" class="indexterm"></a></span> <a href="#LIBPQ-PQGETRESULT" class="id_link">#</a></dt><dd><p>144 Waits for the next result from a prior145 <a class="xref" href="libpq-async.html#LIBPQ-PQSENDQUERY"><code class="function">PQsendQuery</code></a>,146 <a class="xref" href="libpq-async.html#LIBPQ-PQSENDQUERYPARAMS"><code class="function">PQsendQueryParams</code></a>,147 <a class="xref" href="libpq-async.html#LIBPQ-PQSENDPREPARE"><code class="function">PQsendPrepare</code></a>,148 <a class="xref" href="libpq-async.html#LIBPQ-PQSENDQUERYPREPARED"><code class="function">PQsendQueryPrepared</code></a>,149 <a class="xref" href="libpq-async.html#LIBPQ-PQSENDDESCRIBEPREPARED"><code class="function">PQsendDescribePrepared</code></a>,150 <a class="xref" href="libpq-async.html#LIBPQ-PQSENDDESCRIBEPORTAL"><code class="function">PQsendDescribePortal</code></a>, or151 <a class="xref" href="libpq-pipeline-mode.html#LIBPQ-PQPIPELINESYNC"><code class="function">PQpipelineSync</code></a>152 call, and returns it.153 A null pointer is returned when the command is complete and there154 will be no more results.155</p><pre class="synopsis">156PGresult *PQgetResult(PGconn *conn);157</pre><p>158 </p><p>159 <a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a> must be called repeatedly until160 it returns a null pointer, indicating that the command is done.161 (If called when no command is active,162 <a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a> will just return a null pointer163 at once.) Each non-null result from164 <a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a> should be processed using the165 same <code class="structname">PGresult</code> accessor functions previously166 described. Don't forget to free each result object with167 <a class="xref" href="libpq-exec.html#LIBPQ-PQCLEAR"><code class="function">PQclear</code></a> when done with it. Note that168 <a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a> will block only if a command is169 active and the necessary response data has not yet been read by170 <a class="xref" href="libpq-async.html#LIBPQ-PQCONSUMEINPUT"><code class="function">PQconsumeInput</code>171 </a>.172 </p><p>173 In pipeline mode, <code class="function">PQgetResult</code> will return normally174 unless an error occurs; for any subsequent query sent after the one175 that caused the error until (and excluding) the next synchronization point,176 a special result of type <code class="literal">PGRES_PIPELINE_ABORTED</code> will177 be returned, and a null pointer will be returned after it.178 When the pipeline synchronization point is reached, a result of type179 <code class="literal">PGRES_PIPELINE_SYNC</code> will be returned.180 The result of the next query after the synchronization point follows181 immediately (that is, no null pointer is returned after182 the synchronization point.)183 </p><div class="note"><h3 class="title">Note</h3><p>184 Even when <a class="xref" href="libpq-exec.html#LIBPQ-PQRESULTSTATUS"><code class="function">PQresultStatus</code></a> indicates a fatal185 error, <a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a> should be called until it186 returns a null pointer, to allow <span class="application">libpq</span> to187 process the error information completely.188 </p></div></dd></dl></div><p>189 </p><p>190 Using <a class="xref" href="libpq-async.html#LIBPQ-PQSENDQUERY"><code class="function">PQsendQuery</code></a> and191 <a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a> solves one of192 <a class="xref" href="libpq-exec.html#LIBPQ-PQEXEC"><code class="function">PQexec</code></a>'s problems: If a command string contains193 multiple <acronym class="acronym">SQL</acronym> commands, the results of those commands194 can be obtained individually. (This allows a simple form of overlapped195 processing, by the way: the client can be handling the results of one196 command while the server is still working on later queries in the same197 command string.)198 </p><p>199 Another frequently-desired feature that can be obtained with200 <a class="xref" href="libpq-async.html#LIBPQ-PQSENDQUERY"><code class="function">PQsendQuery</code></a> and <a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a>201 is retrieving large query results a row at a time. This is discussed202 in <a class="xref" href="libpq-single-row-mode.html" title="34.6. Retrieving Query Results Row-by-Row">Section 34.6</a>.203 </p><p>204 By itself, calling <a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a>205 will still cause the client to block until the server completes the206 next <acronym class="acronym">SQL</acronym> command. This can be avoided by proper207 use of two more functions:208 209 </p><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PQCONSUMEINPUT"><span class="term"><code class="function">PQconsumeInput</code><a id="id-1.7.3.11.7.3.1.1.2" class="indexterm"></a>210 </span> <a href="#LIBPQ-PQCONSUMEINPUT" class="id_link">#</a></dt><dd><p>211 If input is available from the server, consume it.212</p><pre class="synopsis">213int PQconsumeInput(PGconn *conn);214</pre><p>215 </p><p>216 <a class="xref" href="libpq-async.html#LIBPQ-PQCONSUMEINPUT"><code class="function">PQconsumeInput</code>217 </a> normally returns 1 indicating218 <span class="quote">“<span class="quote">no error</span>”</span>, but returns 0 if there was some kind of219 trouble (in which case <a class="xref" href="libpq-status.html#LIBPQ-PQERRORMESSAGE"><code class="function">PQerrorMessage</code></a> can be220 consulted). Note that the result does not say whether any input221 data was actually collected. After calling222 <a class="xref" href="libpq-async.html#LIBPQ-PQCONSUMEINPUT"><code class="function">PQconsumeInput</code>223 </a>, the application can check224 <a class="xref" href="libpq-async.html#LIBPQ-PQISBUSY"><code class="function">PQisBusy</code></a> and/or225 <code class="function">PQnotifies</code> to see if their state has changed.226 </p><p>227 <a class="xref" href="libpq-async.html#LIBPQ-PQCONSUMEINPUT"><code class="function">PQconsumeInput</code>228 </a> can be called even if the229 application is not prepared to deal with a result or notification230 just yet. The function will read available data and save it in231 a buffer, thereby causing a <code class="function">select()</code>232 read-ready indication to go away. The application can thus use233 <a class="xref" href="libpq-async.html#LIBPQ-PQCONSUMEINPUT"><code class="function">PQconsumeInput</code>234 </a> to clear the235 <code class="function">select()</code> condition immediately, and then236 examine the results at leisure.237 </p></dd><dt id="LIBPQ-PQISBUSY"><span class="term"><code class="function">PQisBusy</code><a id="id-1.7.3.11.7.3.2.1.2" class="indexterm"></a></span> <a href="#LIBPQ-PQISBUSY" class="id_link">#</a></dt><dd><p>238 Returns 1 if a command is busy, that is,239 <a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a> would block waiting for input.240 A 0 return indicates that <a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a> can be241 called with assurance of not blocking.242</p><pre class="synopsis">243int PQisBusy(PGconn *conn);244</pre><p>245 </p><p>246 <a class="xref" href="libpq-async.html#LIBPQ-PQISBUSY"><code class="function">PQisBusy</code></a> will not itself attempt to read data247 from the server; therefore <a class="xref" href="libpq-async.html#LIBPQ-PQCONSUMEINPUT"><code class="function">PQconsumeInput</code>248 </a>249 must be invoked first, or the busy state will never end.250 </p></dd></dl></div><p>251 </p><p>252 A typical application using these functions will have a main loop that253 uses <code class="function">select()</code> or <code class="function">poll()</code> to wait for254 all the conditions that it must respond to. One of the conditions255 will be input available from the server, which in terms of256 <code class="function">select()</code> means readable data on the file257 descriptor identified by <a class="xref" href="libpq-status.html#LIBPQ-PQSOCKET"><code class="function">PQsocket</code></a>. When the main258 loop detects input ready, it should call259 <a class="xref" href="libpq-async.html#LIBPQ-PQCONSUMEINPUT"><code class="function">PQconsumeInput</code>260 </a> to read the input. It can then261 call <a class="xref" href="libpq-async.html#LIBPQ-PQISBUSY"><code class="function">PQisBusy</code></a>, followed by262 <a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a> if <a class="xref" href="libpq-async.html#LIBPQ-PQISBUSY"><code class="function">PQisBusy</code></a>263 returns false (0). It can also call <code class="function">PQnotifies</code>264 to detect <code class="command">NOTIFY</code> messages (see <a class="xref" href="libpq-notify.html" title="34.9. Asynchronous Notification">Section 34.9</a>).265 </p><p>266 A client that uses267 <a class="xref" href="libpq-async.html#LIBPQ-PQSENDQUERY"><code class="function">PQsendQuery</code></a>/<a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a>268 can also attempt to cancel a command that is still being processed269 by the server; see <a class="xref" href="libpq-cancel.html" title="34.7. Canceling Queries in Progress">Section 34.7</a>. But regardless of270 the return value of <a class="xref" href="libpq-cancel.html#LIBPQ-PQCANCEL"><code class="function">PQcancel</code></a>, the application271 must continue with the normal result-reading sequence using272 <a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a>. A successful cancellation will273 simply cause the command to terminate sooner than it would have274 otherwise.275 </p><p>276 By using the functions described above, it is possible to avoid277 blocking while waiting for input from the database server. However,278 it is still possible that the application will block waiting to send279 output to the server. This is relatively uncommon but can happen if280 very long SQL commands or data values are sent. (It is much more281 probable if the application sends data via <code class="command">COPY IN</code>,282 however.) To prevent this possibility and achieve completely283 nonblocking database operation, the following additional functions284 can be used.285 286 </p><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PQSETNONBLOCKING"><span class="term"><code class="function">PQsetnonblocking</code><a id="id-1.7.3.11.10.2.1.1.2" class="indexterm"></a></span> <a href="#LIBPQ-PQSETNONBLOCKING" class="id_link">#</a></dt><dd><p>287 Sets the nonblocking status of the connection.288</p><pre class="synopsis">289int PQsetnonblocking(PGconn *conn, int arg);290</pre><p>291 </p><p>292 Sets the state of the connection to nonblocking if293 <em class="parameter"><code>arg</code></em> is 1, or blocking if294 <em class="parameter"><code>arg</code></em> is 0. Returns 0 if OK, -1 if error.295 </p><p>296 In the nonblocking state, successful calls to297 <a class="xref" href="libpq-async.html#LIBPQ-PQSENDQUERY"><code class="function">PQsendQuery</code></a>, <a class="xref" href="libpq-copy.html#LIBPQ-PQPUTLINE"><code class="function">PQputline</code></a>,298 <a class="xref" href="libpq-copy.html#LIBPQ-PQPUTNBYTES"><code class="function">PQputnbytes</code></a>, <a class="xref" href="libpq-copy.html#LIBPQ-PQPUTCOPYDATA"><code class="function">PQputCopyData</code></a>,299 and <a class="xref" href="libpq-copy.html#LIBPQ-PQENDCOPY"><code class="function">PQendcopy</code></a> will not block; their changes300 are stored in the local output buffer until they are flushed.301 Unsuccessful calls will return an error and must be retried.302 </p><p>303 Note that <a class="xref" href="libpq-exec.html#LIBPQ-PQEXEC"><code class="function">PQexec</code></a> does not honor nonblocking304 mode; if it is called, it will act in blocking fashion anyway.305 </p></dd><dt id="LIBPQ-PQISNONBLOCKING"><span class="term"><code class="function">PQisnonblocking</code><a id="id-1.7.3.11.10.2.2.1.2" class="indexterm"></a></span> <a href="#LIBPQ-PQISNONBLOCKING" class="id_link">#</a></dt><dd><p>306 Returns the blocking status of the database connection.307</p><pre class="synopsis">308int PQisnonblocking(const PGconn *conn);309</pre><p>310 </p><p>311 Returns 1 if the connection is set to nonblocking mode and 0 if312 blocking.313 </p></dd><dt id="LIBPQ-PQFLUSH"><span class="term"><code class="function">PQflush</code><a id="id-1.7.3.11.10.2.3.1.2" class="indexterm"></a></span> <a href="#LIBPQ-PQFLUSH" class="id_link">#</a></dt><dd><p>314 Attempts to flush any queued output data to the server. Returns315 0 if successful (or if the send queue is empty), -1 if it failed316 for some reason, or 1 if it was unable to send all the data in317 the send queue yet (this case can only occur if the connection318 is nonblocking).319</p><pre class="synopsis">320int PQflush(PGconn *conn);321</pre><p>322 </p></dd></dl></div><p>323 </p><p>324 After sending any command or data on a nonblocking connection, call325 <a class="xref" href="libpq-async.html#LIBPQ-PQFLUSH"><code class="function">PQflush</code></a>. If it returns 1, wait for the socket326 to become read- or write-ready. If it becomes write-ready, call327 <a class="xref" href="libpq-async.html#LIBPQ-PQFLUSH"><code class="function">PQflush</code></a> again. If it becomes read-ready, call328 <a class="xref" href="libpq-async.html#LIBPQ-PQCONSUMEINPUT"><code class="function">PQconsumeInput</code>329 </a>, then call330 <a class="xref" href="libpq-async.html#LIBPQ-PQFLUSH"><code class="function">PQflush</code></a> again. Repeat until331 <a class="xref" href="libpq-async.html#LIBPQ-PQFLUSH"><code class="function">PQflush</code></a> returns 0. (It is necessary to check for332 read-ready and drain the input with <a class="xref" href="libpq-async.html#LIBPQ-PQCONSUMEINPUT"><code class="function">PQconsumeInput</code>333 </a>,334 because the server can block trying to send us data, e.g., NOTICE335 messages, and won't read our data until we read its.) Once336 <a class="xref" href="libpq-async.html#LIBPQ-PQFLUSH"><code class="function">PQflush</code></a> returns 0, wait for the socket to be337 read-ready and then read the response as described above.338 </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="libpq-exec.html" title="34.3. 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