ok
Direktori : /opt/alt/postgresql11/usr/share/doc/alt-postgresql11-9.2.24/html/ |
Current File : //opt/alt/postgresql11/usr/share/doc/alt-postgresql11-9.2.24/html/spi-memory.html |
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >Memory Management</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK REV="MADE" HREF="mailto:pgsql-docs@postgresql.org"><LINK REL="HOME" TITLE="PostgreSQL 9.2.24 Documentation" HREF="index.html"><LINK REL="UP" TITLE="Server Programming Interface" HREF="spi.html"><LINK REL="PREVIOUS" TITLE="SPI_getnspname" HREF="spi-spi-getnspname.html"><LINK REL="NEXT" TITLE="SPI_palloc" HREF="spi-spi-palloc.html"><LINK REL="STYLESHEET" TYPE="text/css" HREF="stylesheet.css"><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"><META NAME="creation" CONTENT="2017-11-06T22:43:11"></HEAD ><BODY CLASS="SECT1" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="5" ALIGN="center" VALIGN="bottom" ><A HREF="index.html" >PostgreSQL 9.2.24 Documentation</A ></TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A TITLE="SPI_getnspname" HREF="spi-spi-getnspname.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="spi.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" >Chapter 43. Server Programming Interface</TD ><TD WIDTH="20%" ALIGN="right" VALIGN="top" ><A TITLE="SPI_palloc" HREF="spi-spi-palloc.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="SPI-MEMORY" >43.3. Memory Management</A ></H1 ><DIV CLASS="TOC" ><DL ><DT ><B >Table of Contents</B ></DT ><DT ><A HREF="spi-spi-palloc.html" >SPI_palloc</A > -- allocate memory in the upper executor context</DT ><DT ><A HREF="spi-realloc.html" >SPI_repalloc</A > -- reallocate memory in the upper executor context</DT ><DT ><A HREF="spi-spi-pfree.html" >SPI_pfree</A > -- free memory in the upper executor context</DT ><DT ><A HREF="spi-spi-copytuple.html" >SPI_copytuple</A > -- make a copy of a row in the upper executor context</DT ><DT ><A HREF="spi-spi-returntuple.html" >SPI_returntuple</A > -- prepare to return a tuple as a Datum</DT ><DT ><A HREF="spi-spi-modifytuple.html" >SPI_modifytuple</A > -- create a row by replacing selected fields of a given row</DT ><DT ><A HREF="spi-spi-freetuple.html" >SPI_freetuple</A > -- free a row allocated in the upper executor context</DT ><DT ><A HREF="spi-spi-freetupletable.html" >SPI_freetuptable</A > -- free a row set created by <CODE CLASS="FUNCTION" >SPI_execute</CODE > or a similar function</DT ><DT ><A HREF="spi-spi-freeplan.html" >SPI_freeplan</A > -- free a previously saved prepared statement</DT ></DL ></DIV ><P > <SPAN CLASS="PRODUCTNAME" >PostgreSQL</SPAN > allocates memory within <I CLASS="FIRSTTERM" >memory contexts</I >, which provide a convenient method of managing allocations made in many different places that need to live for differing amounts of time. Destroying a context releases all the memory that was allocated in it. Thus, it is not necessary to keep track of individual objects to avoid memory leaks; instead only a relatively small number of contexts have to be managed. <CODE CLASS="FUNCTION" >palloc</CODE > and related functions allocate memory from the <SPAN CLASS="QUOTE" >"current"</SPAN > context. </P ><P > <CODE CLASS="FUNCTION" >SPI_connect</CODE > creates a new memory context and makes it current. <CODE CLASS="FUNCTION" >SPI_finish</CODE > restores the previous current memory context and destroys the context created by <CODE CLASS="FUNCTION" >SPI_connect</CODE >. These actions ensure that transient memory allocations made inside your procedure are reclaimed at procedure exit, avoiding memory leakage. </P ><P > However, if your procedure needs to return an object in allocated memory (such as a value of a pass-by-reference data type), you cannot allocate that memory using <CODE CLASS="FUNCTION" >palloc</CODE >, at least not while you are connected to SPI. If you try, the object will be deallocated by <CODE CLASS="FUNCTION" >SPI_finish</CODE >, and your procedure will not work reliably. To solve this problem, use <CODE CLASS="FUNCTION" >SPI_palloc</CODE > to allocate memory for your return object. <CODE CLASS="FUNCTION" >SPI_palloc</CODE > allocates memory in the <SPAN CLASS="QUOTE" >"upper executor context"</SPAN >, that is, the memory context that was current when <CODE CLASS="FUNCTION" >SPI_connect</CODE > was called, which is precisely the right context for a value returned from your procedure. </P ><P > If <CODE CLASS="FUNCTION" >SPI_palloc</CODE > is called while the procedure is not connected to SPI, then it acts the same as a normal <CODE CLASS="FUNCTION" >palloc</CODE >. Before a procedure connects to the SPI manager, the current memory context is the upper executor context, so all allocations made by the procedure via <CODE CLASS="FUNCTION" >palloc</CODE > or by SPI utility functions are made in this context. </P ><P > When <CODE CLASS="FUNCTION" >SPI_connect</CODE > is called, the private context of the procedure, which is created by <CODE CLASS="FUNCTION" >SPI_connect</CODE >, is made the current context. All allocations made by <CODE CLASS="FUNCTION" >palloc</CODE >, <CODE CLASS="FUNCTION" >repalloc</CODE >, or SPI utility functions (except for <CODE CLASS="FUNCTION" >SPI_copytuple</CODE >, <CODE CLASS="FUNCTION" >SPI_returntuple</CODE >, <CODE CLASS="FUNCTION" >SPI_modifytuple</CODE >, and <CODE CLASS="FUNCTION" >SPI_palloc</CODE >) are made in this context. When a procedure disconnects from the SPI manager (via <CODE CLASS="FUNCTION" >SPI_finish</CODE >) the current context is restored to the upper executor context, and all allocations made in the procedure memory context are freed and cannot be used any more. </P ><P > All functions described in this section can be used by both connected and unconnected procedures. In an unconnected procedure, they act the same as the underlying ordinary server functions (<CODE CLASS="FUNCTION" >palloc</CODE >, etc.). </P ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="spi-spi-getnspname.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="spi-spi-palloc.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >SPI_getnspname</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="spi.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >SPI_palloc</TD ></TR ></TABLE ></DIV ></BODY ></HTML >