<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >Error Handling</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK REL="HOME" TITLE="Database Independent Abstraction Layer for C" HREF="index.html"><LINK REL="PREVIOUS" TITLE="Adding libdbi to your project" HREF="x276.html"><LINK REL="NEXT" TITLE="Connection error numbers and messages" HREF="connerrors.html"></HEAD ><BODY CLASS="CHAPTER" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#840084" ALINK="#0000FF" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" >Database Independent Abstraction Layer for C: libdbi Programmer's Guide</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="x276.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="connerrors.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="CHAPTER" ><H1 ><A NAME="ERRORHANDLING" ></A >Chapter 3. Error Handling</H1 ><DIV CLASS="TOC" ><DL ><DT ><B >Table of Contents</B ></DT ><DT >3.1. <A HREF="connerrors.html" >Connection error numbers and messages</A ></DT ><DT >3.2. <A HREF="errornumbers.html" >Error numbers</A ></DT ></DL ></DIV ><P >Applications should check all libdbi function calls for errors and respond appropriately to avoid entering an undefined status. libdbi uses two mechanisms to indicate errors:</P ><P ></P ><DIV CLASS="VARIABLELIST" ><DL ><DT >Function return values</DT ><DD ><P >Essentially all libdbi functions return a value, a concept not unfamiliar to the seasoned C programmer. For example, the <A HREF="reference-core.html#DBI-INITIALIZE" >dbi_initialize</A > function returns the number of loaded drivers, or -1 if an error occurred. In this case checking the return value is sufficient to detect an error condition. However, other functions like the family of "getters" cannot indicate error conditions with a return value. Consider e.g. the <A HREF="reference-field.html#DBI-RESULT-GET-STRING" >dbi_result_get_string</A > function which is used to retrieve strings from a database. If there was an error in accessing the value, the function will return the string "ERROR". However, this string is a legal value of such a field (the problem is the same for any other conceivable return value, including the empty string and the NULL pointer). Therefore we need an additional mechanism to report errors.</P ></DD ><DT >Error numbers and error messages</DT ><DD ><P >Connections store the status of the most recent operation which can be queried by the accessor function <A HREF="reference-conn.html#DBI-CONN-ERROR" ><CODE CLASS="FUNCTION" >dbi_conn_error</CODE ></A >. This is equivalent to the <CODE CLASS="VARNAME" >errno</CODE > variable of the standard C library which is used by most system calls and can be printed in human-readable form by the <CODE CLASS="FUNCTION" >perror</CODE > system call. This mechanism implies that your program queries the status right after each operation, as the values will be overwritten by subsequent operations.</P ></DD ></DL ></DIV ><P >The return values of all libdbi functions are listed in the <A HREF="reference.html" >reference</A > chapter below. The error numbers will be briefly discussed in the following sections.</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="x276.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="connerrors.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Adding libdbi to your project</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" > </TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Connection error numbers and messages</TD ></TR ></TABLE ></DIV ></BODY ></HTML >