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>Chapter 4. Error Handling</H1
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><DT
><B
>Table of Contents</B
></DT
><DT
>4.1. <A
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>Connection error numbers and messages</A
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>4.2. <A
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><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
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><DT
>Function return values</DT
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><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
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><P
>The return values of all libdbi functions are listed in the <A
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>reference</A
> chapter below. The error numbers will be briefly discussed in the following sections.</P
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