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><H1
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><A
NAME="ERROR-MESSAGE-REPORTING"
>47.2. Reporting Errors Within the Server</A
></H1
><P
> Error, warning, and log messages generated within the server code
should be created using <CODE
CLASS="FUNCTION"
>ereport</CODE
>, or its older cousin
<CODE
CLASS="FUNCTION"
>elog</CODE
>. The use of this function is complex enough to
require some explanation.
</P
><P
> There are two required elements for every message: a severity level
(ranging from <TT
CLASS="LITERAL"
>DEBUG</TT
> to <TT
CLASS="LITERAL"
>PANIC</TT
>) and a primary
message text. In addition there are optional elements, the most
common of which is an error identifier code that follows the SQL spec's
SQLSTATE conventions.
<CODE
CLASS="FUNCTION"
>ereport</CODE
> itself is just a shell function, that exists
mainly for the syntactic convenience of making message generation
look like a function call in the C source code. The only parameter
accepted directly by <CODE
CLASS="FUNCTION"
>ereport</CODE
> is the severity level.
The primary message text and any optional message elements are
generated by calling auxiliary functions, such as <CODE
CLASS="FUNCTION"
>errmsg</CODE
>,
within the <CODE
CLASS="FUNCTION"
>ereport</CODE
> call.
</P
><P
> A typical call to <CODE
CLASS="FUNCTION"
>ereport</CODE
> might look like this:
</P><PRE
CLASS="PROGRAMLISTING"
>ereport(ERROR,
(errcode(ERRCODE_DIVISION_BY_ZERO),
errmsg("division by zero")));</PRE
><P>
This specifies error severity level <TT
CLASS="LITERAL"
>ERROR</TT
> (a run-of-the-mill
error). The <CODE
CLASS="FUNCTION"
>errcode</CODE
> call specifies the SQLSTATE error code
using a macro defined in <TT
CLASS="FILENAME"
>src/include/utils/errcodes.h</TT
>. The
<CODE
CLASS="FUNCTION"
>errmsg</CODE
> call provides the primary message text. Notice the
extra set of parentheses surrounding the auxiliary function calls —
these are annoying but syntactically necessary.
</P
><P
> Here is a more complex example:
</P><PRE
CLASS="PROGRAMLISTING"
>ereport(ERROR,
(errcode(ERRCODE_AMBIGUOUS_FUNCTION),
errmsg("function %s is not unique",
func_signature_string(funcname, nargs,
NIL, actual_arg_types)),
errhint("Unable to choose a best candidate function. "
"You might need to add explicit typecasts.")));</PRE
><P>
This illustrates the use of format codes to embed run-time values into
a message text. Also, an optional <SPAN
CLASS="QUOTE"
>"hint"</SPAN
> message is provided.
</P
><P
> The available auxiliary routines for <CODE
CLASS="FUNCTION"
>ereport</CODE
> are:
<P
></P
></P><UL
><LI
><P
> <CODE
CLASS="FUNCTION"
>errcode(sqlerrcode)</CODE
> specifies the SQLSTATE error identifier
code for the condition. If this routine is not called, the error
identifier defaults to
<TT
CLASS="LITERAL"
>ERRCODE_INTERNAL_ERROR</TT
> when the error severity level is
<TT
CLASS="LITERAL"
>ERROR</TT
> or higher, <TT
CLASS="LITERAL"
>ERRCODE_WARNING</TT
> when the
error level is <TT
CLASS="LITERAL"
>WARNING</TT
>, otherwise (for <TT
CLASS="LITERAL"
>NOTICE</TT
>
and below) <TT
CLASS="LITERAL"
>ERRCODE_SUCCESSFUL_COMPLETION</TT
>.
While these defaults are often convenient, always think whether they
are appropriate before omitting the <CODE
CLASS="FUNCTION"
>errcode()</CODE
> call.
</P
></LI
><LI
><P
> <CODE
CLASS="FUNCTION"
>errmsg(const char *msg, ...)</CODE
> specifies the primary error
message text, and possibly run-time values to insert into it. Insertions
are specified by <CODE
CLASS="FUNCTION"
>sprintf</CODE
>-style format codes. In addition to
the standard format codes accepted by <CODE
CLASS="FUNCTION"
>sprintf</CODE
>, the format
code <TT
CLASS="LITERAL"
>%m</TT
> can be used to insert the error message returned
by <CODE
CLASS="FUNCTION"
>strerror</CODE
> for the current value of <TT
CLASS="LITERAL"
>errno</TT
>.
<A
NAME="AEN97563"
HREF="#FTN.AEN97563"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
>
<TT
CLASS="LITERAL"
>%m</TT
> does not require any
corresponding entry in the parameter list for <CODE
CLASS="FUNCTION"
>errmsg</CODE
>.
Note that the message string will be run through <CODE
CLASS="FUNCTION"
>gettext</CODE
>
for possible localization before format codes are processed.
</P
></LI
><LI
><P
> <CODE
CLASS="FUNCTION"
>errmsg_internal(const char *msg, ...)</CODE
> is the same as
<CODE
CLASS="FUNCTION"
>errmsg</CODE
>, except that the message string will not be
translated nor included in the internationalization message dictionary.
This should be used for <SPAN
CLASS="QUOTE"
>"cannot happen"</SPAN
> cases that are probably
not worth expending translation effort on.
</P
></LI
><LI
><P
> <CODE
CLASS="FUNCTION"
>errmsg_plural(const char *fmt_singular, const char *fmt_plural,
unsigned long n, ...)</CODE
> is like <CODE
CLASS="FUNCTION"
>errmsg</CODE
>, but with
support for various plural forms of the message.
<TT
CLASS="REPLACEABLE"
><I
>fmt_singular</I
></TT
> is the English singular format,
<TT
CLASS="REPLACEABLE"
><I
>fmt_plural</I
></TT
> is the English plural format,
<TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
> is the integer value that determines which plural
form is needed, and the remaining arguments are formatted according
to the selected format string. For more information see
<A
HREF="nls-programmer.html#NLS-GUIDELINES"
>Section 48.2.2</A
>.
</P
></LI
><LI
><P
> <CODE
CLASS="FUNCTION"
>errdetail(const char *msg, ...)</CODE
> supplies an optional
<SPAN
CLASS="QUOTE"
>"detail"</SPAN
> message; this is to be used when there is additional
information that seems inappropriate to put in the primary message.
The message string is processed in just the same way as for
<CODE
CLASS="FUNCTION"
>errmsg</CODE
>.
</P
></LI
><LI
><P
> <CODE
CLASS="FUNCTION"
>errdetail_internal(const char *msg, ...)</CODE
> is the same
as <CODE
CLASS="FUNCTION"
>errdetail</CODE
>, except that the message string will not be
translated nor included in the internationalization message dictionary.
This should be used for detail messages that are not worth expending
translation effort on, for instance because they are too technical to be
useful to most users.
</P
></LI
><LI
><P
> <CODE
CLASS="FUNCTION"
>errdetail_plural(const char *fmt_singular, const char *fmt_plural,
unsigned long n, ...)</CODE
> is like <CODE
CLASS="FUNCTION"
>errdetail</CODE
>, but with
support for various plural forms of the message.
For more information see <A
HREF="nls-programmer.html#NLS-GUIDELINES"
>Section 48.2.2</A
>.
</P
></LI
><LI
><P
> <CODE
CLASS="FUNCTION"
>errdetail_log(const char *msg, ...)</CODE
> is the same as
<CODE
CLASS="FUNCTION"
>errdetail</CODE
> except that this string goes only to the server
log, never to the client. If both <CODE
CLASS="FUNCTION"
>errdetail</CODE
> (or one of
its equivalents above) and
<CODE
CLASS="FUNCTION"
>errdetail_log</CODE
> are used then one string goes to the client
and the other to the log. This is useful for error details that are
too security-sensitive or too bulky to include in the report
sent to the client.
</P
></LI
><LI
><P
> <CODE
CLASS="FUNCTION"
>errhint(const char *msg, ...)</CODE
> supplies an optional
<SPAN
CLASS="QUOTE"
>"hint"</SPAN
> message; this is to be used when offering suggestions
about how to fix the problem, as opposed to factual details about
what went wrong.
The message string is processed in just the same way as for
<CODE
CLASS="FUNCTION"
>errmsg</CODE
>.
</P
></LI
><LI
><P
> <CODE
CLASS="FUNCTION"
>errcontext(const char *msg, ...)</CODE
> is not normally called
directly from an <CODE
CLASS="FUNCTION"
>ereport</CODE
> message site; rather it is used
in <TT
CLASS="LITERAL"
>error_context_stack</TT
> callback functions to provide
information about the context in which an error occurred, such as the
current location in a PL function.
The message string is processed in just the same way as for
<CODE
CLASS="FUNCTION"
>errmsg</CODE
>. Unlike the other auxiliary functions, this can
be called more than once per <CODE
CLASS="FUNCTION"
>ereport</CODE
> call; the successive
strings thus supplied are concatenated with separating newlines.
</P
></LI
><LI
><P
> <CODE
CLASS="FUNCTION"
>errposition(int cursorpos)</CODE
> specifies the textual location
of an error within a query string. Currently it is only useful for
errors detected in the lexical and syntactic analysis phases of
query processing.
</P
></LI
><LI
><P
> <CODE
CLASS="FUNCTION"
>errcode_for_file_access()</CODE
> is a convenience function that
selects an appropriate SQLSTATE error identifier for a failure in a
file-access-related system call. It uses the saved
<TT
CLASS="LITERAL"
>errno</TT
> to determine which error code to generate.
Usually this should be used in combination with <TT
CLASS="LITERAL"
>%m</TT
> in the
primary error message text.
</P
></LI
><LI
><P
> <CODE
CLASS="FUNCTION"
>errcode_for_socket_access()</CODE
> is a convenience function that
selects an appropriate SQLSTATE error identifier for a failure in a
socket-related system call.
</P
></LI
><LI
><P
> <CODE
CLASS="FUNCTION"
>errhidestmt(bool hide_stmt)</CODE
> can be called to specify
suppression of the <TT
CLASS="LITERAL"
>STATEMENT:</TT
> portion of a message in the
postmaster log. Generally this is appropriate if the message text
includes the current statement already.
</P
></LI
></UL
><P>
</P
><P
> There is an older function <CODE
CLASS="FUNCTION"
>elog</CODE
> that is still heavily used.
An <CODE
CLASS="FUNCTION"
>elog</CODE
> call:
</P><PRE
CLASS="PROGRAMLISTING"
>elog(level, "format string", ...);</PRE
><P>
is exactly equivalent to:
</P><PRE
CLASS="PROGRAMLISTING"
>ereport(level, (errmsg_internal("format string", ...)));</PRE
><P>
Notice that the SQLSTATE error code is always defaulted, and the message
string is not subject to translation.
Therefore, <CODE
CLASS="FUNCTION"
>elog</CODE
> should be used only for internal errors and
low-level debug logging. Any message that is likely to be of interest to
ordinary users should go through <CODE
CLASS="FUNCTION"
>ereport</CODE
>. Nonetheless,
there are enough internal <SPAN
CLASS="QUOTE"
>"cannot happen"</SPAN
> error checks in the
system that <CODE
CLASS="FUNCTION"
>elog</CODE
> is still widely used; it is preferred for
those messages for its notational simplicity.
</P
><P
> Advice about writing good error messages can be found in
<A
HREF="error-style-guide.html"
>Section 47.3</A
>.
</P
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN97563"
HREF="error-message-reporting.html#AEN97563"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
> That is, the value that was current when the <CODE
CLASS="FUNCTION"
>ereport</CODE
> call
was reached; changes of <TT
CLASS="LITERAL"
>errno</TT
> within the auxiliary reporting
routines will not affect it. That would not be true if you were to
write <TT
CLASS="LITERAL"
>strerror(errno)</TT
> explicitly in <CODE
CLASS="FUNCTION"
>errmsg</CODE
>'s
parameter list; accordingly, do not do so.
</P
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