Error, warning, and log messages generated within the server code should be created using ereport
, or its older cousin elog
. The use of this function is complex enough to require some explanation.
There are two required elements for every message: a severity level (ranging from DEBUG
to PANIC
) 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. ereport
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 ereport
is the severity level. The primary message text and any optional message elements are generated by calling auxiliary functions, such as errmsg
, within the ereport
call.
A typical call to ereport
might look like this:
ereport(ERROR, (errcode(ERRCODE_DIVISION_BY_ZERO), errmsg("division by zero")));
This specifies error severity level ERROR
(a run-of-the-mill error). The errcode
call specifies the SQLSTATE error code using a macro defined in src/include/utils/errcodes.h
. The errmsg
call provides the primary message text. Notice the extra set of parentheses surrounding the auxiliary function calls — these are annoying but syntactically necessary.
Here is a more complex example:
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.")));
This illustrates the use of format codes to embed run-time values into a message text. Also, an optional "hint" message is provided.
If the severity level is ERROR
or higher, ereport
aborts the execution of the user-defined function and does not return to the caller. If the severity level is lower than ERROR
, ereport
returns normally.
The available auxiliary routines for ereport
are:
-
errcode(sqlerrcode)
specifies the SQLSTATE error identifier code for the condition. If this routine is not called, the error identifier defaults toERRCODE_INTERNAL_ERROR
when the error severity level isERROR
or higher,ERRCODE_WARNING
when the error level isWARNING
, otherwise (forNOTICE
and below)ERRCODE_SUCCESSFUL_COMPLETION
. While these defaults are often convenient, always think whether they are appropriate before omitting theerrcode()
call. -
errmsg(const char *msg, ...)
specifies the primary error message text, and possibly run-time values to insert into it. Insertions are specified bysprintf
-style format codes. In addition to the standard format codes accepted bysprintf
, the format code%m
can be used to insert the error message returned bystrerror
for the current value oferrno
. [1]%m
does not require any corresponding entry in the parameter list forerrmsg
. Note that the message string will be run throughgettext
for possible localization before format codes are processed. -
errmsg_internal(const char *msg, ...)
is the same aserrmsg
, except that the message string will not be translated nor included in the internationalization message dictionary. This should be used for "cannot happen" cases that are probably not worth expending translation effort on. -
errmsg_plural(const char *fmt_singular, const char *fmt_plural, unsigned long n, ...)
is likeerrmsg
, but with support for various plural forms of the message.fmt_singular
is the English singular format,fmt_plural
is the English plural format,n
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 Section 53.2.2. -
errdetail(const char *msg, ...)
supplies an optional "detail" 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 forerrmsg
. -
errdetail_internal(const char *msg, ...)
is the same aserrdetail
, 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. -
errdetail_plural(const char *fmt_singular, const char *fmt_plural, unsigned long n, ...)
is likeerrdetail
, but with support for various plural forms of the message. For more information see Section 53.2.2. -
errdetail_log(const char *msg, ...)
is the same aserrdetail
except that this string goes only to the server log, never to the client. If botherrdetail
(or one of its equivalents above) anderrdetail_log
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. -
errdetail_log_plural(const char *fmt_singular, const char *fmt_plural, unsigned long n, ...)
is likeerrdetail_log
, but with support for various plural forms of the message. For more information see Section 53.2.2. -
errhint(const char *msg, ...)
supplies an optional "hint" 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 forerrmsg
. -
errcontext(const char *msg, ...)
is not normally called directly from anereport
message site; rather it is used inerror_context_stack
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 forerrmsg
. Unlike the other auxiliary functions, this can be called more than once perereport
call; the successive strings thus supplied are concatenated with separating newlines. -
errposition(int cursorpos)
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. -
errtable(Relation rel)
specifies a relation whose name and schema name should be included as auxiliary fields in the error report. -
errtablecol(Relation rel, int attnum)
specifies a column whose name, table name, and schema name should be included as auxiliary fields in the error report. -
errtableconstraint(Relation rel, const char *conname)
specifies a table constraint whose name, table name, and schema name should be included as auxiliary fields in the error report. Indexes should be considered to be constraints for this purpose, whether or not they have an associatedpg_constraint
entry. Be careful to pass the underlying heap relation, not the index itself, asrel
. -
errdatatype(Oid datatypeOid)
specifies a data type whose name and schema name should be included as auxiliary fields in the error report. -
errdomainconstraint(Oid datatypeOid, const char *conname)
specifies a domain constraint whose name, domain name, and schema name should be included as auxiliary fields in the error report. -
errcode_for_file_access()
is a convenience function that selects an appropriate SQLSTATE error identifier for a failure in a file-access-related system call. It uses the savederrno
to determine which error code to generate. Usually this should be used in combination with%m
in the primary error message text. -
errcode_for_socket_access()
is a convenience function that selects an appropriate SQLSTATE error identifier for a failure in a socket-related system call. -
errhidestmt(bool hide_stmt)
can be called to specify suppression of theSTATEMENT:
portion of a message in the postmaster log. Generally this is appropriate if the message text includes the current statement already. -
errhidecontext(bool hide_ctx)
can be called to specify suppression of theCONTEXT:
portion of a message in the postmaster log. This should only be used for verbose debugging messages where the repeated inclusion of context would bloat the log volume too much.
Note: At most one of the functions
errtable
,errtablecol
,errtableconstraint
,errdatatype
, orerrdomainconstraint
should be used in anereport
call. These functions exist to allow applications to extract the name of a database object associated with the error condition without having to examine the potentially-localized error message text. These functions should be used in error reports for which it's likely that applications would wish to have automatic error handling. As of PostgreSQL 9.3, complete coverage exists only for errors in SQLSTATE class 23 (integrity constraint violation), but this is likely to be expanded in future.
There is an older function elog
that is still heavily used. An elog
call:
elog(level, "format string", ...);
is exactly equivalent to:
ereport(level, (errmsg_internal("format string", ...)));
Notice that the SQLSTATE error code is always defaulted, and the message string is not subject to translation. Therefore, elog
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 ereport
. Nonetheless, there are enough internal "cannot happen" error checks in the system that elog
is still widely used; it is preferred for those messages for its notational simplicity.
Advice about writing good error messages can be found in Section 52.3.
Notes
[1] | That is, the value that was current when the |
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