34.8. Error Handling#
This section describes how you can handle exceptional conditions and warnings in an embedded SQL program. There are two nonexclusive facilities for this.
- Callbacks can be configured to handle warning and error conditions using the
WHENEVERcommand. - Detailed information about the error or warning can be obtained from the
sqlcavariable.
34.8.1. Setting Callbacks#
One simple method to catch errors and warnings is to set a specific action to be executed whenever a particular condition occurs. In general:
EXEC SQL WHENEVERconditionaction;
condition can be one of the following:
SQLERROR#The specified action is called whenever an error occurs during the execution of an SQL statement.
SQLWARNING#The specified action is called whenever a warning occurs during the execution of an SQL statement.
NOT FOUND#The specified action is called whenever an SQL statement retrieves or affects zero rows. (This condition is not an error, but you might be interested in handling it specially.)
action can be one of the following:
CONTINUE#This effectively means that the condition is ignored. This is the default.
GOTOlabelGO TO#labelJump to the specified label (using a C
gotostatement).SQLPRINT#Print a message to standard error. This is useful for simple programs or during prototyping. The details of the message cannot be configured.
STOP#Call
exit(1), which will terminate the program.DO BREAK#Execute the C statement
break. This should only be used in loops orswitchstatements.DO CONTINUE#Execute the C statement
continue. This should only be used in loops statements. if executed, will cause the flow of control to return to the top of the loop.CALLname(args)DO#name(args)Call the specified C functions with the specified arguments. (This use is different from the meaning of
CALLandDOin the normal PostgreSQL grammar.)
The SQL standard only provides for the actionsCONTINUE andGOTO (andGO TO).
Here is an example that you might want to use in a simple program. It prints a simple message when a warning occurs and aborts the program when an error happens:
EXEC SQL WHENEVER SQLWARNING SQLPRINT;EXEC SQL WHENEVER SQLERROR STOP;
The statementEXEC SQL WHENEVER is a directive of the SQL preprocessor, not a C statement. The error or warning actions that it sets apply to all embedded SQL statements that appear below the point where the handler is set, unless a different action was set for the same condition between the firstEXEC SQL WHENEVER and the SQL statement causing the condition, regardless of the flow of control in the C program. So neither of the two following C program excerpts will have the desired effect:
/* * WRONG */int main(int argc, char *argv[]){ ... if (verbose) { EXEC SQL WHENEVER SQLWARNING SQLPRINT; } ... EXEC SQL SELECT ...; ...}/* * WRONG */int main(int argc, char *argv[]){ ... set_error_handler(); ... EXEC SQL SELECT ...; ...}static void set_error_handler(void){ EXEC SQL WHENEVER SQLERROR STOP;}34.8.2. sqlca#
For more powerful error handling, the embedded SQL interface provides a global variable with the namesqlca (SQL communication area) that has the following structure:
struct{ char sqlcaid[8]; long sqlabc; long sqlcode; struct { int sqlerrml; char sqlerrmc[SQLERRMC_LEN]; } sqlerrm; char sqlerrp[8]; long sqlerrd[6]; char sqlwarn[8]; char sqlstate[5];} sqlca; (In a multithreaded program, every thread automatically gets its own copy ofsqlca. This works similarly to the handling of the standard C global variableerrno.)
sqlca covers both warnings and errors. If multiple warnings or errors occur during the execution of a statement, thensqlca will only contain information about the last one.
34.8.3. SQLSTATE vs.SQLCODE#
The fieldssqlca.sqlstate andsqlca.sqlcode are two different schemes that provide error codes. Both are derived from the SQL standard, butSQLCODE has been marked deprecated in the SQL-92 edition of the standard and has been dropped in later editions. Therefore, new applications are strongly encouraged to useSQLSTATE.
SQLSTATE is a five-character array. The five characters contain digits or upper-case letters that represent codes of various error and warning conditions.SQLSTATE has a hierarchical scheme: the first two characters indicate the general class of the condition, the last three characters indicate a subclass of the general condition. A successful state is indicated by the code00000. TheSQLSTATE codes are for the most part defined in the SQL standard. ThePostgreSQL server natively supportsSQLSTATE error codes; therefore a high degree of consistency can be achieved by using this error code scheme throughout all applications. For further information seeAppendix A.
SQLCODE, the deprecated error code scheme, is a simple integer. A value of 0 indicates success, a positive value indicates success with additional information, a negative value indicates an error. The SQL standard only defines the positive value +100, which indicates that the last command returned or affected zero rows, and no specific negative values. Therefore, this scheme can only achieve poor portability and does not have a hierarchical code assignment. Historically, the embedded SQL processor forPostgreSQL has assigned some specificSQLCODE values for its use, which are listed below with their numeric value and their symbolic name. Remember that these are not portable to other SQL implementations. To simplify the porting of applications to theSQLSTATE scheme, the correspondingSQLSTATE is also listed. There is, however, no one-to-one or one-to-many mapping between the two schemes (indeed it is many-to-many), so you should consult the globalSQLSTATE listing inAppendix A in each case.
These are the assignedSQLCODE values:
- 0 (
ECPG_NO_ERROR)# Indicates no error. (SQLSTATE 00000)
- 100 (
ECPG_NOT_FOUND)# This is a harmless condition indicating that the last command retrieved or processed zero rows, or that you are at the end of the cursor. (SQLSTATE 02000)
When processing a cursor in a loop, you could use this code as a way to detect when to abort the loop, like this:
while (1){ EXEC SQL FETCH ... ; if (sqlca.sqlcode == ECPG_NOT_FOUND) break;}But
WHENEVER NOT FOUND DO BREAKeffectively does this internally, so there is usually no advantage in writing this out explicitly.- -12 (
ECPG_OUT_OF_MEMORY)# Indicates that your virtual memory is exhausted. The numeric value is defined as
-ENOMEM. (SQLSTATE YE001)- -200 (
ECPG_UNSUPPORTED)# Indicates the preprocessor has generated something that the library does not know about. Perhaps you are running incompatible versions of the preprocessor and the library. (SQLSTATE YE002)
- -201 (
ECPG_TOO_MANY_ARGUMENTS)# This means that the command specified more host variables than the command expected. (SQLSTATE 07001 or 07002)
- -202 (
ECPG_TOO_FEW_ARGUMENTS)# This means that the command specified fewer host variables than the command expected. (SQLSTATE 07001 or 07002)
- -203 (
ECPG_TOO_MANY_MATCHES)# This means a query has returned multiple rows but the statement was only prepared to store one result row (for example, because the specified variables are not arrays). (SQLSTATE 21000)
- -204 (
ECPG_INT_FORMAT)# The host variable is of type
intand the datum in the database is of a different type and contains a value that cannot be interpreted as anint. The library usesstrtol()for this conversion. (SQLSTATE 42804)- -205 (
ECPG_UINT_FORMAT)# The host variable is of type
unsigned intand the datum in the database is of a different type and contains a value that cannot be interpreted as anunsigned int. The library usesstrtoul()for this conversion. (SQLSTATE 42804)- -206 (
ECPG_FLOAT_FORMAT)# The host variable is of type
floatand the datum in the database is of another type and contains a value that cannot be interpreted as afloat. The library usesstrtod()for this conversion. (SQLSTATE 42804)- -207 (
ECPG_NUMERIC_FORMAT)# The host variable is of type
numericand the datum in the database is of another type and contains a value that cannot be interpreted as anumericvalue. (SQLSTATE 42804)- -208 (
ECPG_INTERVAL_FORMAT)# The host variable is of type
intervaland the datum in the database is of another type and contains a value that cannot be interpreted as anintervalvalue. (SQLSTATE 42804)- -209 (
ECPG_DATE_FORMAT)# The host variable is of type
dateand the datum in the database is of another type and contains a value that cannot be interpreted as adatevalue. (SQLSTATE 42804)- -210 (
ECPG_TIMESTAMP_FORMAT)# The host variable is of type
timestampand the datum in the database is of another type and contains a value that cannot be interpreted as atimestampvalue. (SQLSTATE 42804)- -211 (
ECPG_CONVERT_BOOL)# This means the host variable is of type
booland the datum in the database is neither't'nor'f'. (SQLSTATE 42804)- -212 (
ECPG_EMPTY)# The statement sent to thePostgreSQL server was empty. (This cannot normally happen in an embedded SQL program, so it might point to an internal error.) (SQLSTATE YE002)
- -213 (
ECPG_MISSING_INDICATOR)# A null value was returned and no null indicator variable was supplied. (SQLSTATE 22002)
- -214 (
ECPG_NO_ARRAY)# An ordinary variable was used in a place that requires an array. (SQLSTATE 42804)
- -215 (
ECPG_DATA_NOT_ARRAY)# The database returned an ordinary variable in a place that requires array value. (SQLSTATE 42804)
- -216 (
ECPG_ARRAY_INSERT)# The value could not be inserted into the array. (SQLSTATE 42804)
- -220 (
ECPG_NO_CONN)# The program tried to access a connection that does not exist. (SQLSTATE 08003)
- -221 (
ECPG_NOT_CONN)# The program tried to access a connection that does exist but is not open. (This is an internal error.) (SQLSTATE YE002)
- -230 (
ECPG_INVALID_STMT)# The statement you are trying to use has not been prepared. (SQLSTATE 26000)
- -239 (
ECPG_INFORMIX_DUPLICATE_KEY)# Duplicate key error, violation of unique constraint (Informix compatibility mode). (SQLSTATE 23505)
- -240 (
ECPG_UNKNOWN_DESCRIPTOR)# The descriptor specified was not found. The statement you are trying to use has not been prepared. (SQLSTATE 33000)
- -241 (
ECPG_INVALID_DESCRIPTOR_INDEX)# The descriptor index specified was out of range. (SQLSTATE 07009)
- -242 (
ECPG_UNKNOWN_DESCRIPTOR_ITEM)# An invalid descriptor item was requested. (This is an internal error.) (SQLSTATE YE002)
- -243 (
ECPG_VAR_NOT_NUMERIC)# During the execution of a dynamic statement, the database returned a numeric value and the host variable was not numeric. (SQLSTATE 07006)
- -244 (
ECPG_VAR_NOT_CHAR)# During the execution of a dynamic statement, the database returned a non-numeric value and the host variable was numeric. (SQLSTATE 07006)
- -284 (
ECPG_INFORMIX_SUBSELECT_NOT_ONE)# A result of the subquery is not single row (Informix compatibility mode). (SQLSTATE 21000)
- -400 (
ECPG_PGSQL)# Some error caused by thePostgreSQL server. The message contains the error message from thePostgreSQL server.
- -401 (
ECPG_TRANS)# ThePostgreSQL server signaled that we cannot start, commit, or rollback the transaction. (SQLSTATE 08007)
- -402 (
ECPG_CONNECT)# The connection attempt to the database did not succeed. (SQLSTATE 08001)
- -403 (
ECPG_DUPLICATE_KEY)# Duplicate key error, violation of unique constraint. (SQLSTATE 23505)
- -404 (
ECPG_SUBSELECT_NOT_ONE)# A result for the subquery is not single row. (SQLSTATE 21000)
- -602 (
ECPG_WARNING_UNKNOWN_PORTAL)# An invalid cursor name was specified. (SQLSTATE 34000)
- -603 (
ECPG_WARNING_IN_TRANSACTION)# Transaction is in progress. (SQLSTATE 25001)
- -604 (
ECPG_WARNING_NO_TRANSACTION)# There is no active (in-progress) transaction. (SQLSTATE 25P01)
- -605 (
ECPG_WARNING_PORTAL_EXISTS)# An existing cursor name was specified. (SQLSTATE 42P03)