Top  Previous  Next


The READVL statement reads a specific field from a record of a previously opened file, setting a read lock. The READVU statement is similar but sets an update lock.





READVL var {ENCODING name} FROM file.var,, field.expr

{ON ERROR statement(s)}

{LOCKED statement(s)}

{THEN statement(s)}

{ELSE statement(s)}




varis the name of a variable to receive the dynamic array read from the file.


file.varis the file variable associated with the file.


record.idevaluates to the id of the record to be read.


field.exprevaluates to the number of the field to be read.


statement(s)are statements to be executed depending on the outcome of the operation.


At least one of the THEN and ELSE clauses must be present unless the OPTIONAL.THEN.ELSE setting of the $MODE compiler directive is enabled.



The optional ENCODING element to this statement sets the character encoding to be used, overriding any encoding set in the VOC F-type record or when the file was opened. This is relevant only to directory files and is ignored for other file types. A null string as the encoding name is equivalent to not having the ENCODING clause at all. To disable the default encoding, the encoding name should be specified as "NULL".


The specified record is read and the field identified by field.expr is extracted into the named variable. If the field does not exist, var is set to a null string.


READVL sets a read lock is set on the record. READVU sets an update lock on the record. See Locks for full details of QM's locking mechanism.


A field.expr value of zero may be used to determine if the record exists. var will be set to a null string.


The LOCKED clause is executed if the file or record is locked by another process (exclusively in the case of READVL). The STATUS() function will return the user id of a process holding a lock that is blocking this action or zero if the lock table is full. If the LOCKED clause is omitted and the file or record is locked, the program will wait for the lock to be released.


The THEN clause is executed if the record is read successfully regardless of whether the specified field is present.


The ELSE clause is executed if the operation fails because no record with the given id is present on the file. If the PICK.READ mode of the $MODE directive is used var will be left unchanged, otherwise it will be set to a null string. The STATUS() function will indicate the cause of the error.



The ON ERROR clause is executed for serious fault conditions such as errors in a file's internal control structures. The STATUS() function will return an error number. If no ON ERROR clause is present, an abort would occur.






  ...processing statements...



  DISPLAY "Record " : ITEM.ID : " not found"




This program fragment reads field 3 of a record from the file previously opened to file variable STOCK into variable ITEM, setting an update lock. If successful, the processing statements are executed and the modified value is written back to the file. If the record is not found, a message is displayed and the record is unlocked.