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READU

Description

The READU statement allows a program to read a record from a previously opened file into a variable. It respects record locking and locks the specified record for update.

It takes the general form: 

READU variable1 FROM {variable2,} expression {SETTING setvar} 
{ON ERROR statements} {LOCKED statements} {THEN|ELSE statements}

Where: 

variable1 is the identifier into which the record will be read.

variable2 if specified, should be a jBASE BASIC variable that has previously been opened to a file using the OPEN statement. If variable2 is not specified then the default file is assumed.

The expression should evaluate to a valid record key for the file.

If the SETTING clause is specified and the read fails, setvar will be set to one of these values.

If ON ERROR is specified, the statements following the ON ERROR clause will be executed for any of the above Incremental File Errors except error 128.

If the READU is successful then the statements following THEN will be executed. If the READU is unsuccessful, i.e. the record key does not exist in the file, then the statements following ELSE are executed. If the READU is unsuccessful and there is no ELSE then expression is set to "" (null).

Note: 

If the record could not be read because another process already had a lock on the record then one of two actions is taken. If the LOCKED clause was specified in the statement then the statements dependent on it are executed. If no LOCKED clause was specified then the statement blocks (waits) until the other process releases the lock. The SYSTEM(43) function can be used to determine which port has the lock.

The lock taken by the READU statement will be released by any of the following events:

  • The same program with WRITE, WRITEVor MATWRITE statements writes to the record.
  • The same program with the DELETE statement deletes the record.
  • The record lock is released explicitly using the RELEASE statement.
  • The program stops normally or abnormally.

When a file is OPENed to a local file variable in a subroutine then the file is closed when the subroutine RETURNs so all locks taken on that file are released, including locks taken in a calling program. Files that are opened to COMMON variables are not closed so the locks remain intact.

An example of use is as: 

OPEN "Customers" ELSE ABORT 201, "Customers"
OPEN "DICT Customers" TO DCusts ELSE ABORT 201, "DICT Customers"
END
LOOP
    READU Rec FROM DCusts, "Xref" LOCKED
        CRT "Xref locked by port ":SYSTEM(43):" - retrying"
        SLEEP 1; 
        CONTINUE ;* Restart LOOP
    END THEN
    READ DataRec FROM Rec ELSE 
        ABORT 202, Rec
        END
        BREAK ;* Leave the LOOP
    END ELSE
    ABORT 202, "Xref"
    END
REPEAT



See also: WRITE, WRITEU, MATWRITE, MATWRITEU, RELEASE, DELETE

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