12.1 BACKSPACE Statement

The BACKSPACE statement positions a sequential file at the beginning of the preceding record, making it available for subsequent I/O processing. It takes one of the following forms:

BACKSPACE ([UNIT=]io-unit [, ERR=label] [, IOSTAT=i-var])
BACKSPACE io-unit

io-unit
Is an external unit specifier.

label
Is the label of the branch target statement that receives control if an error occurs.

i-var
Is a scalar integer variable that is defined as a positive integer if an error occurs and zero if no error occurs.

Rules and Behavior

The I/O unit number must specify an open file on disk or magnetic tape.

A BACKSPACE statement must not be specified for a file that is open for direct, append, or keyed access, because record n is not available to the RMS I/O system.

If a file is already positioned at the beginning of a file, a BACKSPACE statement has no effect.

Examples

The following statement repositions the file connected to I/O unit 4 back to the preceding record:

  BACKSPACE 4

  BACKSPACE (UNIT=9, IOSTAT=IOS, ERR=10)

This statement positions the file connected to unit 9 back to the preceding record. If an error occurs, control is transferred to the statement labeled 10, and a positive integer is stored in variable IOS.

For More Information:

• On the UNIT control specifier, see Section 10.2.1.1.
  
  
• On the ERR control specifier, see Section 10.2.1.8.
  
  
• On the IOSTAT control specifier, see Section 10.2.1.7.
  
  
• On append access, see Section 12.6.1.
  
  
• On record position, advancement, and transfer, see your user manual.