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:

Syntax

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

io-unit
(Input) Is an external unit specifier.

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

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

Description

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

Backspacing from the current record n is performed by rewinding to the start of the file and then performing n-1 successive READs to reach the previous record.

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

BACKSPACE cannot be used to skip over records that have been written using list-directed or namelist formatting.

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

If the file is positioned between the last record and the end-of-file record, BACKSPACE positions the file at the start of the last record.

See Also

REWIND, ENDFILE, Data Transfer I/O Statements, Branch Specifiers

Examples

  BACKSPACE 5
  BACKSPACE (5)
  BACKSPACE lunit
  BACKSPACE (UNIT = lunit, ERR = 30, IOSTAT = ios)

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

  BACKSPACE 4

Consider the following statement:

  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.