Open Watcom FORTRAN 77 Language Reference

FORTRAN Statements
2.7 BACKSPACE Statement
BACKSPACE u
BACKSPACE (alist)
where:
u
alist
is an external unit identifier.
is a list of backspace specifiers separated by commas:
[UNIT =] u
IOSTAT = ios
ERR = s
Execution of a BACKSPACE statement causes the file connected to the specified unit to be positioned at the
beginning of the preceding record. If the preceding record is an endfile record then the file is positioned at
the beginning of the endfile record.
Backspace Specifiers
[UNIT =] u
IOSTAT = ios
ERR = s
u is an external unit identifier. An external unit identifier is a non-negative integer
expression. If the optional UNIT= specifier is omitted then the specifier must be the first
item in the list of specifiers.
is an input/output status specifier. The integer variable or integer array element ios is
defined with zero if no error condition occurs or a positive integer value if an error
condition occurs.
is an error specifier and s is a statement label. When an error occurs, execution is
transferred to the statement labelled by s.
Example:
LOOP
READ( UNIT=8, END=100, FMT=200 ) RECORD
ENDLOOP
100 BACKSPACE( UNIT=8 )
WRITE( UNIT=8, FMT=200 ) NEWREC
In the previous example, we illustrate how one might append a record to the end of an existing file.
Notes:
1. The unit must be connected for sequential access.
2. If the file is positioned before the first record then the BACKSPACE statement has no effect.
3. It is illegal to backspace a file that does not exist.
BACKSPACE Statement 19