POINT_LUN Procedure

Allows the current position of the specified file to be set to any arbitrary point in the file.

Usage

OpenVMS Users: To use POINT_LUN to specify a byte offset in an OpenVMS file, you must use the Block keyword in the OPEN procedure when you open the file. For example:

OPENR, 1, 'File.dat', /Block 
POINT_LUN, 1, 25B

Example

In this example, POINT_LUN is used to move about within an unformatted file of integers.

a = INDGEN(100)

; Create an integer vector of length 100 that is initialized to the values
; of its one-dimensional subscripts.

OPENW, unit, 'ptlun.dat', /Get_Lun

; Open a file called ptlun.dat for writing.

WRITEU, unit, a

; Write the 100-element integer vector as unformatted binary data to
; the file ptlun.dat.

POINT_LUN, -unit, pos
PRINT, 'Current offset into ptlun.dat is', $
    pos, ' bytes.'

Current offset into ptlun.dat is 200 bytes.

; Retrieve and display the current position within ptlun.dat.

POINT_LUN, unit, 0

; Rewind the file to the beginning.

b = INTARR(2)

; Read two integers from the beginning of the file into a two-element
; integer array, b.

READU, unit, b
PRINT, b

0       1

POINT_LUN, -unit, pos

; Since two integers were just read, each of length 2 bytes, the
; current position within the file should be 4 bytes offset from
; beginning. Retrieve current position.

PRINT, 'Current offset into ptlun.dat is', pos, ' bytes.'

Current offset into ptlun.dat is 4 bytes.

FREE_LUN, unit

; Close the file and free the file unit number.

POINT_LUN Procedure

Usage

Input Parameters

Output Parameters

Keywords

Discussion

Example

See Also