Seek

Seek allows you to move the read pointer on a file opened by Openi or Openio.
When a null value is sent, it also allows you to flush the write operation performed on a file opened by Openo or Openio.

Syntax

  Seek POSITION MOVE_EXPR
  Seek MOVE_EXPR
  Seek POSITION MOVE_EXPR Using [CLASS]
  Seek MOVE_EXPR Using [CLASS]

• POSITION can be one of the following combination of keywords:
  • Last-: to give a position from the end position of the file.
  • First+: to give a position from the beginning of the file.
  • Curr+: to give a relative position ahead of the current position in the file.
  • Curr-: to give a relative backward position compared to the current position in the file.
• MOVE_EXPR is an expression that gives the position in bytes.
• [CLASS] is the class abbreviation used to open the file. If it is not given, the file opened without abbreviation is considered.

Examples

   # Let's open a file and read 10 bytes every 20 bytes
    SChar BUFFERS(10)(MAXLINES)
    Openi "MYFILE"
    fstat = 0 : I = 0
    Repeat
       Getseq 1, BUFFERS(I) : I += 1
       Seek 10
    Until fstat <> 0 or I >= MAXLINES
    Openi
   # Let's rewrite the 50 last bytes of a file
    Tinyint BUFFER(50)
    Openio "MYFILE"
    Seek Last - 50
    Getseq 50, BUFFER
   # ..  BUFFER is modified
    Seek Last - 50
    Putseq 50, BUFFER
    Openio
   # When 2 files are opened:
   # Let's rewrite the 50 last bytes of the second file with the first 50 bytes of the first file
    Tinyint BUFFER(50)
    Openi "MYFILE1" Using [YYY]
    Openio "MYFILE2" Using [ZZZ]
    Getseq 50, BUFFER Using [YYY]
    Seek Last - 50 Using [ZZZ]
    Putseq 50, BUFFER Using [ZZZ]
    Openio Using [ZZZ]
    Openi Using [YYY]
   # Write 1 byte with "flush" on a file opened without abbreviation
    Putseq 1, MY_BYTE
    Seek 0

Description

Seek allows you to move the read or read/write pointer when a file is opened by Openi or Openio. The move can be done relatively at:
* The beginning of the file.
* The current position.
* The end of the file.

On some special files (devices), you may not perform a Seek operation and will therefore receive an error.

The Seek 0 syntax flushes the write buffer when the file has been opened by Openo or Openio. It is important, because the write operation is buffered. This syntax can be used on any abbreviation linked to a sequential file (on a file opened by Openi, nothing will happen).

Comments

The function adxseek(0) or adxseek("CLASS") (for a file opened with the [CLASS] abbreviation) allows you to know the current position on the file (in bytes) relatively to the beginning. If no file has been opened, a -1 value is returned.

This function is updated even if you are unable to perform a Seek on the file because it is a special file. In this case, it counts the number of bytes that have been written since the opening of the file.

Associated errors

Error code	Description
10	Non numeric value for MOVE_EXPR.
24	No file opened in read or read/write mode.
50	Non integer value for MOVE_EXPR.
57	Unable to perform a Seek operation on this file.

See also

Openi, Openo, Openio, Getseq, Rdseq, Putseq, Wrseq, adxifs, adxirs, adxium, adxmso.