HPlogo Accessing Files Programmer's Guide > Chapter 7 Record Selection and Data Transfer

Control Operations

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There may be times when you want to move the record pointer to a particular place without necessarily transferring any data. There are three general categories for this type of record selection:

Spacing: Move the record pointer backward or forward.

Pointing: Reset the record pointer.

Rewinding: Reset the pointer to record 0.

Spacing

To space forward or backward in your file, use the FSPACE intrinsic. Its syntax is 

  FSPACE (filenum, displacement);
The displacement parameter gives the number of records to space from the current record pointer. Use a positive number for spacing forward in the file or a negative number for spacing backward.

You can use the FSPACE intrinsic only with files that contain fixed-length or undefined-length records; variable-length record files are not allowed. The FSPACE intrinsic may not be used when you have opened your file with APPEND access; the file system returns a CCL condition if you attempt to use it in this case. Spacing beyond the EOF results in a CCG condition, and the record pointer is not changed.

Pointing

To request a specific location for the record pointer to indicate, use the FPOINT intrinsic. Its syntax is 

  FPOINT (filenum, recnum);
Use the recnum parameter to specify the new location for the record pointer; recnum is the record number relative to the start of the file (record 0).

You can use The FPOINT intrinsic only with files that contain fixed-length or undefined-length records; variable-length record files are not allowed. The FPOINT intrinsic may not be used when you have opened your file with Append access; the file system returns a CCL condition if you attempt to use it in this case. It is legal to point to any record between the start of file and the file limit. Subsequent reads will fail if the data pointer is positioned beyond the EOF. If the data pointer is positioned beyond the EOF and a subsequent write is done, this will become the new EOF and all data between the old and new EOF will be initialized with the fill character.

Rewinding

When you "rewind" your file, you set the record pointer to indicate record 0, the first record in your file. Use the FCONTROL intrinsic with a control code of 5 to accomplish this. FCONTROL's syntax in this case would be 

  FCONTROL (filenum, 5, dummyparam);
Issuing this intrinsic call sets the record pointer to record 0. You can use FCONTROL with fixed-length, variable-length, or undefined-length record files; you can use it with any access method.

NOTE: FCONTROL's control code 5 has a special meaning when used with Append access. The file system sets the record pointer to record 0, as with other access modes, but at the time of the next write operation to the file, the record pointer is set to the end of the file so that no data is overwritten.

For more information about the FSPACE, FPOINT, and FCONTROL intrinsics, consult the MPE/iX Intrinsics Reference Manual.



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