Sequential Access I/O

Logical record length for unformatted, sequential files is determined by the number of bytes required by the items in the I/O list. The requirements of this form of I/O cause the external physical record size to be somewhat larger than the logical record size.

Example:

	WRITE( 8 ) A, B 

The FORTRAN runtime system embeds the record boundaries in the data by inserting an INTEGER*4 byte count at the beginning and end of each unformatted sequential record during an unformatted sequential WRITE. The trailing byte count enables BACKSPACE to operate on records. The result is that FORTRAN programs can use an unformatted sequential READ only on data that was written by an unformatted sequential WRITE operation. Any attempt to read such a record as formatted would have unpredictable results.

Here are some guidelines:

• Avoid using the unformatted sequential READ unless your file was written that way.

• Because of the extra data at the beginning and end of each unformatted sequential record, you might want to try using the unformatted direct I/O whenever that extra data is significant. It is more significant with short records than with very long ones.