For details about the routines discussed in this chapter, see the individual man pages for each routine. In addition, see the reference manuals for your compiler system.
BUFFER IN and BUFFER OUT statements initiate a data transfer between the specified file or unit at the current record and the specified area of program memory. To allow maximum asynchronous performance, all BUFFER IN and BUFFER OUT operations should begin and end on a sector boundary.
The BUFFER IN and BUFFER OUT statements can perform sequential asynchronous unformatted I/O if the files are assigned as unbuffered files. You must declare the BUFFER IN and BUFFER OUT files as unbuffered by using one of the following assign(1) commands.
assign -s u ... assign -F system ...
If the files are not declared as unbuffered, the BUFFER IN and BUFFER OUT statements may execute synchronously.
For tapes, BUFFER IN and BUFFER OUT operate synchronously; when you execute a BUFFER statement, the data is placed in the buffer before you execute the next statement in the program. Therefore, for tapes, BUFFER IN has no advantage over a read statement or a CALL READ statement; however, the library code is doing asynchronous read-aheads to fill its own buffer.
The F77 format is the default file structure.
The BUFFER IN and BUFFER OUT statements decrease the overhead associated with transferring data through library and system buffers. These statements also offer the advantages of asynchronous I/O. I/O operations for several files can execute concurrently and can also execute concurrently with CPU instructions. This can decrease overall wall-clock time.
In order for this to occur, the program must ensure that the requested asynchronous data movement was completed before accessing the data. The program must also be able to do a significant amount of CPU-intensive work or other I/O during asynchronous I/O to increase the program speed.
Buffer I/O processing waits until any previous buffer I/O operation on the file completes before beginning another buffer I/O operation.
Use the UNIT(3f) and LENGTH(3f) functions with BUFFER IN and BUFFER OUT statements to delay further program execution until the buffer I/O statement completes.
For details about the routines discussed in this section, see the individual man pages for each routine.
The UNIT intrinsic routine waits for the completion of the BUFFER IN or BUFFER OUT statement. A program that uses asynchronous BUFFER IN and BUFFER OUT must ensure that the data movement completes before trying to access the data. The UNIT routine can be called when the program wants to delay further program execution until the data transfer is complete. When the buffer I/O operation is complete, UNIT returns a status indicating the outcome of the buffer I/O operation.
The LENGTH intrinsic routine returns the length of transfer for a BUFFER IN or a BUFFER OUT statement. If the LENGTH routine is called during a BUFFER IN or BUFFER OUT operation, the execution sequence is delayed until the transfer is complete. LENGTH then returns the number of words successfully transferred. A 0 is returned for an end-of-file (EOF).