The FFIO system is based on the concept that for all I/O a list of processing steps must be performed to transfer the user data between the user's memory and the desired I/O device. Computer manufacturers have always provided I/O options to users because I/O is often the slowest part of a computational process. In addition, it is extremely difficult to provide one I/O access method that works optimally in all situations.

The following figure depicts the typical flow of data from the user's variables to and from the I/O device.

Figure 9-1. Typical data flow

It is useful to think of each of these boxes as a stopover for the data, and each transition between stopovers as a processing step.

Each transition has benefits and costs. Different applications might use the total I/O system in different ways. For example, if I/O requests are large, the library buffer is unnecessary because the buffer is used primarily to avoid making system calls for every small request. You can achieve better I/O throughput with large I/O requests by not using library buffering.

If library buffering is not used, I/O requests should be on 4096-byte block boundaries; otherwise, I/O performance will be degraded. On the other hand, if all I/O requests are very small, the library buffer is essential to avoid making a costly system call for each I/O request.

It is useful to be able to modify the I/O process to prevent intermediate steps (such as buffering of data) for existing programs without requiring that the source code be changed. The assign(1) command lets you modify the total user I/O path by establishing an I/O environment.

The FFIO system lets you specify each stopover in Figure 9-1. You can specify a comma-separated list of one or more processing steps by using the assign -F command:

assign -F spec1,spec2,spec3...

Each spec in the list is a processing step that requests one I/O layer, or logical grouping of layers. The layer specifies the operations that are performed on the data as it is passed between the user and the I/O device. A layer refers to the specific type of processing being done. In some cases, the name corresponds directly to the name of one layer. In other cases, however, specifying one layer invokes the routines used to pass the data through multiple layers. See the INTRO_FFIO (3f) man page for details about using the -F option to the assign command.

Processing steps are ordered as if the -F side (the left side) is the user and the system/device is the right side, as in the following example:

assign -F user,bufa,system

With this specification, a WRITE operation first performs the user operation on the data, then performs the bufa operation, and then sends the data to the system. In a READ operation, the process is performed from right to left. The data moves from the system to the user. The layers closest to the user are higher-level layers; those closer to the system are lower-level layers.

The FFIO system has an internal model of the world of data, which it maps to any given actual logical file type. Four of these concepts are basic to understanding the inner workings of the layers.

All files are streams of 0 or more bits that may contain record or file marks.

Individual layers have varying rules about which of these things can appear and in which order they can appear in a file.

Fortran users can use the assign(1) command to specify these FFIO options. For C users, the FFIO layers are available only to programs that call the FFIO routines directly ( ffopen(3c), ffread(3c), and ffwrite(3c)).

You can use FFIO with the following Fortran I/O forms:

The specification list on the assign -F command comprises all of the processing steps that the I/O system performs. If assign -F is specified, any default processing is overridden. The FFIO system provides detailed control over I/O processing requests. However, to effectively use any FFIO option, you must understand the I/O processing details.

As a very simple example, suppose you were making large I/O requests and did not require buffering or blocking on your data. You could specify the following:

assign -F system

The system layer is a generic system interface that chooses an appropriate layer for your file. If the file is on disk, it chooses the syscall layer, which maps each user I/O request directly to the corresponding system call. A Fortran READ statement is mapped to one or more read(2) system calls and a Fortran WRITE statement to one or more write(2) system calls. This results in almost the same processing as would be done if the assign -s u command was used.

If you want your file to be COS blocked, you can specify the following:

assign -F cos,system

If you want your file to be F77 blocked, you can specify the following:

assign -F f77,system

These two specs request that each WRITE request first be blocked (blocking adds control words to the data in the file to delimit records). The cos layer then sends the blocked data to the system layer. The system layer passes the data to the device.

The process is reversed for READ requests. The system layer retrieves blocked data from the file. The blocked data is passed to the next higher layer, the cos layer, where it is deblocked. The deblocked data is then presented to the user.

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