| Application Programmer's I/O Guide - S-3695-35 | ||
|---|---|---|
| Prev Section | Chapter 10. Introduction to FFIO | Next Section |
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. For example, unformatted sequential I/O is assigned a default structure of cos on UNICOS systems and UNICOS/mk systems. The -F cos option provides the same structure. The FFIO system provides detailed control over I/O processing requests. However, to effectively use the cos option (or 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 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 (the default blocking for Fortran unformatted I/O on UNICOS and UNICOS/mk systems), 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.
A COS blocked blank-compressed file can also be read. The following are the processing steps necessary to do this:
Issue system calls to read data from the device.
Deblock the data and deliver blank-compressed characters.
Decompress the characters and deliver them to the user.
In this case, the spec with system is on the right end and would be as follows:
-F blankx,cos,system |
You do not need to specify the system spec because it is always implied on the right end. To read the COS blocked blank-compressed file, use the following specification:
assign -F blankx,cos |
Because the system spec is assumed, it is never required.
Several different layers are available for the spec argument. Each layer invokes one or more layers, which then handles the data it is given in an appropriate manner. For example, the syscall layer essentially passes each request to an appropriate system call. The tape layer uses an array of more sophisticated system calls to handle magnetic tape I/O. The blankx layer passes all data requests to the next lower layer, but it transforms the data before it is passed. The mr layer tries to hold an entire file in a buffer that can change size as the size of the file changes; it also limits actual I/O to lower layers so that I/O occurs only at open, close, and overflow.
The following tables list the classes you can specify for the spec argument to the assign -F option:
You can modify the behavior of each I/O layer. The following spec format shows how you can specify a class and one or more opt and num fields:
class.opt1.opt2:num1:num2:num3 |
For class, you can specify one of the layers listed in the previous tables. Each of the layers has a different set of options and numeric parameter fields that can be specified. This is necessary because each layer performs different duties. The following rules apply to the spec argument:
The class and opt fields are case-insensitive. For example, the following two specs are identical:
Ibm.VBs:100:200 |
IBM.vbS:100:200 |
The opt and num fields are usually optional, but sufficient separators must be specified as placeholders to eliminate ambiguity. For example, the following spec s are identical:
cos..::40, cos.::40 cos::40 |
In this example, opt1, opt2, num1, and num2 can assume default values. Similarly, the sds layer also allows optional opt and num fields and it sets opt1, opt2, num1, num2, and num3 to default values as required.
To specify more than one spec, use commas between specs. Within each spec, you can specify more than one opt and num. Use periods between opt fields, and use colons between num fields.
The following options all have the same effect. They all specify the sds layer on UNICOS systems and set the initial SDS allocation to 100 512-word sectors:
-F sds:100 -F sds.:100 -F sds..:100 |
The following option contains one spec for an sds layer that has an opt field of scr (which requests scratch file behavior):
-F sds.scr |
The following option requests two class es with no opt s:
-F cos,sds |
The following option contains two specs and requests two layers: cos and sds. The cos layer has no options; the sds layer has options scr and ovfl, which specify that the file is a scratch file that is allowed to overflow, and that the maximum SDS allocation is 1000 sectors:
-F cos,sds.scr.ovfl::1000 |
When possible, the default settings of the layers are set so that optional fields are seldom needed.
| Prev Section | Table of Contents | Title Page | Next Section |
| Introduction to FFIO | Up one level | Setting FFIO library parameters (UNICOS systems only) |