2-8 LOGICAL UNITS AND FILES
****************************
(thanks to Timothy Prince for the important comment)
Note that since operating systems treat peripheral devices as
files (see below), this discussion applies not just to files on
disks and tapes, but also to non-file oriented devices.
Logical Unit Numbers
--------------------
A FORTRAN program may read and write several files at the same time
(including the standard input and output), a convenient method to
'label' the files used is to assign small integers to them.
Similar methods are used in other languages, and the labeling
integers are called: LOGICAL UNIT NUMBERS (LUN) or file handles.
The separation of the file interface into two distinct logical
layers (LUN/file-name) makes it possible to perform the association
between the two at run-time, giving an extra flexibility (see below).
FORTRAN I/O is performed to or from a LOGICAL UNIT identified by the
logical unit number (LUN), the LUN is CONNECTED to a file/device
either explicitly with an OPEN statement or implicitly.
LUNs are global entities in a program, a LUN used in one procedure
to open a file, will refer to the same file in other procedures.
Explicit and Implicit OPEN
--------------------------
Using an 'OPEN' statement is not mandatory but is recommended,
otherwise you may work with system defaults that may be non-portable,
or just strange.
Using an OPEN statement is a good programming practice and
makes it possible to create files with optimal characteristics,
and read existing files in the optimal way.
The following standard OPEN keywords are useful:
Keyword Value Comments
------- ------------- ---------------------------------
UNIT A LUN number In the allowed range (see below)
FILE File name May be a constant/variable
STATUS 'OLD' To use an existing file,
'NEW' To create a new file
ACCESS 'SEQUENTIAL'
'DIRECT' Open for direct (random) access
FORM 'FORMATTED' For an ASCII file
'UNFORMATTED' For a binary file
BLANK 'NULL' Ignore blanks
ERR Statement label to jump to if 'open' fails
On an advantage of using the IOSTAT specifier over the ERR specifier
see the chapter on the three I/O methods.
Non-standard keywords can optimize I/O performance, or enable you
to use INDEXED (non-standard!) files.
Redirecting implicitly opened files
-----------------------------------
If you use the 'FILE' keyword in the 'open' statement, you create
a 'connection' between the LUN and the file name. If you don't use
this keyword a default file name is assumed:
System Default file name
------ -----------------
VMS FORTnnn.DAT
UNIX fort.n
These default file names are used to create one more "naming level".
Using an operating system command it is possible to make the program
process another file without having to re-compile it:
System Command syntax Shell
------ ---------------------- -----
VMS define FOR010 new.dat DCL
IRIX set FOR010 = new.dat ????
setenv FOR010 new.dat ????
AIX ln -s new.dat fort.10 ????
This is a common technique, it gives the programmer more flexibility
managing file I/O.
By the way, a more informative term for 'shell' is 'command language
interpreter'.
Preassigned LUNs
----------------
Some LUNs are permanently assigned (PRECONNECTED) to the standard
input and output devices (the keyboard and screen respectively in
an interactive session).
There is a popular (non-standard!) convention for preassigned LUNs
whose origin is not clear, it is probably derived from IBM practice,
not from the FORTRAN 66 standard:
Unit VMS logical C stream Old I/O Interactive Batch mode
no. device name name usage mode usage usage
---- ---------- -------- ----------- ----------- ----------
0 stderr screen log file
5 SYS$INPUT stdin card reader keyboard
6 SYS$OUTPUT stdout line printer screen log file
Using these LUNs explicitly is bad programming. A better way is:
I/O Statement VMS device UNIX dev Interactive VMS Batch
example session job
------------- ---------- --------- ----------- -------------
read(*,...) SYS$INPUT stdin keyboard batch file
write(*,...) SYS$OUTPUT stdout screen log file
Unix-like redirection makes the use of the * specifiers convenient;
failing that, it's hard to get flexibility without relying on the
non-standard numeric values.
Output redirection
------------------
There is no difference between WRITE(6,*) and WRITE(*,*) when you
redirect program output, except on VMS.
VMS redirects output by redefining the logical name SYS$OUTPUT,
there are two ways an ordinary user can do that:
DEFINE/USER_MODE SYS$OUTPUT FILE.OUT (User-mode)
DEFINE SYS$OUTPUT FILE.OUT (Supervisor-mode)
In supervisor mode you will get one file called FILE.OUT containing
the output of both WRITE(6,*) and WRITE(*,*) statements.
In user mode you will get two versions of the file FILE.OUT, the
first version contains the WRITE(*,*) output and the second the
WRITE(6,*) output.
Range of LUNs
-------------
The range of LUNs is limited in FORTRAN to a subset of the positive
integers:
Operating system Minimal LUN Maximal Lun Open by default
---------------- ----------- ----------- ---------------
VMS 0 119 none
Typical UNIX 0 2**31 - 1 0, 5, 6
DEC (and maybe IRIX) compilers use negative LUNs when you don't
explicitly specify a LUN in an I/O statement.
The default DEC unit numbers are:
I/O STATEMENT UNIT
------------- ----
PRINT -1
TYPE -2
ACCEPT -3
READ -4
Negative unit numbers are unavailable to the programmer, so this
"trick" prevents conflicts between I/O statements that use the
default logical unit numbers and those that use explicit logical
unit numbers.
You usually see these negative logical unit numbers only in error
messages, e.g. run-time error messages produced by the Fortran
run-time library for your compiler.
LUN allocation
--------------
Once a LUN was connected to a file, the connection remains in
effect until an explicit CLOSE statement is executed, then the
LUN is 'released' and you can use it again.
That means that you have to keep track of the LUNs you use in
your program, or have some way to ask the system about them.
You can find if you have already used a certain LUN with the
INQUIRE statement:
INTEGER u
LOGICAL log
............................
u = 10
INQUIRE (UNIT=u, OPENED=log)
C ------------------------------------------------------------------
IF (log) THEN
WRITE (*,*) ' Unit ', u, ' is not free '
ELSE
WRITE (*,*) ' Unit ', u, ' is free '
ENDIF
C ------------------------------------------------------------------
Other INQUIRE keywords check if the unit number is valid ('EXIST'),
and supply information on the connected file.
You may call a system routine (non-standard!) to find a 'free'
LUN and allocate it, afterwards you can deallocate the LUN.
LIB$GET_LUN(u) (VMS)
LIB$FREE_LUN(u)
However, you can easily write a PORTABLE such routine in FORTRAN
using the INQUIRE statement.
Reading/Writing from/to peripheral devices
------------------------------------------
Almost, if not all operating-system unified the I/O operations
performed on peripheral devices, so that devices like printers
and serial ports can be used as if they are files.
Some DOS examples (Maybe you'll need sometimes to add a colon
':' to the device name):
copy CON tmp.tmp (copies anything you type to the file tmp.tmp
CON is the name of the "console" device in
this case it's the keyboard. Exit with CTRL/Z,
then a RETURN)
copy tmp.tmp PRN (prints the file tmp.tmp)
copy tmp.tmp COM1 (sends the file tmp.tmp through the COM1
serial port)
type CON (displays on the screen whatever you type,
after you press RETURN. Exit with CTRL/Z
then a RETURN)
Using the peripheral devices from a program is easy, use the
appropriate device as if it was a file, and do an OPEN:
OPEN(UNIT=unitnum, FILE='devname', STATUS='OLD',...)
than just read/write from/to the opened unit.
The examples here used DOS, but PC operating systems made by
a certain company are rather bad, and you may have to use a
routine library supplied with the compiler.
A much more important factor in the social movement than those already mentioned was the ever-increasing influence of women. This probably stood at the lowest point to which it has ever fallen, during the classic age of Greek life and thought. In the history of Thucydides, so far as it forms a connected series of events, four times only during a period of nearly seventy years does a woman cross the scene. In each instance her apparition only lasts for a moment. In three of the four instances she is a queen or a princess, and belongs either to the half-barbarous kingdoms of northern Hellas or to wholly barbarous Thrace. In the one remaining instance208— that of the woman who helps some of the trapped Thebans to make their escape from Plataea—while her deed of mercy will live for ever, her name is for ever lost.319 But no sooner did philosophy abandon physics for ethics and religion than the importance of those subjects to women was perceived, first by Socrates, and after him by Xenophon and Plato. Women are said to have attended Plato’s lectures disguised as men. Women formed part of the circle which gathered round Epicurus in his suburban retreat. Others aspired not only to learn but to teach. Arêtê, the daughter of Aristippus, handed on the Cyrenaic doctrine to her son, the younger Aristippus. Hipparchia, the wife of Crates the Cynic, earned a place among the representatives of his school. But all these were exceptions; some of them belonged to the class of Hetaerae; and philosophy, although it might address itself to them, remained unaffected by their influence. The case was widely different in Rome, where women were far more highly honoured than in Greece;320 and even if the prominent part assigned to them in the legendary history of the city be a proof, among others, of its untrustworthiness, still that such stories should be thought worth inventing and preserving is an indirect proof of the extent to which feminine influence prevailed. With the loss of political liberty, their importance, as always happens at such a conjuncture, was considerably increased. Under a personal government there is far more scope for intrigue than where law is king; and as intriguers women are at least the209 equals of men. Moreover, they profited fully by the levelling tendencies of the age. One great service of the imperial jurisconsults was to remove some of the disabilities under which women formerly suffered. According to the old law, they were placed under male guardianship through their whole life, but this restraint was first reduced to a legal fiction by compelling the guardian to do what they wished, and at last it was entirely abolished. Their powers both of inheritance and bequest were extended; they frequently possessed immense wealth; and their wealth was sometimes expended for purposes of public munificence. Their social freedom seems to have been unlimited, and they formed combinations among themselves which probably served to increase their general influence.321 The old religions of Greece and Italy were essentially oracular. While inculcating the existence of supernatural beings, and prescribing the modes according to which such beings were to be worshipped, they paid most attention to the interpretation of the signs by which either future events in general, or the consequences of particular actions, were supposed to be divinely revealed. Of these intimations, some were given to the whole world, so that he who ran might read, others were reserved for certain favoured localities, and only communicated through the appointed ministers of the god. The Delphic oracle in particular enjoyed an enormous reputation both among Greeks and barbarians for guidance afforded under the latter conditions; and during a considerable period it may even be said to have directed the course of Hellenic civilisation. It was also under this form that supernatural religion suffered most injury from the great intellectual movement which followed the Persian wars. Men who had learned to study the constant sequences of Nature for themselves, and to shape their conduct according to fixed principles of prudence or of justice, either thought it irreverent to trouble the god about questions on which they were competent to form an opinion for themselves, or did not choose to place a well-considered scheme at the mercy of his possibly interested responses. That such a revolution occurred about the middle of the fifth century B.C., seems proved by the great change of tone in reference to this subject which one perceives on passing from Aeschylus to Sophocles. That anyone should question the veracity of an oracle is a supposition which never crosses the mind of the elder dramatist. A knowledge of augury counts among the greatest benefits222 conferred by Prometheus on mankind, and the Titan brings Zeus himself to terms by his acquaintance with the secrets of destiny. Sophocles, on the other hand, evidently has to deal with a sceptical generation, despising prophecies and needing to be warned of the fearful consequences brought about by neglecting their injunctions. The stranger had a pleasant, round face, with eyes that twinkled in spite of the creases around them that showed worry. No wonder he was worried, Sandy thought: having deserted the craft they had foiled in its attempt to get the gems, the man had returned from some short foray to discover his craft replaced by another. “Thanks,” Dick retorted, without smiling. When they reached him, in the dying glow of the flashlight Dick trained on a body lying in a heap, they identified the man who had been warned by his gypsy fortune teller to “look out for a hidden enemy.” He was lying at full length in the mould and leaves. "But that is sport," she answered carelessly. On the retirement of Townshend, Walpole reigned supreme and without a rival in the Cabinet. Henry Pelham was made Secretary at War; Compton Earl of Wilmington Privy Seal. He left foreign affairs chiefly to Stanhope, now Lord Harrington, and to the Duke of Newcastle, impressing on them by all means to avoid quarrels with foreign Powers, and maintain the blessings of peace. With all the faults of Walpole, this was the praise of his political system, which system, on the meeting of Parliament in the spring of 1731, was violently attacked by Wyndham and Pulteney, on the plea that we were making ruinous treaties, and sacrificing British interests, in order to benefit Hanover, the eternal millstone round the neck of England. Pulteney and Bolingbroke carried the same attack into the pages of The Craftsman, but they failed to move Walpole, or to shake his power. The English Government, instead of treating Wilkes with a dignified indifference, was weak enough to show how deeply it was touched by him, dismissed him from his commission of Colonel of the Buckinghamshire Militia, and treated Lord Temple as an abettor of his, by depriving him of the Lord-Lieutenancy of the same county, and striking his name from the list of Privy Councillors, giving the Lord-Lieutenancy to Dashwood, now Lord Le Despencer. "I tell you what I'll do," said the Deacon, after a little consideration. "I feel as if both Si and you kin stand a little more'n you had yesterday. I'll cook two to-day. We'll send a big cupful over to Capt. McGillicuddy. That'll leave us two for to-morrer. After that we'll have to trust to Providence." "Indeed you won't," said the Surgeon decisively. "You'll go straight home, and stay there until you are well. You won't be fit for duty for at least a month yet, if then. If you went out into camp now you would have a relapse, and be dead inside of a week. The country between here and Chattanooga is dotted with the graves of men who have been sent back to the front too soon." "Adone do wud that—though you sound more as if you wur in a black temper wud me than as if you pitied me." "Wot about this gal he's married?" "Don't come any further." "Davy, it 'ud be cruel of us to go and leave him." "Insolent priest!" interrupted De Boteler, "do you dare to justify what you have done? Now, by my faith, if you had with proper humility acknowledged your fault and sued for pardon—pardon you should have had. But now, you leave this castle instantly. I will teach you that De Boteler will yet be master of his own house, and his own vassals. And here I swear (and the baron of Sudley uttered an imprecation) that, for your meddling knavery, no priest or monk shall ever again abide here. If the varlets want to shrieve, they can go to the Abbey; and if they want to hear mass, a priest can come from Winchcombe. But never shall another of your meddling fraternity abide at Sudley while Roland de Boteler is its lord." "My lord," said Edith, in her defence, "this woman has sworn falsely. The medicine I gave was a sovereign remedy, if given as I ordered. Ten drops would have saved the child's life; but the contents of the phial destroyed it. The words I uttered were prayers for the life of the child. My children, and all who know me, can bear witness that I have a custom of asking His blessing upon all I take in hand. I raised my eyes towards heaven, and muttered words; but, my lord, they were words of prayer—and I looked up as I prayed, to the footstool of the Lord. But it is in vain to contend: the malice of the wicked will triumph, and Edith Holgrave, who even in thought never harmed one of God's creatures, must be sacrificed to cover the guilt, or hide the thoughtlessness of another." "Aye, Sir Treasurer, thou hast reason to sink thy head! Thy odious poll-tax has mingled vengeance—nay, blood—with the cry of the bond." HoME古一级毛片免费观看
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