2-7 COMMON BLOCKS
******************
(Thanks to Keith Bierman for the excellent comments, and to
Craig Burley for making clear some subtle points)
Common blocks are considered by some a "dusty" feature of FORTRAN 77,
which is an inevitable source of problems, and should be replaced
by the modules of Fortran 90. The "rational subsets" of Fortran 90,
ELF90 by Lahey, and F by Imagine1, do not support common blocks.
The variables in a common block are re-identified in each procedure
using storage association, a mechanism that is considered inferior
to naming each one.
Another view: Actually, it often isn't, but they should be coherent,
distinct organizations of truly global data, rather than convenient
placeholders for things you don't want to bother passing among a
few procedures.
The uses of common blocks
-------------------------
Common blocks are a FORTRAN-specific mechanism for indirectly passing
variables to procedures, i.e. not by procedure arguments. The common
block mechanism is a refinement of the global variables used in other
programming languages, as you can control which procedures have access
to a common block and which will not (by including or not the common
statement in these procedures).
When you have variables that are passed along a chain of procedure calls
but are used only in some of them, you can avoid repeatedly entering
them into argument and declaration lists by putting them in a common
block and declaring it only in the procedures that actually use them.
Eliminating these argument passing operations may save CPU time as well.
Using common blocks can shorten argument and declaration lists,
but the indirect links created between different parts of the program
are confusing and hard to trace, and a constant source of nasty bugs.
Note that common blocks doesn't have the full functionality of passing
variables by procedure calls, the FORTRAN 77 standard forbids common
block variables from using the adjustable array mechanism (only the
dimensional info may be passed in the block, but not the variable),
so their dimensions must be specified explicitly with constants.
This is a serious limitation, you can't write nice general-purpose
routines that can be put in an object library. A partial solution
is including a file that contains the required dimensional information
in each procedure that use the common block.
+-----------------------------------------+
| AVOID USING COMMON BLOCKS IF POSSIBLE |
+-----------------------------------------+
Losing variable values
----------------------
COMMON variables may LOSE THEIR VALUES after a subprogram return,
you can use the SAVE statement on each re-declaration, or declare
the COMMON block in the main program to avoid such situations.
Losing COMMON (and other) variable values was inevitable with the
oldtime overlay loaders used before the age of virtual memory,
when a COMMON block wasn't used by one of the currently executing
procedures the memory area allocated to it was reallocated for
other purpose. Virtual memory made this technique unnecessary
but as the standard allows this behaviour you should still guard
against it.
A static code checker like FTNCHEK (used with the -volatile option)
can locate COMMON blocks that may have this problem.
Guidelines on COMMON usage
--------------------------
If you decide to use COMMON blocks, the following remarks
may help:
1) Character variables shouldn't be in the same COMMON
block with non-character variables. If you pass both
kinds, create one COMMON block for the character
variables, and one for the numeric variables.
The standard imposes this requirement to ensure that
operations associating variables of different types
will give portable results, without having to specify
storage sizes. See the chapter on memory allocation
for a discussion of this subject.
Mixing character and numeric variables in a common
block may create a problem when you port your program.
Also there might have been an ancient system or two
that allowed easier implementation of FORTRAN without
such mixing, but on most modern systems, it's no problem.
2) Declare array dimensions explicitly in a type declaration,
or in a COMMON block declaration, don't specify the
dimensions with the DIMENSION statement.
3) The best policy is to make all re-declarations of a COMMON
block exact copies, with the same variable names, in the
same order, and with the same data types and sizes.
On all re-declarations of a COMMON block, also make the type
declaration lists of the COMMON block members exact copies.
A possible way to ensure consistent declarations is to use
the non-standard INCLUDE statement or a preprocessor,
(e.g. the #include directive of cpp - the C pre-processor
found in most unix f77 implementations).
4) A good rule is to order the variables of COMMON blocks
in a size decreasing order:
COMPLEX*32, COMPLEX*16, REAL*16
COMPLEX*8, REAL*8, INTEGER*8
REAL*4, INTEGER*4, LOGICAL
INTEGER*2
BYTE, CHARACTER
Ordered that way the 'smaller' variables will not get
the 'larger' ones out of alignment.
Usually INTEGER is INTEGER*4, REAL is REAL*4 and DOUBLE
PRECISION is REAL*8, but some machines have unusual sizes
(e.g. on CRAYs INTEGER is 64 bits but only 46 are used by
default, REAL is REAL*8, DOUBLE PRECISION is REAL*16 but
two bytes are not used), on other machines a character
storage unit may be 2 bytes or even variable-length
instead of the usual 1.
5) All COMMON blocks can only legally be initialized with
a block data subprogram.
6) A blank COMMON block may behave differently from named
COMMON blocks:
a) Blank COMMON variables never become undefined
as a result of procedures return
b) A blank COMMON block may be re-declared with
different size, avoid using that feature.
c) A blank COMMON block may not be initialized by
a DATA statement
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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