puman2.n

早期freebsd实现

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.\"	@(#)puman2.n	8.1 (Berkeley) 6/8/93
.\"
.if !\n(xx \{\
.so tmac.p \}
'if n 'ND
.nr H1 1
.NH
Basic UNIX Pascal
.PP
The following sections
explain the basics of using
.UP .
In examples here we use the text editor
.I ex 
(1).
Users of the text editor
.I ed
should have little trouble following these examples,
as
.I ex
is similar to
.I ed .
We use
.I ex
because it
allows us to make clearer examples.\(dg
.FS
\(dg Users with \s-2CRT\s0 terminals should find the editor
.I vi
more pleasant to use;
we do not show its use here because its display oriented nature
makes it difficult to illustrate.
.FE
The new
.UX
user will find it helpful to read one of the text editor documents
described in section 1.4 before continuing with this section.
.NH 2
A first program
.PP
To prepare a program for
.UP
we first need to have an account on
.UX
and to `login'
to the system on this account.
These procedures are described in the documents
.I "Communicating with UNIX" 
and
.I "UNIX for Beginners".
.PP
Once we are logged in we need to choose a name for our program;
let us call it `first' as this is the first example.
We must also choose a name for the file in which the program will be stored.
The
.UP
system requires that programs reside in files which have names ending with
the sequence `.p' so we will call our file `first.p'.
.PP
A sample editing session to create this file would begin:
.LS
% \*bex first.p\fR
"first.p" [New file]
:
.LE
We didn't expect the file to exist, so the error diagnostic doesn't
bother us.
The editor now knows the name of the file we are creating.
The `:' prompt indicates that it is ready for command input.
We can add the text for our program using the `append'
command as follows.
.LS
:\*bappend\fR
.B
program first(output)
begin
	writeln('Hello, world!')
end.
\&.
.R
:
.LE
The line containing the single `\*b.\fR' character here indicated
the end of the appended text.
The `:' prompt indicates that
.I ex
is ready for another command.
As the editor operates in a temporary work space we must now store the contents
of this work space in the file `first.p'
so we can use the Pascal 
translator and executor
.IX
on it.
.LS
:\*bwrite\fR
"first.p" [New file] 4 lines, 59 characters
:\*bquit\fR
% 
.LE
We wrote out the file from the edit buffer here with the
`write'
command, and
.I ex
indicated the number of lines and characters written.
We then quit the editor, and now have a prompt from the shell.\(dd
.FS
\(dd Our examples here assume you are using
.I csh.
.FE
.KS
.PP
We are ready to try
to translate and execute our program.
.DS
.tr '\(aa^\(ua
% \*bpix first.p\fR
.so firstout
.tr ''^^
%
.DE
.KE
.PP
The translator first printed a syntax error diagnostic.
The number 2 here indicates that the rest of the line is an image
of the second line of our program.
The translator is saying that it expected to find a `;' before the
keyword
.B begin
on this line.
If we look at the Pascal syntax charts in the Jensen-Wirth
.I "User Manual" ,
or at some of the sample programs therein, we will see that
we have omitted the terminating `;' of the
.B program 
statement on the first
line of our program.
.PP
One other thing to notice about the error diagnostic is the letter `e'
at the beginning.
It stands for `error',
indicating that our input was not legal Pascal.
The fact that it is an `e' rather than an `E'
indicates that the translator managed to recover from this error well
enough that generation of code and execution could take place.
Execution is possible whenever no fatal `E' errors
occur during translation.
The other classes of diagnostics are `w' warnings,
which do not necessarily indicate errors in the program,
but point out inconsistencies which are likely to be due to program bugs,
and `s' standard-Pascal violations.\*(dg
.FS
\*(dgThe standard Pascal warnings occur only when the associated
.B s
translator option is enabled.
The
.B s
option is discussed in sections 5.1 and A.6 below.
Warning diagnostics are discussed at the end of section 3.2,
the associated
.B w
option is described in section 5.2.
.FE
.PP
After completing the translation of the program to interpretive code,
the Pascal system indicates that execution of the translated program began.
The output from the execution of the program then appeared.
At program termination, the Pascal runtime system indicated the
number of statements executed, and the amount of cpu time
used, with the resolution of the latter being 1/60'th of a second.
.PP
Let us now fix the error in the program and translate it to a permanent
object code file
.I obj
using
.PI .
The program
.PI
translates Pascal programs but stores the object code instead of executing it\*(dd.
.FS
\*(ddThis script indicates some other useful approaches to debugging
Pascal programs.
As in
.I ed
we can shorten commands in
.I ex
to an initial prefix of the command name as we did
with the
.I substitute
command here.
We have also used the `!' shell escape command here to execute other
commands with a shell without leaving the editor.
.FE
.LS
% \*bex first.p\fR
"first.p" 4 lines, 59 characters
:\*b1 print\fR
program first(output)
:\*bs/$/;\fR
program first(output);
:\*bwrite\fR
"first.p" 4 lines, 60 characters
:\*bquit\fR
% \*bpi first.p\fR
%
.LE
If we now use the
.UX
.I ls
list files command we can see what files we have:
.LS
% \*bls\fR
first.p
obj
%
.LE
The file `obj' here contains the Pascal interpreter code.
We can execute this by typing:
.LS
% \*bpx obj\fR
.so firstobjout
%
.LE
Alternatively, the command:
.LS
% \*bobj\fR
.LE
will have the same effect.
Some examples of different ways to execute the program follow.
.LS
% \*bpx\fR
.so firstobjout
% \*bpi -p first.p\fR
% \*bpx obj\fR
.so firstobjout2
% \*bpix -p first.p\fR
.so firstobjout2
%
.LE
.PP
Note that
.I px
will assume that `obj' is the file we wish to execute
if we don't tell it otherwise.
The last two translations use the
.B \-p
no-post-mortem option to eliminate
execution statistics and
`Execution begins'
and
`Execution terminated'
messages.
See section 5.2 for more details.
If we now look at the files in our directory we will see:
.LS
% \*bls\fR
first.p
obj
%
.LE
We can give our object program a name other than `obj' by using the move
command
.I mv
(1).
Thus to name our program `hello':
.LS
% \*bmv obj hello\fR
% \*bhello\fR
Hello, world!
% \*bls\fR
first.p
hello
%
.LE
Finally we can get rid of the Pascal object code by using the
.I rm
(1) remove file command, e.g.:
.LS
% \*brm hello\fR
% \*bls\fR
first.p
%
.LE
.PP
For small programs which are being developed
.IX
tends to be more convenient to use than
.PI
and
.X .
Except for absence of the
.I obj
file after a
.IX
run,
a
.IX
command is equivalent to a
.PI
command followed by a
.X
command.
For larger programs,
where a number of runs testing different parts of the program are
to be made,
.PI
is useful as this
.I obj
file can be executed any desired number of times.
.. >>> INSERT SECTION FOR PC <<<
.NH 2
A larger program
.PP
Suppose that we have used the editor to put a larger program
in the file `bigger.p'.
We can list this program with line numbers by using the program
.I cat -n
i.e.:
.LS
% \*bcat -n bigger.p\fR
.so bigger3.p
%
.LE
This program is similar to program 4.9 on page 30 of the
Jensen-Wirth
.I "User Manual" .
A number of problems have been introduced into this example for
pedagogical reasons.
.br
.PP
If we attempt to translate and execute the program using
.IX
we get the following response:
.LS
% \*bpix bigger.p\fR
.so bigout1
%
.LE
.PP
Since there were fatal `E' errors in our program,
no code was generated and execution was necessarily suppressed.
One thing which would be useful at this point is a listing of the
program with the error messages.
We can get this by using the command:
.LS
% \*bpi -l bigger.p\fR
.LE
There is no point in using
.IX
here, since we know there are fatal errors in the program.
This command will produce the output at our terminal.
If we are at a terminal which does not produce a hard copy
we may wish to print this
listing off-line on a line printer.
We can do this with the command:
.LS
% \*bpi -l bigger.p | lpr\fR
.LE
.PP
In the next few sections we will illustrate various aspects of the
Berkeley
Pascal system by correcting this program.
.NH 2
Correcting the first errors
.PP
Most of the errors which occurred in this program were
.I syntactic
errors, those in the format and structure of the program rather than
its content.
Syntax errors are flagged by printing the offending line, and then a line
which flags the location at which an error was detected.
The flag line also gives an explanation
stating either a possible cause of the error,
a simple action which can be taken to recover from the error so
as to be able to continue the analysis,
a symbol which was expected at the point of error,
or an indication that the input was `malformed'.
In the last case, the recovery may skip ahead in the input
to a point where analysis of the program can continue.
.PP
In this example,
the first error diagnostic indicates that the translator detected
a comment within a comment.
While this is not considered an error in `standard'
Pascal, it usually corresponds to an error in the program which
is being translated.
In this case, we have accidentally omitted the trailing `*)' of the comment
on line 8.
We can begin an editor session to correct this problem by doing:
.LS
% \*bex bigger.p\fR
"bigger.p" 24 lines, 512 characters
:\*b8s/$/ *)\fR
        s = 32;       (* 32 character width for interval [x, x+1] *)
:
.LE
.PP
The second diagnostic, given after line 16,
indicates that the keyword
.B do