.PP
This section describes features of the Pascal input/output environment,
with special consideration of the features peculiar to an
interactive implementation.
.PP
Our first sample programs, in section 2, used the file
.I output .
We gave examples there of redirecting the output to a file and to the line
printer using the shell.
Similarly, we can read the input from a file or another program.
Consider the following Pascal program which is similar to the program
.I cat
(1).
.LS
% \*bpix -l kat.p
.PP
An extremely common problem encountered by new users of Pascal, especially
in the interactive environment offered by
.UX ,
relates to the definitions of
.I eof
and
.I eoln .
These functions are supposed to be defined at the beginning of execution of
a Pascal program, indicating whether the input device is at the end of a
line or the end of a file.
Setting
.I eof
or
.I eoln
actually corresponds to an implicit read in which the input is
inspected, but no input is ``used up''.
In fact, there is no way the system can know whether the input is
at the end-of-file or the end-of-line unless it attempts to read a line from it.
If the input is from a previously created file,
then this reading can take place without run-time action by the user.
However, if the input is from a terminal, then the input
is what the user types.\*(dg
If the system were to do an initial read
automatically at the beginning of program execution,
and if the input were a terminal,
the user would have to type some input before execution could begin.
.FS
\*(dgIt is not possible to determine whether the input is
a terminal, as the input may appear to be a file but actually be a
.I pipe,
the output of a program which is reading from the terminal.
.FE
This would make it impossible for the program to begin by prompting
for input or printing a herald.
.PP
.UP
has been designed so that an initial read is not necessary.
At any given time, the Pascal system may or may not know whether the
end-of-file or end-of-line conditions are true.
Thus, internally, these functions can have three values \-
true, false, and ``I don't know yet; if you ask me I'll have to
find out''.
All files remain in this last, indeterminate state until the Pascal
program requires a value for
.I eof
or
.I eoln
either explicitly or implicitly, e.g. in a call to
.I read .
The important point to note here is that if you force the Pascal
system to determine whether the input is at the end-of-file or the end-of-line,
it will be necessary for it to attempt to read from the input.
.PP
Thus consider the following example code
.LS
\*bwhile not\fP eof \*bdo\fP \*bbegin\fP
write('number, please? ');
read(i);
writeln('that was a ', i: 2)
\*bend\fP
.LE
At first glance, this may be appear to be a correct program
for requesting, reading and echoing numbers.
Notice, however, that the
.B while
loop asks whether
.I eof
is true
.I before
the request is printed.
This will force the Pascal system to decide whether the input is at the
end-of-file.
The Pascal system will give no messages;
it will simply wait for the user to type a line.
By producing the desired prompting before testing
.I eof,
the following code avoids this problem:
.LS
write('number, please ?');
\*bwhile not\fP eof \*bdo\fP \*bbegin\fP
read(i);
writeln('that was a ', i:2);
write('number, please ?')
\*bend\fP
.LE
The user must still type a line before the
.B while
test is completed, but the prompt will ask for it.
This example, however, is still not correct.
To understand why, it is first necessary to know, as we will discuss below,
that there is a blank character at the end of each line in a Pascal text
file.
The
.I read
procedure, when reading integers or real numbers,
is defined so that,
if there are only blanks left in the file,
it will return a zero value and set the end-of-file condition.
If, however, there is a number remaining in the file, the end-of-file
condition will not be set even if it is the last number, as
.I read
never reads the blanks after the number, and there is always at least
one blank.
Thus the modified code will still put out a spurious
.LS
that was a 0
.LE
at the end of a session with it when the end-of-file is reached.
The simplest way to correct the problem in this example is to use the procedure
.I readln
instead of
.I read
here.
In general, unless we test the end-of-file condition both before and
after calls to
.I read
or
.I readln ,
there will be inputs for which our program will attempt
to read past end-of-file.
.PP
To have a good understanding of when
.I eoln
will be true it is necessary to know that in any file
there is a special character indicating end-of-line,
and that, in effect, the Pascal system always reads one character ahead of the
Pascal
.I read
commands.\*(dg
.FS
\*(dgIn Pascal terms,
`read(ch)'
corresponds to
`ch := input^; get(input)'
.FE
For instance,
in response to `read(ch)',
the system sets
.I ch
to the current input character and gets the next input character.
If the current input character is the last character of the line,
then the next input character from the file is the new-line character,
the normal
.UX
line separator.
When the read routine gets the new-line character,
it replaces that character by a blank
(causing every line to end with a blank)
and sets
.I eoln
to true.
.I Eoln
will be true as soon as we read the last character of the line and
.B before
we read the blank character corresponding to the end of line.
Thus it is almost always a mistake to write a program which deals with
input in the following way:
.LS
read(ch);
\*bif\fP eoln \*bthen\fP
\fIDone with line\fP
\*belse\fP
\fINormal processing\fP
.LE
as this will almost surely have the effect of ignoring the last character
in the line.
The `read(ch)' belongs as part of the normal processing.
.PP
Given this framework, it is not hard to explain the function of a
.I readln
call, which is defined as:
.LS
\*bwhile not\fP eoln \*bdo\fP
get(input);
get(input);
.LE
This advances the file until the blank corresponding to the end-of-line
is the current input symbol and then discards this blank.
The next character available from
.I read
will therefore be the first character of the next line,
if one exists.
.PP
A final point about Pascal input-output must be noted here.
This concerns the buffering of the file
.I output .
It is extremely inefficient for the Pascal system to send each character
to the user's terminal as the program generates it for output;
even less efficient if the output is the input of another
program such as the line printer daemon
.I lpr
(1).
To gain efficiency, the Pascal system ``buffers'' the output characters
(i.e. it saves them in memory until the buffer is full and then emits
the entire buffer in one system interaction.)
However, to allow interactive prompting to work as in the example given
above, this prompt must be printed before the Pascal system waits for a
response.
For this reason, Pascal normally prints all the output which has
been generated for the file
.I output
whenever
.HP
.RS
.IP 1)
A
.I writeln
occurs, or
.IP 2)
The program reads from the terminal, or
.IP 3)
The procedure
.I message
or
.I flush
is called.
.RE
.LP
Thus, in the code sequence
.ne 5
.LS
\*bfor\fP i := 1 to 5 \*bdo begin\fP
write(i: 2);
\fICompute a lot with no output\fP
\*bend;\fP
writeln
.LE
the output integers will not print until the
.I writeln
occurs.
The delay can be somewhat disconcerting, and you should be aware
that it will occur.
By setting the
.B b
option to 0 before the
.B program
statement by inserting a comment of the form
.LS
(*$b0*)
.LE
we can cause
.I output
to be completely unbuffered, with a corresponding horrendous degradation
in program efficiency.
Option control in comments is discussed in section 5.
.PP
It is possible to use extended forms of the built-in functions
.I reset
and
.I rewrite
to get more general associations of
.UX
file names with Pascal file variables.
When a file other than
.I input
or
.I output
is to be read or written, then the reading or writing must be preceded
by a
.I reset
or
.I rewrite
call.
In general, if the Pascal file variable has never been used before,
there will be no
.UX
filename associated with it.
As we saw in section 2.9,
by mentioning the file in the
.B program
statement,
we could cause a
.UX
file with the same name as the Pascal variable to be associated with it.
If we do not mention a file in the
.B program
statement and use it for the first time with the statement
.LS
reset(f)
.LE
or
.LS
rewrite(f)
.LE
then the Pascal system will generate a temporary name of the form
`tmp.x'
for some character `x',
and associate this
.UX
file name name with the Pascal file.
The first such generated name will be `tmp.1'
and the names continue by incrementing their last character through the
.SM ASCII
set.
The advantage of using such temporary files is that they are automatically
.I remove d
by the Pascal system as soon as they become inaccessible.
They are not removed, however, if a runtime error causes termination
while they are in scope.
.PP
To cause a particular
.UX
pathname to be associated with a Pascal file variable
we can give that name in the
.I reset
or
.I rewrite
call, e.g. we could have associated the Pascal file
.I data
with the file
`primes'
in our example in section 3.1 by doing:
.LS
reset(data, 'primes')
.LE
instead of a simple
.LS
reset(data)
.LE
In this case it is not essential to mention `data'
in the program statement, but it is still a good idea
because is serves as an aid to program documentation.
The second parameter to
.I reset
and
.I rewrite
may be any string value, including a variable.
Thus the names of
.UX
files to be associated with Pascal file variables can be read
in at run time.
Full details on file name/file variable associations are given in
section A.3.
.PP
Each
.UX
process receives a variable
length sequence of arguments each of which is a variable length
character string.
The built-in function
.I argc
and the built-in procedure
.I argv
can be used to access and process these arguments.
The value of the function
.I argc
is the number of arguments to the process.
By convention,
the arguments are treated as an array,
and indexed from 0 to
.I argc \-1,
with the zeroth argument being the name of the program being executed.
The rest of the
arguments are those passed to the command on the command line.
Thus, the command
.LS
% \*bobj /etc/motd /usr/dict/words hello\fR
.LE
will invoke the program in the file
.I obj
with
.I argc
having a value of 4.
The zeroth element accessed by
.I argv
will be `obj', the first `/etc/motd', etc.
.PP
Pascal does not provide variable size arrays, nor does it allow
character strings of varying length.
For this reason,
.I argv
is a procedure and has the syntax
.LS
argv(i, a)
.LE
where
.I i
is an integer and
.I a
is a string variable.
This procedure call assigns the (possibly truncated or blank padded)
.I i \|'th
argument of the current process to the string variable
.I a .
The file manipulation routines
.I reset
and
.I rewrite
will strip trailing blanks from their optional second arguments
so that this blank padding is not a problem in the usual case
where the arguments are file names.
.PP
We are now ready to give a
Berkeley
Pascal program `kat',
based on that given in section 3.1 above,
which can be used with the same syntax as the
.UX
system program
.I cat
(1).
.LS
% \*bcat kat.p\fR
.so kat3.p
%
.LE
Note that the
.I reset
call to the file
.I input
here, which is necessary for a clear program,
may be disallowed on other systems.
As this program deals mostly with
.I argc
and
.I argv
and
.UX
system dependent considerations,
portability is of little concern.
.PP
If this program is in the file `kat.p', then we can do
.LS
% \*bpi kat.p\fR
% \*bmv obj kat\fR
% \*bkat primes\fR
.so kat2out
% \*bkat\fR
.so katscript
%
.LE
Thus we see that, if it is given arguments, `kat' will,
like
.I cat,
copy each one in turn.
If no arguments are given, it copies from the standard input.
Thus it will work as it did before, with
.LS
% \*bkat < primes\fR
.LE
now equivalent to
.LS
% \*bkat primes\fR
.LE
although the mechanisms are quite different in the two cases.
Note that if `kat' is given a bad file name, for example:
.LS
% \*bkat xxxxqqq\fR
.so xxxxqqqout
%
.LE
it will give a diagnostic and a post-mortem control flow backtrace
for debugging.
If we were going to use `kat', we might want to translate it
differently, e.g.:
.LS
% \*bpi -pb kat.p\fR
% \*bmv obj kat\fR
.LE
Here we have disabled the post-mortem statistics printing, so
as not to get the statistics or the traceback on error.
The
.B b
option will cause the system to block buffer the input/output so that
the program will run more efficiently on large files.
We could have also specified the
.B t
option to turn off runtime tests if that was felt to be a speed hindrance
to the program.
Thus we can try the last examples again:
.LS
% \*bkat xxxxqqq\fR
.so xxxxqqqout2
% \*bkat primes\fR
.so primes-d
%
.LE
.PP
The interested reader may wish to try writing a program which
accepts command line arguments like
.PI
does, using
.I argc
and
.I argv
to process them.