ack.7

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.MX
Pascal deviates from the standard in the following ways:
.IP 1.
Standard procedures and functions are not allowed as parameters in 
.MX
Pascal.
You can obtain the same result with negligible loss of performance
by declaring some user routines like:
.XS
.CW
function sine(x:real):real;
begin
    sine:=sin(x)
end;
.ft R
.XE
.IP 2.
The standard procedures read, readln, write and writeln are implemented as
word-symbols, and can therefore not be redeclared.
.SS "Compiler options"
.PP
Some options of the compiler may be controlled by using '{$....}'.
Each option consists of a lower case letter followed by +, \(mi or an unsigned
number.
Options are separated by commas.
The following options exist:
.IP a+/\(mi
This option switches assertions on and off.
If this option is on, then code is included to test these assertions
at run time.
Default +.
.IP c+/\(mi
This option, if on, allows you to use C-type string constants
surrounded by double quotes.
Moreover, a new type identifier 'string' is predefined.
Default \(mi.
.IP d+/\(mi
This option, if on, allows you to use variables of type 'long'.
Default \(mi.
.IP i<num>
.br
With this flag the setsize for a set of integers can be
manipulated.
The number must be the number of bits per set.
The default value is 16.
.IP l+/\(mi
If + then code is inserted to keep track of the source line number.
When this flag is switched on and off, an incorrect line number may appear
if the error occurs in a part of your program for which this flag is off.
Default +.
.IP r+/\(mi
If + then code is inserted to check subrange variables against
lower and upper subrange limits.
Default +.
.IP s+/\(mi
If + then the compiler will hunt for places in your program
where non-standard features are used, and for each place found
it will generate a warning.
Default \(mi.
.IP t+/\(mi
If + then each time a procedure is entered, the routine 'procentry' is
called, and each time a procedure exits, the procedure 'procexit' is
called.
Both 'procentry' and 'procexit' have a 'string' as parameter.
This means that when a user specifies his or her own procedures, the c-option
must be used.
Default procedures are present in the run time library.
Default \(mi.
.IP u+/\(mi
If + then the character '_' is treated like a letter,
so that it may be used in identifiers.
Procedure and function identifiers are not allowed to start with an
underscore because they may collide with library routine names.
Default \(mi.
.PP
Some of these flags (c, d, i, s, u, C and U) are only effective when
they appear before the 'program' symbol.
The others may be switched
on and off.
.PP
A very powerful debugging tool is the knowledge that inaccessible statements
and useless tests are removed by the optimizer.
For instance, a statement like:
.XS
.CW
if debug then
    writeln('initialization done');
.ft R
.XE
is completely removed by the optimizer if debug is a constant with
value false.
The first line is removed if debug is a constant with value true.
Of course, if debug is a variable nothing can be removed.
.SS "Library routines"
.PP
The following library of external routines for Pascal programs is available:
.nf
.SP
.CW
.ta 12n
const	bufsize = ?;
type	br1 =  1..bufsize;
	br2 =  0..bufsize;
	br3 = -1..bufsize;
	ok = -1..0;
	buf = packed array[br1] of char;
	alfa = packed array[1..8] of char;
	string = ^packed array[1..?] of char;
	filetype = file of ?;
	long = ?;
.SP
{all routines must be declared extern}
.SP
function	argc:integer;
function	argv(i:integer):string;
function	environ(i:integer):string;
procedure	argshift;
.SP
procedure	buff(var f:filetype);
procedure	nobuff(var f:filetype);
procedure	notext(var f:text);
procedure	diag(var f:text);
procedure	pcreat(var f:text; s:string);
procedure	popen(var f:text; s:string);
procedure	pclose(var f:filetype);
.SP
procedure	trap(err:integer);
procedure	encaps(procedure p; procedure q(n:integer));
.SP
function	perrno:integer;
function	uread(fd:integer; var b:buf; len:br1):br3;
function	uwrite(fd:integer; var b:buf; len:br1):br3;
.SP
function	strbuf(var b:buf):string;
function	strtobuf(s:string; var b:buf; len:br1):br2;
function	strlen(s:string):integer;
function	strfetch(s:string; i:integer):char;
procedure	strstore(s:string; i:integer; c:char);
.SP
function	clock:integer;
.fi
.ft R
.PP
This library contains some often used external routines for Pascal programs.
The routines can be divided into several categories:
.PP
.ti -2
Argument control:
.RS
.IP argc 10
Gives the number of arguments provided when the program is called.
.IP argv
Selects the specified argument from the argument list and returns a
pointer to it.
This pointer is nil if the index is out of bounds (<0 or >=argc).
.IP environ
Returns a pointer to the i-th environment string (i>=0).
Returns nil
if i is beyond the end of the environment list (UNIX version 7).
.IP argshift
Effectively deletes the first argument from the argument list.
Its function is equivalent to \fIshift\fR in the UNIX shell: argv[2] becomes
argv[1], argv[3] becomes argv[2], etc.
It is a useful procedure to skip optional flag arguments.
Note that the matching of arguments and files
is done at the time a file is opened by a call to reset or rewrite.
.PP
.ti -2
Additional file handling routines:
.IP buff
Turn on buffering of a file.
Not very useful, because all
files are buffered except standard output to a terminal and diagnostic output.
Input files are always buffered.
.IP nobuff
Turn off buffering of an output file.
It causes the current contents of the
buffer to be flushed.
.IP notext
Only useful for input files.
End of line characters are not replaced by a space and character codes out of
the ASCII range (0..127) do not cause an error message.
.IP diag
Initialize a file for output on the diagnostic output stream (fd=2).
Output is not buffered.
.IP pcreat
The same as rewrite(f), except that you must provide the file name yourself.
The name must be zero terminated.
Only text files are allowed.
.IP popen
The same as reset(f), except that you must provide the file name yourself.
The name must be zero terminated.
Only text files are allowed.
.IP pclose
Gives you the opportunity to close files hidden in records or arrays.
All other files are closed automatically.
.PP
.ti -2
String handling:
.IP strbuf
Type conversion from character array to string.
It is your own responsibility that the string is zero terminated.
.IP strtobuf
Copy string into buffer until the string terminating zero byte
is found or until the buffer if full, whatever comes first.
The zero byte is also copied.
The number of copied characters, excluding the zero byte, is returned.
So if
the result is equal to the buffer length, then the end of buffer is reached
before the end of string.
.IP strlen
Returns the string length excluding the terminating zero byte.
.IP strfetch
Fetches the i-th character from a string.
There is no check against the string length.
.IP strstore
Stores a character in a string.
There is no check against
string length, so this is a dangerous procedure.
.PP
.ti -2
Trap handling:
.PP
These routines allow you to handle almost all
the possible error situations yourself.
You may define your own trap handler, replacing the
default handler that produces an error message and quits.
You may also generate traps yourself.
.IP trap
Trap generates the trap passed as argument (0..252).
The trap numbers 128..252 may be used freely.
The others are reserved.
.IP encaps
Encapsulate the execution of \fIp\fR with the trap handler \fIq\fR.
Encaps replaces the previous trap handler by \fIq\fR, calls \fIp\fR
and restores
the previous handler when \fIp\fR returns.
If, during the execution of \fIp\fR, a trap occurs,
then \fIq\fR is called with the trap number as parameter.
For the duration of \fIq\fR the previous trap handler is restored, so that
you may handle only some of the errors in \fIq\fR.
All the other errors must
then be raised again by a call to \fItrap\fR.
.br
Encapsulations may be nested: you may encapsulate a procedure while executing
an encapsulated routine.
.br
Jumping out of an encapsulated procedure (non-local goto) is dangerous,
because the previous trap handler must be restored.
Therefore, you may only jump out of procedure \fIp\fR from inside \fIq\fR and
you may only jump out of one level of encapsulation.
If you want to exit several levels of encapsulation, use traps.
See pc_prlib(7) for lists of trap numbers
for EM machine errors and Pascal run time system errors.
Note that \fIp\fR may not have parameters.
.PP
.ti -2
UNIX system calls:
.IP uread
Equal to the read system call.
Its normal name is blocked by the standard Pascal routine read.
.IP uwrite
As above but for write(2).
.IP perrno
Because external data references are not possible in Pascal,
this routine returns the global variable \fIerrno\fR, indicating the result of
the last system call.
.PP
.ti -2
Miscellaneous:
.IP clock
Return the number of ticks of user and system time consumed by the program.
.PP
The following program presents an example of how these routines can be used.
This program is equivalent to the UNIX command cat(1).
.nf
.SP
.CW
{$c+}
.CW
program cat(input,inp,output);
.CW
var	inp:text;
.CW
	s:string;
.SP
.CW
function argc:integer; extern;
.CW
function argv(i:integer):string; extern;
.CW
procedure argshift; extern;
.CW
function strlen(s:string):integer; extern;
.CW
function strfetch(s:string; i:integer):char; extern;
.SP
.CW
procedure copy(var fi:text);
.CW
var c:char;
.CW
begin reset(fi);
.CW
  while not eof(fi) do
.CW
  begin
.CW
    while not eoln(fi) do
.CW
    begin
.CW
      read(fi,c);
.CW
      write(c)
.CW
    end;
.CW
    readln(fi);
.CW
    writeln
.CW
  end
.CW
end;
.SP
.CW
begin  {main}
.CW
  if argc = 1 then
.CW
    	copy(input)
.CW
  else
.CW
    repeat
.CW
      s := argv(1);
.CW
      if (strlen(s) = 1) and (strfetch(s,1) = '-')
.CW
      then copy(input)
.CW
      else copy(inp);
.CW
      argshift;
.CW
    until argc <= 1;
.CW
end.
.fi
.ft R
.PP
Another example gives some idea of the way to manage trap handling:
.nf
.SP
.CW
program bigreal(output);
.CW
const EFOVFL=4;
.CW
var trapped:boolean;
.CW
.SP
.CW
procedure encaps(procedure p; procedure q(n:integer)); extern;
.CW
procedure trap(n:integer); extern;
.CW
.SP
.CW
procedure traphandler(n:integer);
.CW
begin if n=EFOVFL then trapped:=true else trap(n) end;
.CW
.SP
.CW
procedure work;
.CW
var i,j:real;
.CW
begin trapped:=false; i:=1;
.CW
  while not trapped do
.CW
    begin j:=i; i:=i*2 end;
.CW
  writeln('bigreal = ',j);
.CW
end;
.CW
.SP
.CW
begin
.CW
  encaps(work,traphandler);
.CW
end.
.fi
.ft R
.PP
Two routines may cause fatal error messages to be generated.
These are:
.IP pcreat
Rewrite error (trap 77) if the file cannot be created.
.IP popen
Reset error (trap 76) if the file cannot be opened for reading
.SS References
.IP [1]
BSI standard BS 6192: 1982 (ISO 7185).
.IP [2]
A.S.Tanenbaum, J.W.Stevenson, Hans van Staveren, E.G.Keizer,
"Description of a machine architecture for use with block structured languages",
Informatica rapport IR-81.