ack.7

来自「minix操作系统最新版本（3.1.1）的源代码」· 7 代码 · 共 2,091 行 · 第 1/5 页

.ti -3
non-ASCII char read, trap 106, non-fatal:
.br
the character value of the next character-type
file element is out of range (0..127).
Only for text files.
.in -6
.IT 6.6.5.3
It shall be an error if a variant of a variant-part within the new
variable becomes active and a different variant of the variant-part is
one of the specified variants.
.IS
This error is not detected.
.IT 6.6.5.3
It shall be an error to use dispose(q) if the identifying variable has been
allocated using the form new(p,c1,...,cn).
.IS
This error is not detected.
However, this error can cause more memory
to be freed then was allocated.
Dispose causes a fatal trap 73 when memory already on the free
list is freed again.
.IT 6.6.5.3
It shall be an error to use dispose(q,k1,...,km) if the identifying
variable has been allocated using the form new(p,c1,...,cn) and m is not
equal to n.
.IS
This error is not detected.
However, this error can cause more memory
to be freed then was allocated.
Dispose causes a fatal trap 73 when memory already on the free
list is freed again.
.IT 6.6.5.3
It shall be an error if the variants of a variable to be disposed
are different from those specified by the case-constants to dispose.
.IS
This error is not detected.
.IT 6.6.5.3
It shall be an error if the value of the pointer parameter of dispose has
nil-value or is undefined.
.IS
This error is detected for nil-value (dispose error, trap 73, fatal).
.IT 6.6.5.3
It shall be an error if a variable created using the second form of new is
accessed by the identified variable of the variable-access of a factor,
of an assignment-statement, or of an actual-parameter.
.IS
This error is not detected.
.IT 6.6.6.2
It shall be an error if the value of sqr(x) does not exist.
.IS
This error is detected for real-type arguments (real overflow,
trap 4, non-fatal).
.IT 6.6.6.2
It shall be an error if x in ln(x) is smaller than or equal to 0.
.IS
This error is detected (error in ln, trap 66, non-fatal)
.IT 6.6.6.2
It shall be an error if x in sqrt(x) is smaller than 0.
.IS
This error is detected (error in sqrt, trap 67, non-fatal)
.SP
In addition to these errors, overflow in the expression exp(x) is
detected (error in exp, trap 65, non-fatal; real overflow, trap 4, non-fatal)
.IT 6.6.6.3
It shall be an error if
the integer value of trunc(x) does not exist.
.IS
This error is detected (conversion error, trap 10, non-fatal).
.IT 6.6.6.3
It shall be an error if
the integer value of round(x) does not exist.
.IS
This error is detected (conversion error, trap 10, non-fatal).
.IT 6.6.6.4
It shall be an error if
the integer value of ord(x) does not exist.
.IS
This error can not occur, because the compiler will not allow
such ordinal types.
.IT 6.6.6.4
It shall be an error if
the character value of chr(x) does not exist.
.IS
This error is detected (range bound error, trap 1, non-fatal).
.IT 6.6.6.4
It shall be an error if the value of succ(x) does not exist.
.IS
Same comments as for chr(x).
.IT 6.6.6.4
It shall be an error if the value of pred(x) does not exist.
.IS
Same comments as for chr(x).
.IT 6.6.6.5
It shall be an error if f in eof(f) is undefined.
.IS
This error is detected (file not yet open, trap 72, fatal).
.IT 6.6.6.5
It shall be an error if
f in eoln(f) is undefined, or if eof(f) is true at that time.
.IS
The following errors may occur:
.IS
file not yet open, trap 72, fatal;
.br
not readable, trap 97, fatal;
.br
end of file, trap 98, fatal.
.IT 6.7.1
It shall be an error if a variable-access used as an operand
in an expression is undefined at the time of its use.
.IS
The compiler performs some limited checks to see if identifiers are
used before they are set.
Since it can not always be sure (one could, for
instance, jump out of a loop), only a warning is generated.
When an
expression contains a function-call, an error occurs if the
function is not assigned at run-time.
.IT 6.7.2.2
A term of the form x/y shall be an error if y is zero.
.IS
This error is detected (divide by 0.0, trap 7, non-fatal).
.IT 6.7.2.2
It shall be an error if j is zero in 'i div j'.
.IS
This error is detected (divide by 0, trap 6, non-fatal).
.IT 6.7.2.2
It shall be an error if
j is zero or negative in i MOD j.
.IS
This error is detected (only positive j in 'i mod j', trap 71, non-fatal).
.IT 6.7.2.2
It shall be an error if the result of any operation on integer
operands is not performed according to the mathematical
rules for integer arithmetic.
.IS
This implementation does not detect integer overflow.
.IT 6.8.3.5
It shall be an error if none of the case-constants is equal to the
value of the case-index upon entry to the case-statement.
.IS
This error is detected (case error, trap 20, fatal).
.IT 6.9.1
It shall be an error if the sequence of characters read looking for an
integer does not form a signed-integer as specified in 6.1.5.
.IS
This error is detected (digit expected, trap 105, non-fatal).
.IT 6.9.1
It shall be an error if the sequence of characters read looking for a
real does not form a signed-number as specified in 6.1.5.
.IS
This error is detected (digit expected, trap 105, non-fatal).
.IT 6.9.1
When read is applied to f, it shall be an error if the buffer-variable f^
is undefined or the pre-assertions for get do not hold.
.IS
This error is detected (see get(f)).
.IT 6.9.3
When write is applied to a text file f, it shall be an error if f is
undefined or f is opened for reading.
.IS
This error is detected (see put(f)).
Furthermore, this error is also
detected when f is not a text file.
.IT 6.9.3.1
The values of TotalWidth or FracDigits shall be greater than or equal to
one; it shall be an error if either value is less then one.
.IS
When either value is less than zero, an error (illegal field width, trap
75, non-fatal) occurs.
Zero values are allowed, in order to maintain some
compatibility with the old 
.MX
Pascal compiler.
.IT 6.9.5
It shall be an error if the pre-assertion required for writeln(f) doe not
hold prior to the invocation of page(f);
.IS
This error is detected (see put(f)).
.in 0
.SS "Extensions to the standard"
.LP
.ft I
1. External routines
.LP
Except for the required directive 'forward' the 
.MX
Pascal compiler recognizes
the directive 'extern'.
This directive tells the compiler that the procedure block of this
procedure will not be present in the current program.
The code for the body of this procedure must be included at a later
stage of the compilation process.
.PP
This feature allows one to build libraries containing often used routines.
These routines do not have to be included in all the programs using them.
Maintenance is much simpler if there is only one library module to be
changed instead of many Pascal programs.
.PP
Another advantage is that these library modules may be written in a different
language, for instance C.
.PP
The use of external routines, however, is dangerous.
The compiler normally checks for the correct number and type of parameters
when a procedure is called and for the result type of functions.
If an external routine is called these checks are not sufficient,
because the compiler can not check whether the procedure heading of the
external routine as given in the Pascal program matches the actual routine
implementation.
It should be the loader's task to check this.
However, the current loaders are not that smart.
.PP
For those who wish the use the interface between C and Pascal we
give an incomplete list of corresponding formal parameters in C and Pascal.
.SP
.XS
.ta +\w'function a(pars):type   'u
\fBPascal	C\fR
a:integer	int a
a:char	int a
a:boolean	int a
a:real	double a
a:^type	type *a
var a:type	type *a
procedure a(pars)	struct {
	     void (*a)() ;
	     char *static_link ;
	}
function a(pars):type	struct {
	     type (*a)() ;
	     char *static_link ;
	}
.DT
.XE
The Pascal runtime system uses the following algorithm when calling
function/procedures passed as parameters.
.XS
if (static_link) {
	(*a)(static_link, pars);
} else {
	(*a)(pars);
}
.XE
.LP
.ft I
2. Separate compilation.
.LP
The compiler is able to (separately) compile a collection of declarations,
procedures and functions to form a library.
The library may be linked with the main program, compiled later.
The syntax of these modules is
.XS
.in +\w'module = 'u
.ti -\w'module = 'u
module = [constant-definition-part]
[type-definition-part]
[var-declaration-part]
[procedure-and-function-declaration-part]
.in -\w'module = 'u
.XE
The compiler accepts a program or a module:
.XS
unit = program | module
.XE
All variables declared outside a module must be imported
by parameters, even the files input and output.
Access to a variable declared in a module is only possible
using the procedures and functions declared in that same module.
By giving the correct procedure/function heading followed by the
directive 'extern' you may use procedures and functions declared in
other units.
.LP
.ft I
3. Assertions.
.LP
When the s-option is off, 
.MX
Pascal compiler recognizes an additional
statement, the assertion.
Assertions can be used as an aid in debugging
and documentation.
The syntax is:
.XS
assertion = 'assert' Boolean-expression
.XE
An assertion is a simple-statement, so
.XS
.in +\w'simple-statement = ['u
.ti -\w'simple-statement = ['u
simple-statement = [assignment-statement |
procedure-statement |
goto-statement |
assertion
.in -\w'['u
]
.in -\w'simple-statement = 'u
.XE
An assertion causes an error if the Boolean-expression is false.
That is its only purpose.
It does not change any of the variables, at least it should not.
Therefore, do not use functions with side-effects in the Boolean-expression.
If the a-option is turned on, then assertions are skipped by the
compiler. 'assert' is not a word-symbol (keyword) and may be used as identifier.
However, assignment to a variable and calling of a procedure with that
name will be impossible.
If the s-option is turned on, the compiler will not know a thing about
assertions, so using assertions will then give a parse error.
.LP
.ft I
4. Additional procedures.
.LP
Three additional standard procedures are available:
.IP "halt:"
a call of this procedure is equivalent to jumping to the
end of your program.
It is always the last statement executed.
The exit status of the program may be supplied
as optional argument.
If not, it will be zero.
.IP release:
.IP mark:
for most applications it is sufficient to use the heap as second stack.
Mark and release are suited for this type of use, more suited than dispose.
mark(p), with p of type pointer, stores the current value of the
heap pointer in p. release(p), with p initialized by a call
of mark(p), restores the heap pointer to its old value.
All the heap objects, created by calls of new between the call of
mark and the call of release, are removed and the space they used
can be reallocated.
Never use mark and release together with dispose!
.RE
.LP
.ft I
5. UNIX interfacing.
.LP
If the c-option is turned on, then some special features are available
to simplify an interface with the UNIX environment.
First of all, the compiler allows you to use a different type
of string constants.
These string constants are delimited by double quotes ('"').
To put a double quote into these strings, you must repeat the double quote,
like the single quote in normal string constants.
These special string constants are terminated by a zero byte (chr(0)).
The type of these constants is a pointer to a packed array of characters,
with lower bound 1 and unknown upper bound.
.br
Secondly, the compiler predefines a new type identifier 'string' denoting
this just described string type.
.PP
The only thing you can do with these features is declaration of
constants and variables of type 'string'.
String objects may not be allocated on the heap and string pointers
may not be de-referenced.
Still these strings are very useful in combination with external routines.
The procedure write is extended to print these zero-terminated
strings correctly.
.LP
.ft I
6. Double length (32 bit) integers.
.LP
If the d-option is turned on, then the additional type 'long' is known
to the compiler.
Long variables have integer values in the
range \(mi2147483648 .. [phone].
Long constants can not be declared.
Longs can not be used as control-variables.
It is not allowed to form subranges of type long.
All operations allowed on integers are also
allowed on longs and are indicated by the same
operators: '+', '-', '*', '/', 'div', 'mod'.
The procedures read and write have been extended to handle long
arguments correctly.
It is possible to read longs from a file of integers
and vice-versa, but only if longs and integers have the same size.
The default width for longs is 11.
The standard procedures 'abs' and 'sqr' have been extended to work
on long arguments.
Conversion from integer to long, long to real,
real to long and long to integer are automatic, like the conversion
from integer to real.
These conversions may cause a
.PP
.RS
conversion error, trap 10, non-fatal
.RE
.LP
.ft I
7. Underscore as letter.
.LP
The character '_' may be used in forming identifiers, if the u- or U-option
is turned on.
It is forbidden to start identifiers with underscores, since
this may cause name-clashes with run-time routines.
.LP
.ft I
8. Zero field width in write.
.LP
Zero TotalWidth arguments are allowed.
In this case, no characters are written for
character, string or Boolean type arguments.
A zero FracDigits
argument for fixed-point representation of reals causes the fraction and
the character '.' to be suppressed.
.LP
.ft I
9. Pre-processing.
.LP
If the very first character of a file containing a Pascal
program is the sharp ('#', ASCII 23(hex)) the file is preprocessed
in the same way as C programs.
Lines beginning with a '#' are taken as preprocessor command lines
and not fed to the Pascal compiler proper.
C style comments, /*......*/, are removed by the C preprocessor,
thus C comments inside Pascal programs are also removed when they
are fed through the preprocessor.
.in 0
.SS "Deviations from the standard"
.PP