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

characters, some possibly without graphic representations.
.IS
The 7-bit ASCII character set is used, where LF (10) denotes the
end-of-line marker on text-files.
.IT 6.4.2.2
The ordinal numbers of the character values shall be values of integer-type,
that are implementation-defined, and that are determined by mapping
the character values on to consecutive non-negative integer values
starting at zero.
.IS
The normal ASCII ordering is used: ord('0')=48, ord('A')=65, ord('a')=97, etc.
.IT 6.6.5.2
The post-assertions imply corresponding activities on the external entities,
if any, to which the file-variables are bound.
These activities, and the
point at which they are actually performed, shall be
implementation-defined.
.IS
The reading and writing writing of objects on files is buffered.
This means that when a program terminates abnormally, I/O may be
unfinished.
Terminal I/O is unbuffered.
Files are closed whenever they are rewritten or reset, or on
program termination.
.IT 6.7.2.2
The predefined constant \fImaxint\fR shall be of integer-type and shall denote
an implementation-defined value, that satisfies the following conditions:
.IP (a)
All integral values in the closed interval from \fI\(mimaxint\fR to \fI+maxint\fR
shall be values of the integer-type.
.IP (b)
Any monadic operation performed on an integer value in this interval
shall be correctly performed according to the mathematical rules for
integer arithmetic.
.IP (c)
Any dyadic integer operation on two integer values in this same interval
shall be correctly performed according to the mathematical rules for
integer arithmetic, provided that the result is also in this interval.
.IP (d)
Any relational operation on two integer values in this same interval
shall be correctly performed according to the mathematical rules for
integer arithmetic.
.SP
The representation of integers under 16-bit \*(Mp or under 68000 \*(Mx
is a 16-bit word using two's complement arithmetic.  The integers range
from \(mi32768 to +32767.  Under 32-bit \*(Mp a 32-bit integer is used
ranging from \(mi2147483648 to [phone].
.IT 6.7.2.2
The result of the real arithmetic operators and functions shall be
approximations to the corresponding mathematical results.
The accuracy of
this approximation shall be implementation-defined
.IS
The default size of reals is 8 bytes, the accuracy is 11 bits for the exponent,
and 53 bits for the mantissa.
This gives an accuracy of about 16 digits.
and exponents ranging from \(mi307 to +307.
.IT 6.9.3.1
The default TotalWidth values for integer, Boolean and real types
shall be implementation-defined.
.IS
The defaults are:
.XS
.ta +\w'Boolean   'u +\w'14  'u
integer	6	(16-bit)
integer	11	(32-bit)
Boolean	5
real	14
.DT
.XE
.IT 6.9.3.4.1
ExpDigits, the number of digits written in an exponent part of a real,
shall be implementation-defined.
.IS
ExpDigits is defined as 3.
.IT 6.9.3.4.1
The character written as part of the representation of
a real to indicate the beginning of the exponent part shall be
implementation-defined, either 'E' or 'e'.
.IS
The exponent part starts with 'e'.
.IT 6.9.3.5
The case of the characters written as representation of the
Boolean values shall be implementation-defined.
.IS
The representations of true and false are 'true' and 'false'.
.IT 6.9.5
The effect caused by the standard procedure page
on a text file shall be implementation-defined.
.IS
The ASCII character form feed FF (12) is written.
.IT 6.10
The binding of the variables denoted by the program-parameters
to entities external to the program shall be implementation-defined if
the variable is of a file-type.
.IS
The program parameters must be files and all, except input and output,
must be declared as such in the program block.
.PP
The program parameters input and output, if specified, will correspond
with the UNIX streams 'standard input' and 'standard output'.
.PP
The other program parameters will be mapped to the argument strings
provided by the caller of this program.
The argument strings are supposed to be path names of the files to be
opened or created.
The order of the program parameters determines the mapping:
the first parameter is mapped onto the first argument string, etc.
Note that input and output are ignored in this mapping.
.PP
The mapping is recalculated each time a program parameter
is opened for reading or writing by a call to the standard procedures
reset or rewrite.
This gives the programmer the opportunity to manipulate the list
of string arguments using the external procedures argc, argv and argshift
available in the Pascal library.
.IT 6.10
The effect of an explicit use of reset or rewrite
on the standard text files input or output shall be implementation-defined.
.IS
The procedures reset and rewrite are no-ops
if applied to input or output.
.in 0
.SS "Implementation-dependent features"
.PP
For each implementation-dependent feature mentioned in the BSI standard,
we give the section number, the quotation from that section and the way
this feature is treated by the 
.MX
Pascal system.
First we quote the definition of 'implementation-dependent':
.PP
.RS
Possibly differing between processors and not necessarily defined for any
particular processor.
.RE
.IT 6.7.2.1
The order of evaluation of the operands of a dyadic operator
shall be implementation-dependent.
.IS
Operands are always evaluated, so the program part
.XS
if (p<>nil) and (p^.value<>0) then
.XE
is probably incorrect.
.PP
The left-hand operand of a dyadic operator is almost always evaluated
before the right-hand side.
Some peculiar evaluations exist for the following cases:
.IP 1.
The modulo operation is performed by a library routine to
check for negative values of the right operand.
.IP 2.
The expression
.XS
set1 <= set2
.XE
where set1 and set2 are compatible set types is evaluated in the
following steps:
.XS
.ta +\w'\-  'u
\-	evaluate set2;
\-	evaluate set1;
\-	compute set2+set1;
\-	test set2 and set2+set1 for equality.
.DT
.XE
.IP 3.
The expression
.XS
set1 >= set2
.XE
where set1 and set2 are compatible set types is evaluated in the following steps:
.XS
.ta +\w'\-  'u
\-	evaluate set1;
\-	evaluate set2;
\-	compute set1+set2;
\-	test set1 and set1+set2 for equality.
.DT
.XE
.IT 6.7.3
The order of evaluation, accessing and binding
of the actual-parameters for functions
shall be implementation-dependent.
.IS
The order of evaluation is from right to left.
.IT 6.8.2.2
The decision as to the order of accessing the variable and evaluating
the expression in an assignment-statement, shall be
implementation-dependent.
.IS
The expression is evaluated first.
.IT 6.8.2.3
The order of evaluation and binding of the actual-parameters for procedures
shall be implementation-dependent.
.IS
The same as for functions.
.IT 6.9.5
The effect of inspecting a text file to which the page
procedure was applied during generation is
implementation-dependent.
.IS
The formfeed character written by page is
treated like a normal character, with ordinal value 12.
.IT 6.10
The binding of the variables denoted by the program-parameters
to entities external to the program shall be implementation-dependent unless
the variable is of a file-type.
.IS
Only variables of a file-type are allowed as program parameters.
.in 0
.SS "Error handling"
.PP
There are three classes of errors to be distinguished.
In the first class are the error messages generated by the compiler.
The second class consists of the occasional errors generated by the other
programs involved in the compilation process.
Errors of the third class are the errors as defined in the standard by:
.PP
.RS
An error is a violation by a program of the requirements of this standard
that a processor is permitted to leave undetected.
.RE
.LP
.ft I
Compiler errors
.PP
Error are written on the standard error output.
Each line has the form:
.XS
<file>, line <number>: <description>
.XE
Every time the compiler detects an error that does not have influence
on the code produced by the compiler or on the syntax decisions, a warning
messages is given.
If only warnings are generated, compilation proceeds and probably results
in a correctly compiled program.
.PP
Sometimes the compiler produces several errors for the same line.
They are only shown up to a maximum of 5 errors per line.
Warning are also shown up to a maximum of 5 per line.
.PP
Extensive treatment of these errors is outside the scope of this manual.
.LP
.ft I
Runtime errors
.PP
Errors detected at run time cause an error message to be generated on the
diagnostic output stream (UNIX file descriptor 2).
The message consists of the name of the program followed by a message
describing the error, possibly followed by the source line number.
Unless the \fI\(enn\fR option is turned on, the compiler generates code to keep track
of which source line causes which instructions to be generated.
.PP
For each error mentioned in the standard we give the section number,
the quotation from that section and the way it is processed by the
Pascal-compiler or runtime system.
.PP
For detected errors the corresponding message
and trap number are given.
Trap numbers are useful for exception-handling routines.
Normally, each error causes the program to terminate.
By using exception-handling routines one can
ignore errors or perform alternate actions.
Only some of the errors can be ignored
by restarting the failing instruction.
These errors are marked as non-fatal,
all others as fatal.
A list of errors with trap number between 0 and 63
(EM errors) can be found in [2].
Errors with trap number between 64 and 127 (Pascal errors) are listed below.
.IT 6.4.6
It shall be an error if a value of type T2 must be
assignment-compatible with type T1, while
T1 and T2 are compatible ordinal-types and the value of
type T2 is not in the closed interval specified by T1.
.IS
The compiler distinguishes between array-index expressions and the other
places where assignment-compatibility is required.
.PP
Array subscripting errors are only detected when the 'A' option is used.
In the other cases, a range bound error occurs when the value of type T2
is not in the closed interval specified by T1, unless range checks are
disabled.
.IT 6.4.6
It shall be an error if a value of type T2 must be
assignment-compatible with type T1, while T1 and T2 are compatible
set-types and any member of the value of type T2
is not in the closed interval specified by the base-type
of the type T1.
.IS
This error is not detected.
.IT 6.5.3.3
It shall be an error if a component of a variant-part of a variant,
where the selector of the variant-part is not a field,
is accessed unless the variant is active for the entirety of each
reference and access to each component of the variant.
.IS
This error is not detected.
.IT 6.5.4
It shall be an error if
the pointer-variable of an identified-variable either denotes a
nil-value or is undefined.
.IS
This error is not detected.
.IT 6.5.4
It shall be an error to remove the identifying-value of an identified
variable from its pointer-type when a reference to the variable exists.
.IS
When the identified variable is an element of the record-variable-list of
a with-statement, a warning is given at compile-time.
Otherwise, this error is not detected.
.IT 6.5.5
It shall be an error to alter the value of a file-variable f when a
reference to the buffer-variable f^ exists.
.IS
When f is altered when it is an element of the record-variable-list of a
with-statement, a warning is given.
When a buffer-variable is used as a
variable-parameter, an error is given.
This is done at compile-time.
.IT 6.6.5.2
It shall be an error if
the stated pre-assertion does not hold immediately
prior to any use of the file handling procedures
rewrite, put, reset and get.
.IS
For each of these four operations the pre-assertions
can be reformulated as:
.XS
.ta +\w'rewrite(f):  'u
rewrite(f):	no pre-assertion.
put(f):	f is opened for writing and f^ is not undefined.
reset(f):	f exists.
get(f):	f is opened for reading and eof(f) is false.
.DT
.XE
The following errors are detected for these operations:
.SP
rewrite(f):
.in +6
.ti -3
more args expected, trap 64, fatal:
.br
f is a program-parameter and the corresponding
file name is not supplied by the caller of the program.
.ti -3
rewrite error, trap 101, fatal:
.br
the caller of the program lacks the necessary
access rights to create the file in the file system
or operating system problems like table overflow
prevent creation of the file.
.in -6
.SP
put(f):
.in +6
.ti -3
file not yet open, trap 72, fatal:
.br
reset or rewrite are never applied to the file.
The checks performed by the run time system are not foolproof.
.ti -3
not writable, trap 96, fatal:
.br
f is opened for reading.
.ti -3
write error, trap 104, fatal:
.br
probably caused by file system problems.
For instance, the file storage is exhausted.
Because I/O is buffered to improve performance,
it might happen that this error occurs if the
file is closed.
Files are closed whenever they are rewritten or reset, or on
program termination.
.in -6
.SP
reset(f):
.in +6
.ti -3
more args expected, trap 64, fatal:
.br
same as for rewrite(f).
.ti -3
reset error, trap 100, fatal:
.br
f does not exist, or the caller has insufficient access rights, or
operating system tables are exhausted.
.in -6
.SP
get(f):
.in +6
.ti -3
file not yet open, trap 72, fatal:
.br
as for put(f).
.ti -3
not readable, trap 97, fatal:
.br
f is opened for writing.
.ti -3
end of file, trap 98, fatal:
.br
eof(f) is true just before the call to get(f).
.ti -3
read error, trap 103, fatal:
.br
unlikely to happen.
Probably caused by hardware problems
or by errors elsewhere in your program that destroyed
the file information maintained by the run time system.
.ti -3
truncated, trap 99, fatal:
.br
the file is not properly formed by an integer
number of file elements.
For instance, the size of a file of integer is odd.