freebsd-dev/share/doc/papers/px/pxin3.n

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.\"	@(#)pxin3.n	5.2 (Berkeley) 4/17/91
.\" $FreeBSD$
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.ta 8n 16n 24n
.nr H1 2
.if n .ND
.NH
Input/output
.NH 2
The files structure
.PP
Each file in the Pascal environment is represented by a pointer
to a
.I files
structure in the heap.
At the location addressed by the pointer is the element
in the file's window variable.
Behind this window variable is information about the file,
at the following offsets:
.so table3.1.n
.PP
Here
.SM FBUF
is a pointer to the system FILE block for the file.
The standard system I/O library is
used that provides block buffered input/output,
with 1024 characters normally transferred at each read or write.
.PP
The files in the
Pascal environment,
are all linked together on a single file chain through the
.SM FCHAIN
links.
For each file the
.SM FLEV
pointer gives its associated file variable.
These are used to free files at block exit as described in section 3.3
below.
.PP
The
FNAME
and
PFNAME
give the associated
file name for the file and the name to be used when printing
error diagnostics respectively.
Although these names are usually the same,
.I input
and
.I output
usually have no associated
file name so the distinction is necessary.
.PP
The
FUNIT
word contains 
a set of flags.
whose representations are:
.TS
center;
l l l.
EOF	0x0100	At end-of-file
EOLN	0x0200	At end-of-line (text files only)
SYNC	0x0400	File window is out of sync
TEMP	0x0800	File is temporary
FREAD	0x1000	File is open for reading
FWRITE	0x2000	File is open for writing
FTEXT	0x4000	File is a text file; process EOLN
FDEF	0x8000	File structure created, but file not opened
.TE
.PP
The
EOF
and
EOLN
bits here reflect the associated built-in function values.
TEMP
specifies that the file has a generated temporary name and that
it should therefore be removed when its block exits.
FREAD
and
FWRITE
specify that
.I reset
and
.I rewrite
respectively have been done on the file so that
input or output operations can be done.
FTEXT
specifies the file is a text file so that
EOLN
processing should be done,
with newline characters turned into blanks, etc.
.PP
The
SYNC
bit,
when true,
specifies that there is no usable image in the file buffer window.
As discussed in the
.I "Berkeley Pascal User's Manual,"
the interactive environment necessitates having
``input^'' undefined at the beginning
of execution so that a program may print a prompt
before the user is required to type input.
The
SYNC
bit implements this.
When it is set,
it specifies that the element in the window
must be updated before it can be used.
This is never done until necessary.
.NH 2
Initialization of files
.PP
All the variables in the Pascal runtime environment are cleared to zero on
block entry.
This is necessary for simple processing of files.
If a file is unused, its pointer will be
.B nil.
All references to an inactive file are thus references through a
.B nil
pointer.
If the Pascal system did not clear storage to zero before execution
it would not be possible to detect inactive files in this simple way;
it would probably be necessary to generate (possibly complicated)
code to initialize
each file on block entry.
.PP
When a file is first mentioned in a
.I reset
or
.I rewrite
call,
a buffer of the form described above is associated with it,
and the necessary information about the file is placed in this
buffer.
The file is also linked into the active file chain.
This chain is kept sorted by block mark address, the
FLEV
entries.
.NH 2
Block exit
.PP
When block exit occurs the interpreter must free the files that are in
use in the block
and their associated buffers.
This is simple and efficient because the files in the active file chain are 
sorted by increasing block mark address.
This means that the files for the current block will be at the front
of the chain.
For each file that is no longer accessible
the interpreter first flushes the files buffer
if it is an output file.
The interpreter then returns the file buffer and the files structure and window
to the free space in the heap and removes the file from the active file chain.
.NH 2
Flushing
.PP
Flushing all the file buffers at abnormal termination,
or on a call to the procedure
.I flush
or
.I message
is done by flushing 
each file on the file chain that has the
FWRITE
bit set in its flags word.
.NH 2
The active file
.PP
For input-output,
.I px
maintains a notion of an active file.
Each operation that references a file makes the file
it will be using the active file and then does its operation.
A subtle point here is that one may do a procedure call to
.I write
that involves a call to a function that references another file,
thereby destroying the active file set up before the
.I write.
Thus the active file is saved at block entry
in the block mark and restored at block exit.\*(Dg
.FS
\*(dg\ It would probably be better to dispense with the notion of
active file and use another mechanism that did not involve extra
overhead on each procedure and function call.
.FE
.NH 2
File operations
.PP
Files in Pascal can be used in two distinct ways:
as the object of
.I read,
.I write,
.I get,
and
.I put
calls, or indirectly as though they were pointers.
The second use as pointers must be careful
not to destroy the active file in a reference such as
.LS
write(output, input\(ua)
.LE
or the system would incorrectly write on the input device.
.PP
The fundamental operator related to the use of a file is
.SM FNIL.
This takes the file variable, as a pointer,
insures that the pointer is not
.B nil,
and also that a usable image is in the file window,
by forcing the
.SM SYNC
bit to be cleared.
.PP
A simple example that demonstrates the use of the file operators
is given by
.LS
writeln(f)
.LE
that produces
.DS
.mD
.TS
lp-2w(8) l.
RV:\fIl	f\fP
UNIT
WRITLN
.TE
.DE
.NH 2
Read operations
.SH
GET
.IP
Advance the active file to the next input element.
.SH
FNIL
.IP
A file pointer is on the stack. Insure that the associated file is active
and that the file is synced so that there is input available in the window.
.SH
READ*
.IP
If the file is a text file, read a block of text
and convert it to the internal type of the specified
operand. If the file is not a text file then 
do an unformatted read of the next record.
The procedure
.SM READLN
reads upto and including the next end of line character.
.SH
READE A
.IP
The operator
.SM READE
reads a string name of an enumerated type and converts it
to its internal value.
.SM READE 
takes a pointer to a data structure as shown in figure 3.2.
.so fig3.2.n
See the description of
.SM NAM
in the next section for an example.
.NH 2
Write operations
.SH
PUT
.IP
Output the element in the active file window.
.SH
WRITEF s
.IP
The argument(s) on the stack are output
by the
.I fprintf
standard
.SM I/O
library routine.
The sub-opcode 
.I s
specifies the number
of longword arguments on the stack.
.SH
WRITEC
.IP
The character on the top of the stack is output
without formatting. Formatted characters must be output with 
.SM WRITEF .
.SH
WRITES
.IP
The string specified by the pointer on the top of the stack is output
by the
.I fwrite
standard
.SM I/O
library routine.
All characters including nulls are printed.
.SH
WRITLN
.IP
A linefeed is output to the active file.
The line-count for the file is
incremented and checked against the line limit.
.SH
PAGE
.IP
A formfeed is output to the active file.
.SH
NAM A
.IP
The value on the top of the stack is converted to a pointer
to an enumerated type string name.
The address 
.SM A
points to an enumerated type structure identical
to that used by
.SM READE .
An error is raised if the value is out of range.
The form of this structure for the predefined type
.B boolean
is shown in figure 3.3.
.so fig3.3.n
The code for 
.SM NAM 
is
.DS
.mD
_NAM:
	\fBincl\fR	lc
	\fBaddl3\fR	(lc)+,ap,r6	#r6 points to scalar name list
	\fBmovl\fR	(sp)+,r3	#r3 has data value
	\fBcmpw\fR	r3,(r6)+	#check value out of bounds
	\fBbgequ\fR	enamrng
	\fBmovzwl\fR	(r6)[r3],r4	#r4 has string index
	\fBpushab\fR	(r6)[r4]	#push string pointer
	\fBjmp\fR	(loop)
enamrng:
	\fBmovw\fR	$ENAMRNG,_perrno
	\fBjbr\fR	error
.DE
The address of the table is calculated by adding the base address
of the interpreter code,
.I ap
to the offset pointed to by
.I lc .
The first word of the table gives the number of records and
provides a range check of the data to be output.
The pointer is then calculated as
.DS
.mD
tblbase = ap + A;
size = *tblbase++;
return(tblbase + tblbase[value]);
.DE
.SH
MAX s,w
.IP
The sub-opcode
.I s
is subtracted from the integer on the top of the stack.
The maximum of the result and the second argument,
.I w ,
replaces the value on the top of the stack.
This function verifies that variable specified
width arguments are non-negative, and meet certain minimum width
requirements.
.SH
MIN s
.IP
The minimum of the value on the top of the stack
and the sub-opcode replaces the value on the top
of the stack.
.sp 1
.LP
The uses of files and the file operations are summarized
in an example which outputs a real variable (r) with a variable
width field (i).
.LS
writeln('r =',r:i,' ',true);
.LE
that generates the code
.DS
.mD
.TS
lp-2w(8) l.
UNITOUT
FILE
CON14:1
CON14:3
LVCON:4	"r ="
WRITES
RV8\fI:l	r\fP
RV4\fI:l	i\fP
MAX:8	1 
RV4\fI:l	i\fP
MAX:1	1 
LVCON:8	" %*.*E"
FILE
WRITEF:6
CONC4	\' \'
WRITEC
CON14:1
NAM	\fIbool\fP 
LVCON:4	"%s"
FILE
WRITEF:3
WRITLN
.TE
.DE
.PP
Here the operator
.SM UNITOUT
is an abbreviated form of the operator
.SM UNIT
that is used when the file to be made active is
.I output .
A file descriptor, record count, string size, and a pointer
to the constant string ``r ='' are pushed 
and then output by
.SM WRITES .
Next the value of 
.I r
is pushed on the stack
and the precision size is calculated by taking
seven less than the width, but not less than one.
This is followed by the width that is reduced by
one to leave space for the required leading blank.
If the width is too narrow, it
is expanded by
.I fprintf .
A pointer to the format string is pushed followed
by a file descriptor and the operator
.SM WRITEF
that prints out
.I r .
The value of six on 
.SM WRITEF
comes from two longs for
.I r
and a long each for the precision, width, format string pointer,
and file descriptor.
The operator
.SM CONC4
pushes the
.I blank
character onto a long on the stack that is then printed out by
.SM WRITEC .
The internal representation for
.I true
is pushed as a long onto the stack and is
then replaced by a pointer to the string ``true''
by the operator
.SM NAM
using the table
.I bool
for conversion.
This string is output by the operator
.SM WRITEF
using the format string ``%s''.
Finally the operator
.SM WRITLN
appends a newline to the file.
.NH 2
File activation and status operations
.SH
UNIT*
.IP
The file pointed to by the file pointer on the top
of the stack is converted to be the active file.
The opcodes
.SM UNITINP
and
.SM UNITOUT
imply standard input and output respectively
instead of explicitly pushing their file pointers.
.SH
FILE
.IP
The standard 
.SM I/O
library file descriptor associated with the active file 
is pushed onto the stack.
.SH
EOF
.IP
The file pointed to by the file pointer on the top
of the stack is checked for end of file. A boolean
is returned with 
.I true
indicating the end of file condition.
.SH
EOLN
.IP
The file pointed to by the file pointer on the top
of the stack is checked for end of line. A boolean
is returned with
.I true
indicating the end of line condition.
Note that only text files can check for end of line.
.NH 2
File housekeeping operations
.SH
DEFNAME
.IP
Four data items are passed on the stack;
the size of the data type associated with the file,
the maximum size of the file name,
a pointer to the file name,
and a pointer to the file variable.
A file record is created with the specified window size
and the file variable set to point to it.
The file is marked as defined but not opened.
This allows
.B program
statement association of file names with file variables
before their use by a 
.SM RESET
or a
.SM REWRITE .
.SH
BUFF s
.IP
The sub-opcode is placed in the external variable
.I _bufopt
to specify the amount of I/O buffering that is desired.
The current options are:
.DS
0 \- character at a time buffering
1 \- line at a time buffering
2 \- block buffering
.DE
The default value is 1.
.SH
RESET
.br
REWRITE
.IP
Four data items are passed on the stack;
the size of the data type associated with the file,
the maximum size of the name (possibly zero),
a pointer to the file name (possibly null),
and a pointer to the file variable.
If the file has never existed it is created as in 
.SM DEFNAME .
If no file name is specified and no previous name exists
(for example one created by
.SM DEFNAME
) then a system temporary name is created.
.SM RESET
then opens the file for input, while
.SM REWRITE
opens the file for output.
.sp 1
.PP
The three remaining file operations are
.SM FLUSH 
that flushes the active file,
.SM REMOVE
that takes the pointer to a file name and removes the
specified file, and
.SM MESSAGE
that flushes all the output files and sets the 
standard error file to be the active file.