freebsd-dev/share/doc/psd/04.uprog/p4

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.\"	@(#)p4	8.1 (Berkeley) 6/8/93
.\"
.NH
LOW-LEVEL I/O
.PP
This section describes the 
bottom level of I/O on the
.UC UNIX
system.
The lowest level of I/O in
.UC UNIX
provides no buffering or any other services;
it is in fact a direct entry into the operating system.
You are entirely on your own,
but on the other hand,
you have the most control over what happens.
And since the calls and usage are quite simple,
this isn't as bad as it sounds.
.NH 2
File Descriptors
.PP
In the
.UC UNIX
operating system,
all input and output is done
by reading or writing files,
because all peripheral devices, even the user's terminal,
are files in the file system.
This means that a single, homogeneous interface
handles all communication between a program and peripheral devices.
.PP
In the most general case,
before reading or writing a file,
it is necessary to inform the system
of your intent to do so,
a process called
``opening'' the file.
If you are going to write on a file,
it may also be necessary to create it.
The system checks your right to do so
(Does the file exist?
Do you have permission to access it?),
and if all is well,
returns a small positive integer
called a
.ul
file descriptor.
Whenever I/O is to be done on the file,
the file descriptor is used instead of the name to identify the file.
(This is roughly analogous to the use of
.UC READ(5,...)
and
.UC WRITE(6,...)
in Fortran.)
All
information about an open file is maintained by the system;
the user program refers to the file
only
by the file descriptor.
.PP
The file pointers discussed in section 3
are similar in spirit to file descriptors,
but file descriptors are more fundamental.
A file pointer is a pointer to a structure that contains,
among other things, the file descriptor for the file in question.
.PP
Since input and output involving the user's terminal
are so common,
special arrangements exist to make this convenient.
When the command interpreter (the
``shell'')
runs a program,
it opens
three files, with file descriptors 0, 1, and 2,
called the standard input,
the standard output, and the standard error output.
All of these are normally connected to the terminal,
so if a program reads file descriptor 0
and writes file descriptors 1 and 2,
it can do terminal I/O
without worrying about opening the files.
.PP
If I/O is redirected 
to and from files with
.UL < 
and
.UL > ,
as in
.P1
prog <infile >outfile
.P2
the shell changes the default assignments for file descriptors
0 and 1
from the terminal to the named files.
Similar observations hold if the input or output is associated with a pipe.
Normally file descriptor 2 remains attached to the terminal,
so error messages can go there.
In all cases,
the file assignments are changed by the shell,
not by the program.
The program does not need to know where its input
comes from nor where its output goes,
so long as it uses file 0 for input and 1 and 2 for output.
.NH 2
Read and Write
.PP
All input and output is done by
two functions called
.UL read
and
.UL write .
For both, the first argument is a file descriptor.
The second argument is a buffer in your program where the data is to
come from or go to.
The third argument is the number of bytes to be transferred.
The calls are
.P1
n_read = read(fd, buf, n);

n_written = write(fd, buf, n);
.P2
Each call returns a byte count
which is the number of bytes actually transferred.
On reading,
the number of bytes returned may be less than
the number asked for,
because fewer than
.UL n
bytes remained to be read.
(When the file is a terminal,
.UL read
normally reads only up to the next newline,
which is generally less than what was requested.)
A return value of zero bytes implies end of file,
and
.UL -1
indicates an error of some sort.
For writing, the returned value is the number of bytes
actually written;
it is generally an error if this isn't equal
to the number supposed to be written.
.PP
The number of bytes to be read or written is quite arbitrary.
The two most common values are 
1,
which means one character at a time
(``unbuffered''),
and
512,
which corresponds to a physical blocksize on many peripheral devices.
This latter size will be most efficient,
but even character at a time I/O
is not inordinately expensive.
.PP
Putting these facts together,
we can write a simple program to copy
its input to its output.
This program will copy anything to anything,
since the input and output can be redirected to any file or device.
.P1
#define	BUFSIZE	512	/* best size for PDP-11 UNIX */

main()	/* copy input to output */
{
	char	buf[BUFSIZE];
	int	n;

	while ((n = read(0, buf, BUFSIZE)) > 0)
		write(1, buf, n);
	exit(0);
}
.P2
If the file size is not a multiple of
.UL BUFSIZE ,
some 
.UL read
will return a smaller number of bytes
to be written by
.UL write ;
the next call to 
.UL read
after that
will return zero.
.PP
It is instructive to see how
.UL read
and
.UL write
can be used to construct
higher level routines like
.UL getchar ,
.UL putchar ,
etc.
For example,
here is a version of
.UL getchar
which does unbuffered input.
.P1
#define	CMASK	0377	/* for making char's > 0 */

getchar()	/* unbuffered single character input */
{
	char c;

	return((read(0, &c, 1) > 0) ? c & CMASK : EOF);
}
.P2
.UL c
.ul
must
be declared
.UL char ,
because
.UL read
accepts a character pointer.
The character being returned must be masked with
.UL 0377
to ensure that it is positive;
otherwise sign extension may make it negative.
(The constant
.UL 0377
is appropriate for the
.UC PDP -11
but not necessarily for other machines.)
.PP
The second version of
.UL getchar
does input in big chunks,
and hands out the characters one at a time.
.P1
#define	CMASK	0377	/* for making char's > 0 */
#define	BUFSIZE	512

getchar()	/* buffered version */
{
	static char	buf[BUFSIZE];
	static char	*bufp = buf;
	static int	n = 0;

	if (n == 0) {	/* buffer is empty */
		n = read(0, buf, BUFSIZE);
		bufp = buf;
	}
	return((--n >= 0) ? *bufp++ & CMASK : EOF);
}
.P2
.NH 2
Open, Creat, Close, Unlink
.PP
Other than the default
standard input, output and error files,
you must explicitly open files in order to
read or write them.
There are two system entry points for this,
.UL open
and
.UL creat 
[sic].
.PP
.UL open
is rather like the
.UL  fopen
discussed in the previous section,
except that instead of returning a file pointer,
it returns a file descriptor,
which is just an
.UL int .
.P1
int fd;

fd = open(name, rwmode);
.P2
As with
.UL fopen ,
the
.UL name
argument
is a character string corresponding to the external file name.
The access mode argument
is different, however:
.UL rwmode
is 0 for read, 1 for write, and 2 for read and write access.
.UL open
returns
.UL -1
if any error occurs;
otherwise it returns a valid file descriptor.
.PP
It is an error to 
try to
.UL open
a file that does not exist.
The entry point
.UL creat
is provided to create new files,
or to re-write old ones.
.P1
fd = creat(name, pmode);
.P2
returns a file descriptor
if it was able to create the file
called
.UL name ,
and
.UL -1
if not.
If the file
already exists,
.UL creat
will truncate it to zero length;
it is not an error to
.UL creat
a file that already exists.
.PP
If the file is brand new,
.UL creat
creates it with the
.ul
protection mode 
specified by
the
.UL pmode
argument.
In the
.UC UNIX
file system,
there are nine bits of protection information
associated with a file,
controlling read, write and execute permission for
the owner of the file,
for the owner's group,
and for all others.
Thus a three-digit octal number
is most convenient for specifying the permissions.
For example,
0755
specifies read, write and execute permission for the owner,
and read and execute permission for the group and everyone else.
.PP
To illustrate,
here is a simplified version of
the
.UC UNIX
utility
.IT cp ,
a program which copies one file to another.
(The main simplification is that our version
copies only one file,
and does not permit the second argument
to be a directory.)
.P1
#define NULL 0
#define BUFSIZE 512
#define PMODE 0644 /* RW for owner, R for group, others */

main(argc, argv)	/* cp: copy f1 to f2 */
int argc;
char *argv[];
{
	int	f1, f2, n;
	char	buf[BUFSIZE];

	if (argc != 3)
		error("Usage: cp from to", NULL);
	if ((f1 = open(argv[1], 0)) == -1)
		error("cp: can't open %s", argv[1]);
	if ((f2 = creat(argv[2], PMODE)) == -1)
		error("cp: can't create %s", argv[2]);

	while ((n = read(f1, buf, BUFSIZE)) > 0)
		if (write(f2, buf, n) != n)
			error("cp: write error", NULL);
	exit(0);
}
.P2
.P1
error(s1, s2)	/* print error message and die */
char *s1, *s2;
{
	printf(s1, s2);
	printf("\en");
	exit(1);
}
.P2
.PP
As we said earlier,
there is a limit (typically 15-25)
on the number of files which a program
may have open simultaneously.
Accordingly, any program which intends to process
many files must be prepared to re-use
file descriptors.
The routine
.UL close
breaks the connection between a file descriptor
and an open file,
and frees the
file descriptor for use with some other file.
Termination of a program
via
.UL exit
or return from the main program closes all open files.
.PP
The function
.UL unlink(filename)
removes the file
.UL filename
from the file system.
.NH 2
Random Access \(em Seek and Lseek
.PP
File I/O is normally sequential:
each
.UL read
or
.UL write
takes place at a position in the file
right after the previous one.
When necessary, however,
a file can be read or written in any arbitrary order.
The
system call
.UL lseek
provides a way to move around in
a file without actually reading
or writing:
.P1
lseek(fd, offset, origin);
.P2
forces the current position in the file
whose descriptor is
.UL fd
to move to position
.UL offset ,
which is taken relative to the location
specified by
.UL origin .
Subsequent reading or writing will begin at that position.
.UL offset
is
a
.UL long ;
.UL fd
and
.UL origin
are
.UL int 's.
.UL origin
can be 0, 1, or 2 to specify that 
.UL offset
is to be
measured from
the beginning, from the current position, or from the
end of the file respectively.
For example,
to append to a file,
seek to the end before writing:
.P1
lseek(fd, 0L, 2);
.P2
To get back to the beginning (``rewind''),
.P1
lseek(fd, 0L, 0);
.P2
Notice the
.UL 0L
argument;
it could also be written as
.UL (long)\ 0 .
.PP
With 
.UL lseek ,
it is possible to treat files more or less like large arrays,
at the price of slower access.
For example, the following simple function reads any number of bytes
from any arbitrary place in a file.
.P1
get(fd, pos, buf, n) /* read n bytes from position pos */
int fd, n;
long pos;
char *buf;
{
	lseek(fd, pos, 0);	/* get to pos */
	return(read(fd, buf, n));
}
.P2
.PP
In pre-version 7
.UC UNIX ,
the basic entry point to the I/O system
is called
.UL seek .
.UL seek
is identical to
.UL lseek ,
except that its
.UL  offset 
argument is an
.UL int
rather than  a
.UL long .
Accordingly,
since
.UC PDP -11
integers have only 16 bits,
the
.UL offset
specified
for
.UL seek
is limited to 65,535;
for this reason,
.UL origin
values of 3, 4, 5 cause
.UL seek
to multiply the given offset by 512
(the number of bytes in one physical block)
and then interpret
.UL origin
as if it were 0, 1, or 2 respectively.
Thus to get to an arbitrary place in a large file
requires two seeks, first one which selects
the block, then one which
has
.UL origin
equal to 1 and moves to the desired byte within the block.
.NH 2
Error Processing
.PP
The routines discussed in this section,
and in fact all the routines which are direct entries into the system
can incur errors.
Usually they indicate an error by returning a value of \-1.
Sometimes it is nice to know what sort of error occurred;
for this purpose all these routines, when appropriate,
leave an error number in the external cell
.UL errno .
The meanings of the various error numbers are
listed
in the introduction to Section II
of the
.I
.UC UNIX
Programmer's Manual,
.R
so your program can, for example, determine if
an attempt to open a file failed because it did not exist
or because the user lacked permission to read it.
Perhaps more commonly,
you may want to print out the
reason for failure.
The routine
.UL perror
will print a message associated with the value
of
.UL errno ;
more generally,
.UL sys\_errno
is an array of character strings which can be indexed
by
.UL errno
and printed by your program.