freebsd-dev/share/doc/psd/04.uprog/p3

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.\" $FreeBSD$
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.\"	@(#)p3	8.1 (Berkeley) 6/8/93
.\"
.NH
THE STANDARD I/O LIBRARY
.PP
The ``Standard I/O Library''
is a collection of routines
intended to provide
efficient
and portable
I/O services
for most C programs.
The standard I/O library is available on each system that supports C,
so programs that confine
their system interactions
to its facilities
can be transported from one system to another essentially without change.
.PP
In this section, we will discuss the basics of the standard I/O library.
The appendix contains a more complete description of its capabilities.
.NH 2
File Access
.PP
The programs written so far have all
read the standard input and written the standard output,
which we have assumed are magically pre-defined.
The next step
is to write a program that accesses
a file that is
.ul
not
already connected to the program.
One simple example is
.IT wc ,
which counts the lines, words and characters
in a set of files.
For instance, the command
.P1
wc x.c y.c
.P2
prints the number of lines, words and characters
in
.UL x.c
and
.UL y.c
and the totals.
.PP
The question is how to arrange for the named files
to be read \(em
that is, how to connect the file system names 
to the I/O statements which actually read the data.
.PP
The rules are simple.
Before it can be read or written
a file has to be
.ul
opened
by the standard library function
.UL fopen .
.UL fopen
takes an external name
(like
.UL x.c
or
.UL y.c ),
does some housekeeping and negotiation with the operating system,
and returns an internal name
which must be used in subsequent
reads or writes of the file.
.PP
This internal name is actually a pointer,
called a
.IT file
.IT pointer ,
to a structure
which contains information about the file,
such as the location of a buffer,
the current character position in the buffer,
whether the file is being read or written,
and the like.
Users don't need to know the details,
because part of the standard I/O definitions
obtained by including
.UL stdio.h
is a structure definition called
.UL FILE .
The only declaration needed for a file pointer
is exemplified by
.P1
FILE	*fp, *fopen();
.P2
This says that
.UL fp
is a pointer to a
.UL FILE ,
and
.UL fopen
returns a pointer to
a
.UL FILE .
.UL FILE \& (
is a type name, like
.UL int ,
not a structure tag.
.PP
The actual call to
.UL fopen
in a program
is
.P1
fp = fopen(name, mode);
.P2
The first argument of
.UL fopen
is the
name
of the file,
as a character string.
The second argument is the
mode,
also as a character string,
which indicates how you intend to
use the file.
The only allowable modes are
read
.UL \&"r" ), (
write
.UL \&"w" ), (
or append
.UL \&"a" ). (
.PP
If a file that you open for writing or appending does not exist,
it is created
(if possible).
Opening an existing file for writing causes the old contents
to be discarded.
Trying to read a file that does not exist
is an error,
and there may be other causes of error
as well
(like trying to read a file
when you don't have permission).
If there is any error,
.UL fopen
will return the null pointer
value
.UL NULL 
(which is defined as zero in
.UL stdio.h ).
.PP
The next thing needed is a way to read or write the file
once it is open.
There are several possibilities,
of which
.UL getc
and
.UL putc
are the simplest.
.UL getc
returns the next character from a file;
it needs the file pointer to tell it what file.
Thus
.P1
c = getc(fp)
.P2
places in 
.UL c
the next character from the file referred to by
.UL fp ;
it returns
.UL EOF
when it reaches end of file.
.UL putc
is the inverse of
.UL getc :
.P1
putc(c, fp)
.P2
puts the character
.UL c
on the file
.UL fp 
and returns
.UL c .
.UL getc
and
.UL putc
return
.UL EOF
on error.
.PP
When a program is started, three files are opened automatically,
and file pointers are provided for them.
These files are the standard input,
the standard output,
and the standard error output;
the corresponding file pointers are
called
.UL stdin ,
.UL stdout ,
and
.UL stderr .
Normally these are all connected to the terminal,
but
may be redirected to files or pipes as described in
Section 2.2.
.UL stdin ,
.UL stdout
and
.UL stderr
are pre-defined in the I/O library
as the standard input, output and error files;
they may be used anywhere an object of type
.UL FILE\ *
can be.
They are 
constants, however,
.ul
not
variables,
so don't try to assign to them.
.PP
With some of the preliminaries out of the way,
we can now write
.IT wc .
The basic design 
is one that has been found
convenient for many programs:
if there are command-line arguments, they are processed in order.
If there are no arguments, the standard input
is processed.
This way the program can be used stand-alone
or as part of a larger process.
.P1
#include <stdio.h>

main(argc, argv)	/* wc: count lines, words, chars */
int argc;
char *argv[];
{
	int c, i, inword;
	FILE *fp, *fopen();
	long linect, wordct, charct;
	long tlinect = 0, twordct = 0, tcharct = 0;

	i = 1;
	fp = stdin;
	do {
		if (argc > 1 && (fp=fopen(argv[i], "r")) == NULL) {
			fprintf(stderr, "wc: can't open %s\en", argv[i]);
			continue;
		}
		linect = wordct = charct = inword = 0;
		while ((c = getc(fp)) != EOF) {
			charct++;
			if (c == '\en')
				linect++;
			if (c == ' ' || c == '\et' || c == '\en')
				inword = 0;
			else if (inword == 0) {
				inword = 1;
				wordct++;
			}
		}
		printf("%7ld %7ld %7ld", linect, wordct, charct);
		printf(argc > 1 ? " %s\en" : "\en", argv[i]);
		fclose(fp);
		tlinect += linect;
		twordct += wordct;
		tcharct += charct;
	} while (++i < argc);
	if (argc > 2)
		printf("%7ld %7ld %7ld total\en", tlinect, twordct, tcharct);
	exit(0);
}
.P2
The function
.UL fprintf
is identical to
.UL printf ,
save that the first argument is a file pointer
that specifies the file to be
written.
.PP
The function
.UL fclose
is the inverse of
.UL fopen ;
it breaks the connection between the file pointer and the external name
that was established by
.UL fopen ,
freeing the
file pointer for another file.
Since there is a limit on the number
of files
that a program may have open simultaneously,
it's a good idea to free things when they are no longer needed.
There is also another reason to call
.UL fclose 
on an output file
\(em it flushes the buffer
in which
.UL putc
is collecting output.
.UL fclose \& (
is called automatically for each open file
when a program terminates normally.)
.NH 2
Error Handling \(em Stderr and Exit
.PP
.UL stderr
is assigned to a program in the same way that
.UL stdin
and
.UL stdout
are.
Output written on 
.UL stderr
appears on the user's terminal
even if the standard output is redirected.
.IT wc
writes its diagnostics on
.UL stderr
instead of
.UL stdout
so that if one of the files can't
be accessed for some reason,
the message
finds its way to the user's terminal instead of disappearing
down a pipeline
or into an output file.
.PP
The program actually signals errors in another way,
using the function
.UL exit 
to terminate program execution.
The argument of
.UL exit
is available to whatever process
called it (see Section 6),
so the success or failure
of the program can be tested by another program
that uses this one as a sub-process.
By convention, a return value of 0
signals that all is well;
non-zero values signal abnormal situations.
.PP
.UL exit
itself
calls
.UL fclose
for each open output file,
to flush out any buffered output,
then calls
a routine named
.UL _exit .
The function
.UL _exit
causes immediate termination without any buffer flushing;
it may be called directly if desired.
.NH 2
Miscellaneous I/O Functions
.PP
The standard I/O library provides several other I/O functions
besides those we have illustrated above.
.PP
Normally output with
.UL putc ,
etc., is buffered (except to
.UL stderr );
to force it out immediately, use
.UL fflush(fp) .
.PP
.UL fscanf
is identical to
.UL scanf ,
except that its first argument is a file pointer
(as with
.UL fprintf )
that specifies the file from which the input comes;
it returns
.UL EOF
at end of file.
.PP
The functions
.UL sscanf
and
.UL sprintf
are identical to
.UL fscanf
and
.UL fprintf ,
except that the first argument names a character string
instead of a file pointer.
The conversion is done from the string
for 
.UL sscanf 
and into it for
.UL sprintf .
.PP
.UL fgets(buf,\ size,\ fp)
copies the next line from
.UL fp ,
up to and including a newline,
into 
.UL buf ;
at most
.UL size-1
characters are copied;
it returns
.UL NULL
at end of file.
.UL fputs(buf,\ fp)
writes the string in
.UL buf
onto file
.UL fp .
.PP
The function
.UL ungetc(c,\ fp)
``pushes back'' the character
.UL c
onto the input stream
.UL fp ;
a subsequent call to
.UL getc ,
.UL fscanf ,
etc.,
will encounter 
.UL c .
Only one character of pushback per file is permitted.