path: root/share/doc/psd/04.uprog/p2

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.\" $FreeBSD$
.\"
.\"	@(#)p2	8.1 (Berkeley) 6/8/93
.\"
.NH
BASICS
.NH 2
Program Arguments
.PP
When a C program is run as a command,
the arguments on the command line are made available
to the
function
.UL main
as an argument count
.UL argc
and an array
.UL argv
of
pointers to
character strings
that contain
the arguments.
By convention,
.UL argv[0]
is the command name itself,
so
.UL argc
is always greater than 0.
.PP
The following program illustrates the mechanism:
it simply echoes its arguments
back to the terminal.
(This is essentially the
.UL echo
command.)
.P1
main(argc, argv)	/* echo arguments */
int argc;
char *argv[];
{
	int i;

	for (i = 1; i < argc; i++)
		printf("%s%c", argv[i], (i<argc-1) ? ' ' : '\en');
}
.P2
.UL argv
is a pointer to an array
whose individual elements are pointers to arrays of characters;
each is terminated by
.UL \e0 ,
so they can be treated as strings.
The program starts by printing 
.UL argv[1]
and loops until it has printed them all.
.PP
The argument count and the arguments
are parameters to
.UL main .
If you want to keep them around so other
routines can get at them, you must
copy them to external variables.
.NH 2
The ``Standard Input'' and ``Standard Output''
.PP
The simplest input mechanism is to read the ``standard input,''
which is generally the user's terminal.
The function
.UL getchar
returns the next input character each time it is called.
A file may be substituted for the terminal by
using the
.UL <
convention: 
if
.UL prog
uses 
.UL getchar ,
then
the command line
.P1
prog <file
.P2
causes
.UL prog
to read
.UL file
instead of the terminal.
.UL prog
itself need know nothing about where its input
is coming from.
This is also true if the input comes from another program via
the 
.U 
pipe mechanism:
.P1
otherprog | prog
.P2
provides the standard input for
.UL prog
from the standard output of
.UL otherprog.
.PP
.UL getchar
returns the value
.UL EOF
when it encounters the end of file
(or an error)
on whatever you are reading.
The value of
.UL EOF
is normally defined to be
.UL -1 ,
but it is unwise to take any advantage
of that knowledge.
As will become clear shortly,
this value is automatically defined for you when
you compile a program,
and need not be of any concern.
.PP
Similarly,
.UL putchar(c)
puts the character
.UL c
on the ``standard output,''
which is also by default the terminal.
The output can be captured on a file
by using
.UL > :
if
.UL prog
uses
.UL putchar ,
.P1
prog >outfile
.P2
writes the standard output on
.UL outfile 
instead of the terminal.
.UL outfile
is created if it doesn't exist;
if it already exists, its previous contents are overwritten.
And a pipe can be used:
.P1
prog | otherprog
.P2
puts the standard output of
.UL prog
into the standard input of
.UL otherprog.
.PP
The function
.UL printf ,
which formats output in various ways,
uses
the same mechanism as
.UL putchar
does,
so calls to
.UL printf
and
.UL putchar
may be intermixed in any order;
the output will appear in the order of the calls.
.PP
Similarly, the function
.UL scanf
provides for formatted input conversion;
it will read the standard input and break it
up into strings, numbers, etc.,
as desired.
.UL scanf
uses the same mechanism as
.UL getchar ,
so calls to them may also be intermixed.
.PP
Many programs
read only one input and write one output;
for such programs I/O
with
.UL getchar ,
.UL putchar ,
.UL scanf ,
and
.UL printf
may be entirely adequate,
and it is almost always enough to get started.
This is particularly true if
the
.UC UNIX
pipe facility is used to connect the output of
one program to the input of the next.
For example, the following program
strips out all ascii control characters
from its input
(except for newline and tab).
.P1
#include <stdio.h>

main()	/* ccstrip: strip non-graphic characters */
{
	int c;
	while ((c = getchar()) != EOF)
		if ((c >= ' ' && c < 0177) || c == '\et' || c == '\en')
			putchar(c);
	exit(0);
}
.P2
The line
.P1
#include <stdio.h>
.P2
should appear at the beginning of each source file.
It causes the C compiler to read a file
.IT /usr/include/stdio.h ) (
of
standard routines and symbols
that includes the definition of
.UL EOF .
.PP
If it is necessary to treat multiple files,
you can use
.UL cat
to collect the files for you:
.P1
cat file1 file2 ... | ccstrip >output
.P2
and thus avoid learning how to access files from a program.
By the way,
the call to
.UL exit
at the end is not necessary to make the program work
properly,
but it assures that any caller
of the program will see a normal termination status
(conventionally 0)
from the program when it completes.
Section 6 discusses status returns in more detail.