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a 
carriage return
without a line feed, so that you can overwrite 
the current line.
The character
<TT>'\b'</TT>
is a backspace,
and will usually move the cursor one
position
to the left.
</p><p>Using these characters,
you can print a percentage-done indicator:
<pre>
	for(i = 0; i &lt; lotsa; i++) {
		printf("\r%3d%%", (int)(100L * i / lotsa));
		fflush(stdout);
		do_timeconsuming_work();
	}
	printf("\ndone.\n");
</pre>
or a baton:
<pre>
	printf("working: ");
	for(i = 0; i &lt; lotsa; i++) {
		printf("%c\b", "|/-\\"[i%4]);
		fflush(stdout);
		do_timeconsuming_work();
	}
	printf("done.\n");
</pre>
</p><p>See also question <a href="faqcat1d60.html?sec=stdio#fflush">12.4</a>.
</p>
<p>References:

ISO Sec. 5.2.2
<hr><hr><hr>
<a name="termcap">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/termcap.html"><!-- qtag -->Question 19.4</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I clear the screen?
<br>How can I
print
text in color?
<br>
How can I
move the cursor
to a specific x,&nbsp;y position?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
Such things depend on the
terminal type (or display)
you're using.
You will have to use a library such as
termcap, terminfo, or curses,
or some system-specific routines, to perform these operations.
</p><p>Functions
in the curses library
to look for are
<TT>clear</TT>,
<TT>move</TT>,
<TT>standout</TT>/<TT>standend</TT>,
and
<TT>attron</TT>/<TT>attroff</TT>/<TT>attrset</TT>;
the last three work with
attribute codes such as A_REVERSE.
In MS-DOS libraries,
there are typically functions named
<TT>gotoxy</TT>
and
<TT>clrscr</TT>
or
<TT>_clearscreen</TT>;
you can also use the ANSI.SYS driver or low-level interrupts.
Under termcap or terminfo,
use <TT>tgetstr</TT>
to retrieve strings like
<TT>cl</TT>,
<TT>so</TT>/<TT>se</TT>,
and <TT>cm</TT>
for clear screen, standout mode, and cursor motion respectively,
then output the strings;
using <TT>cm</TT> additionally requires calling <TT>tgoto</TT>.




Some baroque terminals require
attention to other ``capabilities'' as well;
study the
documentation carefully.
Be aware that some older terminals
may
not support the desired capabilities
at all.
</p><p>Most modern terminal emulation schemes
support the ANSI escape sequences
for cursor motion and visual attributes,
so if you're willing to sacrifice portability,
you can print those sequences directly.
Here is a tiny example to whet your appetite:
<pre>
printf("\033[2J");		/* clear screen */
printf("\033[%d;%dH", 10, 20);	/* move cursor (row 10, col 20) */
printf("Hello, ");
printf("\033[7mworld\033[0m!");	/* inverse video */
</pre>
Here is a link to some
<a href="../../osdep/sd21.html" rel=subdocument>more explanation, and brief lists of codes.</a>
The portable way of emitting these sequences
(if you're not going whole-hog and using curses)
is to use termcap or terminfo;
here is
<a href="../../osdep/sd22.html" rel=subdocument>an example.</a>
</p><p>For clearing the screen,
a halfway portable solution is to print
a form-feed character
(<TT>'\f'</TT>),
which will cause
some
displays
to clear.
Even more portable
(albeit even more gunky)
might be
to print enough newlines
to scroll everything away
(although of course
this leaves the cursor at the bottom of the screen,
not the top).
As a last resort,
you could use <TT>system</TT>
(see question <a href="faqcatea63.html?sec=osdep#system">19.27</a>)
to invoke
an operating system clear-screen command.

</p>

<p>References:

PCS Sec. 5.1.4 pp. 54-60, Sec. 5.1.5 pp. 60-62
<br>
Strang, <I>Programming with curses</I>

<br>
Strang, Mui, and O'Reilly, <I>termcap &amp; terminfo</I>

<hr><hr><hr>
<a name="waitforkey">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/waitforkey.html"><!-- qtag -->Question 19.4b</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
I'm compiling some test programs on a windows-based system,
and the windows containing my program's output
are closing so quickly
after my program calls <TT>exit</TT>
that I can't see the output.
How can I make it pause before closing?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
After wondering why the author of your compiler's run-time system
didn't take care of this for you,
simply add the lines
<pre>
	printf("Hit RETURN to exit"\n");
	fflush(stdout);
	(void)getchar();
</pre>
just before the end of your program.
(If you want to wait for any keystroke,
not just the RETURN key,
see question <a href="faqcatea63.html?sec=osdep#cbreak">19.1</a>.)
<hr><hr><hr>
<a name="specialkeys">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/specialkeys.html"><!-- qtag -->Question 19.5</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How do I read the arrow keys?
What about function keys?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
Terminfo,
some versions of termcap,
and some versions of curses
have support for these non-ASCII keys.
Typically,
a special key sends a multicharacter sequence
(usually beginning with ESC, <TT>'\033'</TT>);

parsing these can be tricky.
(curses will do the parsing for you,
if you call <TT>keypad</TT> first.)
</p><p>Under MS-DOS, if you receive a
character with
value 0
(<em>not</em> <TT>'0'</TT>!)
while reading the keyboard,
it's a flag indicating
that the next character read will be a code indicating a 
special key.
See any DOS programming guide for lists of keyboard scan codes.
(Very briefly:
the up, left, right, and down arrow keys are 72, 75, 77, and 80,
and the function keys

are 59 through 68.)
</p>
<p>References:

PCS Sec. 5.1.4 pp. 56-7
<hr><hr><hr>
<a name="mouse">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/mouse.html"><!-- qtag -->Question 19.6</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How do I read the mouse?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
Consult your system documentation,
or ask on an appropriate system-specific newsgroup
(but check its FAQ list first).
Mouse handling is completely different under the
X window system,
MS-DOS,
the Macintosh,
and probably every other system.
</p>
<p>References:

PCS Sec. 5.5 pp. 78-80
<hr><hr><hr>
<a name="serial">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/serial.html"><!-- qtag -->Question 19.7</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I do serial (``comm'') port I/O?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
It's system-dependent.
Under Unix,
you typically open, read, and write
a device file in <TT>/dev</TT>,
and use the
facilities of the terminal driver
to adjust its characteristics.
(See also questions <a href="faqcatea63.html?sec=osdep#cbreak">19.1</a>
and <a href="faqcatea63.html?sec=osdep#readavail">19.2</a>.)
Under MS-DOS, you can
use
the predefined stream <TT>stdaux</TT>,
or a special file like COM1,
or
some primitive BIOS interrupts,
or (if you require
decent
performance)

any number of interrupt-driven serial I/O packages.
Several
netters
recommend the book
<I>C Programmer's Guide to Serial Communications</I>,
by Joe Campbell.
<hr><hr><hr>
<a name="printer">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/printer.html"><!-- qtag -->Question 19.8</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I direct output to the printer?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
Under Unix,
either use <TT>popen</TT>
(see question <a href="faqcatea63.html?sec=osdep#popen">19.30</a>)
to write to the
<TT>lp</TT>
or
<TT>lpr</TT> program,
or perhaps open

a special file like <TT>/dev/lp</TT>.
Under MS-DOS, write to the
(nonstandard)
predefined stdio stream <TT>stdprn</TT>,
or open the special files PRN or LPT1.
Under some circumstances,
another
(and perhaps the only)
possibility is to use a window manager's screen-capture function,
and print the resulting bitmap.

</p>
<p>References:

PCS Sec. 5.3 pp. 72-74
<hr><hr><hr>
<a name="escseqs">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/escseqs.html"><!-- qtag -->Question 19.9</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How do I send escape sequences
to control a terminal or other 
device?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
If you can figure out how to send characters to the device at all
(see question <a href="faqcatea63.html?sec=osdep#printer">19.8</a>),
it's easy enough to send escape sequences.
In ASCII,
the ESC code is 033
(27 decimal),
so
code like
<pre>
	fprintf(ofd, "\033[J");
</pre>
sends the sequence <TT>ESC&nbsp;[&nbsp;J

.
</TT></p><p>Some programmers prefer to parameterize the ESC code,
like this:
<pre>
	#define ESC 033

	fprintf(ofd, "%c[J", ESC);
</pre>
<hr><hr><hr>
<a name="portio">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/portio.html"><!-- qtag -->Question 19.9b</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I access an I/O board directly?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
At one level, at least, it's quite simple: you have a device
register which is actually wired up so that the bits written to
it get coverted to actual voltage levels in the real world that
you can do interesting things with.
In general, there are two ways to
get the bits in and out.
(A particular I/O board will
use one method or the other;
you'll need to consult its documentation for details.)
<OL><li>If the device is accessed via a dedicated ``I/O port'',
use system-specific functions
to communicate with it.
Under MS-DOS, for example, there were quasistandard
``inport'' and ``outport''
instructions.
<li>If the device uses ``memory-mapped I/O'',
that is,
if the device register(s) are accessed as if they were normal memory
at particular, known locations within the processor's addressing space,
use contrived pointer variables to access
those
locations.
See question <a href="faqcatea63.html?sec=osdep#rawmemadr">19.25</a>.
<hr><hr><hr>
<a name="graphics">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/graphics.html"><!-- qtag -->Question 19.10</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I do graphics?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
Once upon a time,
Unix
had a fairly nice little set of device-independent plot
functions described in plot(3) and plot(5).
The GNU libplot
library,
written by
<a href="http://uranium.math.arizona.edu/~rsm/">Robert Maier</a>,
maintains
the same spirit
and supports many modern plot devices;
see
<a href="http://www.gnu.org/software/plotutils/plotutils.html"><TT>http://www.gnu.org/software/plotutils/plotutils.html</TT></a>.
</p><p>A modern, platform-independent graphics library
(which also supports 3D graphics and animation)
is OpenGL.
Other graphics standards which may be of interest are GKS and PHIGS.

</p><p>If you're programming for MS-DOS,
you'll probably
want to use libraries
conforming to the VESA or BGI
standards.
</p><p>If you're trying to talk to a particular plotter,
making it draw is usually a matter of sending it the appropriate
escape sequences;
see also question <a href="faqcatea63.html?sec=osdep#escseqs">19.9</a>.
The vendor may supply a C-callable library,
or you may be able to find one on the net.
</p><p>If you're programming for a particular
window system
(Macintosh, X windows, Microsoft Windows),
you will use its facilities;
see the relevant
documentation
or
newsgroup
or
FAQ list.
</p>
<p>References:

PCS Sec. 5.4 pp. 75-77
<hr><hr><hr>
<a name="gifjpeg">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/gifjpeg.html"><!-- qtag -->Question 19.10b</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I
display GIF and JPEG images?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
It
will depend
on your display environment,
which may already provide

these functions.
Reference JPEG software is at
<a href="http://www.ijg.org/files/"><TT>http://www.ijg.org/files/</TT></a>.

<hr><hr><hr>
<a name="fonts">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/fonts.html"><!-- qtag -->Question 19.10c</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I load new fonts for display?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
It's system-dependent.
<hr><hr><hr>
<a name="sendmail">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/sendmail.html"><!-- qtag -->Question 19.10d</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I send mail from within a C program?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
Under Unix,
open
a pipe to the <TT>mail</TT> program,
or perhaps <TT>/usr/lib/sendmail</TT>.
See question <a href="faqcatea63.html?sec=osdep#popen">19.30</a>.
<hr><hr><hr>
<a name="fileexists">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/fileexists.html"><!-- qtag -->Question 19.11</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I check whether a file exists?
I want to
warn the user
if a requested input file is missing.
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
It's surprisingly difficult to
make this determination
reliably and portably.
Any test you make can be invalidated
if the file is created or deleted
(i.e.
by some other process)
between the time you make the test
and the time you try to open the file.
</p><p>Three possible test functions are
<TT>stat</TT>,
<TT>access</TT>,
and
<TT>fopen</TT>.
(To
make an approximate
test
using
<TT>fopen</TT>,
just open for reading and close immediately,
although failure does not necessarily indicate nonexistence.)
Of these,
only <TT>fopen</TT> is widely portable,
and <TT>access</TT>,
where it exists,
must be used carefully if
the program
uses the Unix
set-UID
feature.