Expert C Programming

variable errno that is part of ANSI standard C.

If a library or system call encounters problems, it will set errnoto indicate the cause of the
problem. However, the value of errno is only valid if there was a problem—the call will
have some way of indicating this (usually by its return code).

A typical use might be:

errno=0;

if (ioctl(0, FIONREAD, &i)<0) {

if (errno==EBADF) printf("errno: bad file number");

if (errno==EINVAL) printf("errno: invalid

argument");}

You can get as fancy as you like, and encapsulate the checking in a single function that is
called after each system call while you are debugging your program. This really helps a lot
in isolating the errors. The library call perror() will print out an error message when you
know you have one.

If you're interested in single-character I/O like this, you're often also interested in doing other display
control, and the curses library provides various portable routines for both. Curses (think "cursor") is a
library of screen management calls, with implementations on all popular platforms. Rewriting the
main function above to use curses instead of stty gives:

#include <curses.h>

/* uses curses library, and the kbhit() function defined above

*/

main()

{

int c=' ', i=0;

initscr(); / initialize curses functions /

cbreak();

noecho(); / do not echo pressed character /

mvprintw(0, 0, "Press 'q' to quit\n");

refresh();

while (c!='q')

if (kbhit()) {

c = getch(); /* won't block, as we know a character is

waiting */

mvprintw(1, 0, "got char '%c' on iteration %d \n",c, ++i);

refresh();

}

nocbreak();