This is altio/demo_sigio.c (Listing 63-3, page 1348), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/* demo_sigio.c

   A trivial example of the use of signal-driven I/O.
#include <signal.h>
#include <ctype.h>
#include <fcntl.h>
#include <termios.h>
#include "tty_functions.h"      /* Declaration of ttySetCbreak() */
#include "tlpi_hdr.h"

static volatile sig_atomic_t gotSigio = 0;
                                /* Set nonzero on receipt of SIGIO */
static void
sigioHandler(int sig)
    gotSigio = 1;
main(int argc, char *argv[])
    int flags, j, cnt;
    struct termios origTermios;
    char ch;
    struct sigaction sa;
    Boolean done;

    /* Establish handler for "I/O possible" signal */

    sa.sa_flags = SA_RESTART;
    sa.sa_handler = sigioHandler;
    if (sigaction(SIGIO, &sa, NULL) == -1)

    /* Set owner process that is to receive "I/O possible" signal */

    if (fcntl(STDIN_FILENO, F_SETOWN, getpid()) == -1)

    /* Enable "I/O possible" signaling and make I/O nonblocking
       for file descriptor */

    flags = fcntl(STDIN_FILENO, F_GETFL);
    if (fcntl(STDIN_FILENO, F_SETFL, flags | O_ASYNC | O_NONBLOCK) == -1)

    /* Place terminal in cbreak mode */

    if (ttySetCbreak(STDIN_FILENO, &origTermios) == -1)

    for (done = FALSE, cnt = 0; !done ; cnt++) {
        for (j = 0; j < 100000000; j++)
            continue;                   /* Slow main loop down a little */

        if (gotSigio) {                 /* Is input available? */
            gotSigio = 0;

            /* Read all available input until error (probably EAGAIN)
               or EOF (not actually possible in cbreak mode) or a
               hash (#) character is read */

            while (read(STDIN_FILENO, &ch, 1) > 0 && !done) {
                printf("cnt=%d; read %c\n", cnt, ch);
                done = ch == '#';

    /* Restore original terminal settings */

    if (tcsetattr(STDIN_FILENO, TCSAFLUSH, &origTermios) == -1)


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