The Linux Programming Interface

510 Chapter 23

the maximum number of expirations of the timer that the program should wait for

before terminating; the default for this argument is 1.

The program creates a timer using timerfd_create(), and arms it using

timerfd_settime(). It then loops, reading expiration notifications from the file

descriptor until the specified number of expirations has been reached. After each

read(), the program displays the time elapsed since the timer was started, the num-

ber of expirations read, and the total number of expirations so far.

In the following shell session log, the command-line arguments specify a timer

with a 1-second initial value and 1-second interval, and a maximum of 100 expirations.

$ ./demo_timerfd 1:1 100

1.000: expirations read: 1; total=1

2.000: expirations read: 1; total=2

3.000: expirations read: 1; total=3

Type Control-Z to suspend program in background for a few seconds

[1]+ Stopped ./demo_timerfd 1:1 100

$ fg Resume program in foreground

./demo_timerfd 1:1 100

14.205: expirations read: 11; total=14 Multiple expirations since last read()

15.000: expirations read: 1; total=15

16.000: expirations read: 1; total=16

Type Control-C to terminate the program

From the above output, we can see that multiple timer expirations occurred while

the program was suspended in the background, and all of these expirations were

returned on the first read() after the program resumed execution.

Listing 23-8: Using the timerfd API

––––––––––––––––––––––––––––––––––––––––––––––––––––– timers/demo_timerfd.c

#include <sys/timerfd.h>

#include <time.h>

#include <stdint.h> /* Definition of uint64_t */

#include "itimerspec_from_str.h" /* Declares itimerspecFromStr() */

#include "tlpi_hdr.h"

int

main(int argc, char *argv[])

{

struct itimerspec ts;

struct timespec start, now;

int maxExp, fd, secs, nanosecs;

uint64_t numExp, totalExp;

ssize_t s;

if (argc < 2 || strcmp(argv[1], "--help") == 0)

usageErr("%s secs[/nsecs][:int-secs[/int-nsecs]] [max-exp]\n", argv[0]);

itimerspecFromStr(argv[1], &ts);

maxExp = (argc > 2)? getInt(argv[2], GN_GT_0, "max-exp") : 1;