alarm and pause Functions

The alarm function allows us to set a timer that will expire（中止） at a specified time in the future.

When the timer expires, the SIGALRM signal is generated. If we ignore or don’t catch this signal, its default action is to terminate the process.

#include <unistd.h>

unsigned int alarm(unsigned int seconds);
        //Return：0 or number of seconds until previously set alarm

alarm参数

调用alarm会使得之前调用alarm的参数 seconds 返回。如果seconds参数为0，之前的alarm clock取消。

The pause function suspends the calling process until a signal is caught.

#include <unistd.h>

int pause(void);
        //Returns: −1 with errno set to EINTR

Example

1. 利用stejmp和longjmp解决sleep的race condition问题

static jmp_buf env_alrm;
static void sig_alrm(int signo)
{
    longjmp(env_alrm, 1);
}
unsigned int sleep2(unsigned int seconds)
{
    if (signal(SIGALRM, sig_alrm) == SIG_ERR)
        return(seconds);
    if (setjmp(env_alrm) == 0) {
        alarm(seconds); /* start the timer */
        pause(); /* next caught signal wakes us up */
    }
    return(alarm(0)); /* turn off timer, return unslept time */
}

解决了在调用pause之前就已经触发了signal handler，而导致pause永久阻塞下去的问题。

潜在问题

1. 使用setjmp和longjmp会导致sleep中止掉其他signal handlers
2. 调用者已经有alarm set，之前的alarm会被清除
3. 我们该变了SIGALRM的disposition(序列)

2. reading one line from standard input and writing it to standard output.

我们有时在read会需要当超过一定时间直接返回的功能。

static void sig_alrm(int);
int main(void)
{
    int n;
    char line[MAXLINE];
    if (signal(SIGALRM, sig_alrm) == SIG_ERR)
        err_sys("signal(SIGALRM) error");
    alarm(10);
    if ((n = read(STDIN_FILENO, line, MAXLINE)) < 0)
        err_sys("read error");
    alarm(0);
    write(STDOUT_FILENO, line, n);
    exit(0);
}
static void sig_alrm(int signo)
{
    /* nothing to do, just return to interrupt the read */
}

问题如下：

1. alarm和read可能会出现竞争导致，read一直阻塞
2. 如果read能自动重启，定时将毫无意义