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