C：SIGALRM - 报警信息显示每秒 [英] C: SIGALRM - alarm to display message every second

问题描述

所以我想打电话报警，显示信息仍然在工作。每一秒钟。
我包括signal.h中。

So I'm trying to call an alarm to display a message "still working.." every second. I included signal.h.

我主外，我有我的功能：（我从来没有申报/定义S表示的int）

Outside of my main I have my function: (I never declare/define s for int s)

void display_message(int s);   //Function for alarm set up
void display_message(int s) {
     printf("copyit: Still working...\n" );
     alarm(1);    //for every second
     signal(SIGALRM, display_message);
}

然后，在我的主要

Then, in my main

while(1)
{
    signal(SIGALRM, display_message);
    alarm(1);     //Alarm signal every second.

这只要循环开始的在那里。但该计划从未输出仍在工作......消息。我在做什么错误？谢谢你，多版本AP preciated。

That's in there as soon as the loop begins. But the program never outputs the 'still working...' message. What am I doing incorrectly? Thank you, ver much appreciated.

推荐答案

信号处理是不应该含有商业逻辑，或让库调用，如的printf 。见C11§7.1.4/ 4和脚注：

Signal handlers are not supposed to contain "business logic" or make library calls such as printf. See C11 §7.1.4/4 and its footnote:

因此​​，信号处理程序不是在一般情况下，调用标准库函数。

Thus, a signal handler cannot, in general, call standard library functions.

所有的信号处理程序应该做的是设置为通过非中断code采取行动的标志。该程序运行正常，并没有崩溃的风险，即使是增加了一些I / O的其他功能：

All the signal handler should do is set a flag to be acted upon by non-interrupt code. This program runs correctly and does not risk crashing, even if some I/O other functionality were added:

#include <signal.h>
#include <stdio.h>
#include <stdbool.h>
#include <unistd.h>

volatile sig_atomic_t print_flag = false;

void handle_alarm( int sig ) {
    print_flag = true;
}

int main() {
    signal( SIGALRM, handle_alarm ); // Install handler first,
    alarm( 1 ); // before scheduling it to be called.

    for (;;) {
        if ( print_flag ) {
            printf( "Hello\n" );
            print_flag = false;
            alarm( 1 );
        }
    }
}

请注意，虽然，旋转循环是编程一个可怕的方式。本例中使用100％的CPU电力，因为它从来不睡觉。此外报警没有出现，虽然POSIX将其标记为这样的，我记得这是K＆安培的一部分要由C标准定义; R。因此，在便携性方面，你会一样好了与另一个POSIX设施。

Note, though, that spin loops are an awful way to program. This example uses 100% CPU power because it never sleeps. Furthermore alarm does not appear to be defined by the C standard although POSIX marks it as such and I recall it was part of K&R. So in terms of portability, you would be just as well off with another POSIX facility.

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