Detailed explanation of Linux signal set operation functions

Let’s first review what a pending signal set is. When a signal is generated and reaches its destination, this process is called signal delivery. The intermediate state from signal generation to delivery is called the pending state of the signal. The reason for the pending state may be that the signal is blocked, that is, the corresponding bit in the signal mask word (or blocked signal set) is set to 1. Both the blocking signal set and the pending signal set are maintained by the kernel. The whole process is shown in the figure below:

Sometimes we need to block a certain signal, so we need to modify the blocking signal set. So, how do we modify the blocking signal set? One method provided by the system is that we first create a set that is the same as the blocking signal set, and then use it to modify the blocking signal set.

The system provides a series of signal set setting functions. These functions are as follows:

sigset_t  set;

信号集数据类型，本质是typedef unsigned long sigset_t; 

int sigemptyset(sigset_t *set);

将某个信号集清0

int sigfillset(sigset_t *set);

将某个信号集置1

int sigaddset(sigset_t *set, int signum);

将某个信号加入信号集

int sigdelset(sigset_t *set, int signum);

将某个信号清出信号集

以上几个函数返回值均是：成功：0；失败：-1

int sigismember(const sigset_t *set, int signum);

判断某个信号是否在信号集中

返回值：在集合：1；不在：0；出错：-1

After using the above functions to create the signal set, how to change the blocking signal set? The system provides another function: sigprocmask function. The sigprocmask function can be used to block signals or unblock signals. Its essence is to use the signal set we created to change the blocking signal set.

Function prototype:

int sigprocmask(int how, const sigset_t *set, sigset_t *oldset);

return value:

Success: 0; Failure: -1, set errno

Parameter explanation:

set: The incoming parameter is a bitmap. Which bit in the set is 1 indicates which signal is blocked by the current process.

oldset: Outgoing parameters, save the old signal mask set. This is somewhat similar to setitimer.

how parameter value:

Assume that the current signal mask word is mask

1.SIG_BLOCK: When how is set to this value, set indicates the signal that needs to be blocked. Equivalent to mask = mask | set

2.SIG_UNBLOCK: When how is set to this, set indicates the signal that needs to be unblocked. Equivalent to mask = mask & ~set

3.SIG_SETMASK: When how is set to this, set represents the new mask set used to replace the original mask. Equivalent to mask = set. If calling sigprocmask unblocks several current signals, at least one of the signals will be delivered before sigprocmask returns.

How do we read the set of pending signals? The system provides the sigpending function.

Function prototype:

int sigpending(sigset_t *set);

Parameter Description:

set outgoing parameters.

return value:

Return value: Success: 0; Failure: -1, set errno

Example: Print the pending status of all regular signals to the screen.

#include 
#include 
#include 

void printPending(sigset_t *set)
{
    int i = 0;

    for (i = 0; i if (sigismember(set, i) == 1)
            printf("1");
        else
            printf("0");
    }
    printf("\n");
}

int main()
{
    sigset_t set, oldset, pendset;
    sigemptyset(&set);
    sigaddset(&set, SIGQUIT);   // ctrl + \ 将产生SIGQUIT信号
    sigprocmask(SIG_BLOCK, &set, &oldset);
    while (1) {
        sigpending(&pendset);
        printPending(&pendset);     // 写一个函数打印未决信号集
        sleep(1);
    }
}

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