今天看到了信号有关的章节,对其中的返回值的描述不太理解,也包括后面例程中提供的正规写法:
if (signal(SIGINT, sigint_handler)) == SIG_ERR)
觉得这条if语句也有点搞不懂
原文说的是:若成功则返回指向前次处理程序的指针,若出错则为SIG_ERR
当时想不清楚signal调用后到底返回了什么,这条if语句怎么判断,后来用google找到一篇国外的博客解释的不错。其实主要是记住signal返回的是一个指向某个函数的指针,而这个函数就是上次处理这个信号的信号处理函数,如果未处理过,那就是NULL。测试代码如下:
#include <signal.h>
#include <assert.h>
#include <stdio.h>
void catch1(int signo) {
printf("catch1 received signal %d\n", signo);
}
void catch2(int signo) {
printf("catch2 received signal %d\n", signo);
}
int main(void) {
sig_t prev_sigint_handler1 = signal(SIGINT, catch1);
assert(prev_sigint_handler1 == NULL);
sig_t prev_sighup_handler1 = signal(SIGHUP, catch2);
assert(prev_sighup_handler1 == NULL);
raise(SIGINT); // calls catch1
raise(SIGHUP); // calls catch2
// Now let's swap the handlers
sig_t prev_sigint_handler2 = signal(SIGINT, catch2);
assert(prev_sigint_handler2 == catch1);
sig_t prev_sighup_handler2 = signal(SIGHUP, catch1);
assert(prev_sighup_handler2 == catch2);
raise(SIGINT); // calls catch2
raise(SIGHUP); // calls catch1
return 0;
}
% ./a.out
catch1 received signal 2
catch2 received signal 1
catch2 received signal 2
catch1 received signal 1
不难看出一开始两个handler1都为NULL,分别发送SIGINT和SIGHUP信号后,就指向了catch1和catch2。而再次调用signal后会分别得到两个handler,于是可以让新的handler2交换捕获的信号,就和指针的赋值一样了