信號學習：
1。簡單點用signal（主要就1個api，signal(SIGINT, SignalHandler)和signal(SIGINT, SIG_IGN)就是全部了）；
2。復雜點用sigaction（可以設置在handling期間，允許/不允許哪些信號打斷自己）；
3。當前線程handling過程中，當前線程不會因為相同signal再次進入handler，但當前線程可以被別的signal打斷重新進入handler，別的線程可以進入handler；
例如下面這個例子，只有一個主線程，先被SIGINT打斷進入handler，雖然又被SIGTERM打斷再次進入handler：
#0 0x00007f7cc55dc200 in __nanosleep_nocancel () at ../sysdeps/unix/syscall-template.S:82
#1 0x00007f7cc55dc090 in __sleep (seconds=<value optimized out>) at ../sysdeps/unix/sysv/linux/sleep.c:138
#2 0x0000000000468300 in SignalHandler (sig=15) at src/dispatcher2_server.cpp:78
#3 <signal handler called>
#4 0x00007f7cc55dc200 in __nanosleep_nocancel () at ../sysdeps/unix/syscall-template.S:82
#5 0x00007f7cc55dc090 in __sleep (seconds=<value optimized out>) at ../sysdeps/unix/sysv/linux/sleep.c:138
#6 0x0000000000468300 in SignalHandler (sig=2) at src/dispatcher2_server.cpp:78
#7 <signal handler called>
#8 0x00007f7cc55dc200 in __nanosleep_nocancel () at ../sysdeps/unix/syscall-template.S:82
#9 0x00007f7cc55dc090 in __sleep (seconds=<value optimized out>) at ../sysdeps/unix/sysv/linux/sleep.c:138
#10 0x000000000046844a in main (argc=1, argv=0x7fff048454a8) at src/dispatcher2_server.cpp:102
demo：
static void SignalHandler(int sig) {
if (SIGTERM == sig || SIGINT == sig) {
struct sigaction sa;
memset(&sa, 0, sizeof(sa));
sa.sa_handler = SIG_IGN;
sigaction(SIGINT, &sa, NULL); // keep other threads from entering this handler
sigaction(SIGTERM, &sa, NULL); // keep other threads from entering this handler
//...
}
}
struct sigaction sa;
memset(&sa, 0, sizeof(sa));
sa.sa_handler = SignalHandler;
sigfillset(&sa.sa_mask); // block every signal during handling
sigaction(SIGINT, &sa, NULL);
sigaction(SIGTERM, &sa, NULL);
介紹：http://www.alexonlinux.com/signal-handling-in-linux
demo：http://aspyct.org/blog/2012/08/25/unix-signal-handling-example/