prototype of signal

Unix Network Programming, 5.8 POSIX Signal Handling

void (*signal (int signo, void (*func) (int))) (int);

==>

typedef    void    Sigfunc(int);
Sigfunc *signal (int signo, Sigfunc *func);

signal是这样一个函数指针：它需要两个参数，第一个参数是int的signo， 第二个参数是一个以int作参数并返回void的函数指针；它的返回值也是一个以int作参数并返回void的函数指针。

POSIX Signal Semantics
We summarize the following points about signal handling on a POSIX-compliant system:
Once a signal handler is installed, it remains installed. (Older systems removed the signal handler each time it was executed.)
While a signal handler is executing, the signal being delivered is blocked. Furthermore, any additional signals that were specified in the sa_mask signal set passed to sigaction when the handler was installed are also blocked. In Figure 5.6, we set sa_mask to the empty set, meaning no additional signals are blocked other than the signal being caught.
If a signal is generated one or more times while it is blocked, it is normally delivered only one time after the signal is unblocked. That is, by default, Unix signals are not queued. We will see an example of this in the next section. The POSIX real-time standard, 1003.1b, defines some reliable signals that are queued, but we do not use them in this text.
It is possible to selectively block and unblock a set of signals using the sigprocmask function. This lets us protect a critical region of code by preventing certain signals from being caught while that region of code is executing.