（f）printf（）线程安全，但不是信号处理程序安全

Question

We recently had a discussion at work about signal handlers in C (Unix enviornment).

Someone mentioned that

(f)printf() is certainly thread-safe but not signal-handler safe.

What does the above statement mean? What aspect of (f)printf() make it not signal-handler safe? Is it because it accesses the stdout and stdin which are global and hence the function is not re-entrant?

Or is there some other aspect which I am missing?

Answer 1

There is actually fairly little that is legal to do in a signal handler directly. Instead, one must usually set some flag or trigger to do the real work outside the signal handling context. While handling a signal, the only functions you can call are those which are "async signal safe," which is described in detail here: What constitutes asynchronous-safeness

Answer 2

As mentioned in here: http://www.gnu.org/software/libc/manual/html_node/Nonreentrancy.html#Nonreentrancy

If a function uses and modifies an object that you supply, then it is potentially non-reentrant; two calls can interfere if they use the same object.

In your case that object is stdout . When a signal arrives in the middle of a (f)printf() and if you use (f)printf in handler, both data could be corrupted as they operate on the same stream, stdout . Reentrancy is the root cause in that case. Even if you use the stream in just signal handler, two signal handlers can interfere.