I understand that, from a signal handler function sigaction() I should only call those functions that are "async-safe". But why is so?
Calling an unsafe function may lead to undefined behavior.
The Open Group Base Specifications Issue 7 (POSIX.1-2008), in its treatment of "Signal Concepts", says:
[W]hen a signal interrupts an unsafe function ... and the signal-catching function calls an unsafe function, the behavior is undefined.
As to why unsafe functions are unsafe, there may be many reasons in a given implementation.
However, a previous version of the standard, Issue 6 (POSIX.1-2004), hints at one possible reason on some implementations. That version describes async-signal-safe functions as "either reentrant or non-interruptible by signals". So, consider a function which relies on static data to keep state but is interrupted by itself midway through its execution — can that data be trusted once control returns to the interrupted function?
来源：https://stackoverflow.com/questions/46354925/why-only-async-safe-functions-should-be-called-from-a-signal-handler