当线程/进程处于睡眠状态时，睡眠线程或进程的指令是否继续执行？

Question

When i do my assignment on IPC(Inter Process Communication),a chat application using shared memory between two process,how does a sleep thread checks the condition in the while loop(if *shm != 1 the thread prints the string wakeup) how can is this possible?

#define TRUE 1

while(*shm == 1)
    sleep(1)
printf("wakeup");

Answer 1

The sleep doesn't check shm value.

Right now your thread calls sleep , it will suspend the thread just for a few miliseconds, and then it will be woken up by a signal from OS timer/scheduler. That's all that sleep does. Then the thread wakes up and resumes your code: sleep() returns, loop starts next iteration, checks shm and makes a decision whether to sleep again or continue.

It works, but consumes more than it could if you've used specialized synchronization primitives - like condition variables in c++11/posix. Sleeping while waiting for condition variable signal will truly sleep until signal is received, without wasting cpu cycles for such periodical wake-ups.

Answer 2

unsigned int sleep(unsigned int seconds);

Suspends thread execution for a specified number of seconds. Because of processor delays, the thread can sleep slightly longer than this specified time. An unblocked signal received during this time (for which the action is to invoke a signal handler function or to end the thread) “wakes up” the thread prematurely. When that function returns, sleep() returns immediately even if there is sleep time remaining.

So your while(*shm == 1) doesn't calcs while thread sleeps.