Is it safe to call pthread_cancel() on terminated thread?

Question

I\'m wondering if it is safe to call pthread_cancel() on a terminated thread. I couldn\'t find any hints in the manual page. Thanks in advance for any hints.

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Answer 1

Yes, it is safe to call it before pthread_detach or pthread_join has been called. However, according to my experiments, it can return ESRCH (=3) if the thread has already returned from its thread function. (gcc (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0)

#include "gtest/gtest.h"

static void *S_ThreadEntry1(void* pvarg)
{
     return 0;
}

TEST(ThreadTest, PthreadCancelAfterExit)
{
     pthread_t threadid;
     ASSERT_EQ(pthread_create(&threadid, 0, &S_ThreadEntry1, nullptr), 0);
     sleep(1);

     int cancel_ret = pthread_cancel(threadid);
     std::cout << "pthread_cancel returned " << cancel_ret << std::endl;
     if (cancel_ret != 0)
          EXPECT_EQ(cancel_ret, ESRCH);
     void* res;
     EXPECT_EQ(pthread_join(threadid, &res), 0);
     std::cout << "res=" << res << std::endl;
}

Output:

[ RUN      ] ThreadTest.PthreadCancelAfterExit
pthread_cancel returned 3
res=0
[       OK ] ThreadTest.PthreadCancelAfterExit (1000 ms)