site_archiver@lists.apple.com Delivered-To: darwin-kernel@lists.apple.com The Single Unix Specification v3 (SUSv3) defines cancelation points in http://www.opengroup.org/onlinepubs/000095399/functions/xsh_chap02_09.html#t... int read(...) { pthread_testcancel(); return read_nocancel(...); } Transcript: bash$ gcc cancel_bug.c -Wall && ./a.out Created thread, sleeping calling read..<press return> .called read()==1 calling read..<press return> .called read()==1 calling read..cancelling read() <blocks forever or until more input is available> /* cancel_bug.c */ #include <assert.h> #include <pthread.h> #include <stdio.h> #include <stdlib.h> #include <unistd.h> static int exit_status = EXIT_FAILURE; void * func(void *arg) { char buf; int oldstate; ssize_t res; return NULL; } int main(void) { pthread_t thread; void *retval; CHECK(pthread_create(&thread, NULL, func, NULL) == 0); fprintf(stderr, "Created thread, sleeping\n");fflush(stderr); sleep(10); /* ensure thread reaches blocking read */ fprintf(stderr, "cancelling read()\n");fflush(stderr); CHECK(pthread_cancel(thread) == 0); CHECK(pthread_join(thread, &retval) == 0); CHECK(PTHREAD_CANCELED == retval); return exit_status; } Regards, _______________________________________________ Do not post admin requests to the list. They will be ignored. Darwin-kernel mailing list (Darwin-kernel@lists.apple.com) Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/darwin-kernel/site_archiver%40lists.a... My tests indicate that pthread_cancel still cannot interrupt blocking system calls like read(2) in Intel Mac OS X 10.5.1, despite the claim that it is now UNIX 2003 compliant. A brief look at the Darwin 9.0 kernel sources seems to confirm this. The critical part, and the reason why pthread_testcancel is not sufficient is "If a thread has cancelability enabled and a cancellation request is made with the thread as a target while the thread is suspended at a cancellation point, the thread shall be awakened and the cancellation request shall be acted upon.". That is, pthread_cancel should wake (a well defined list of) blocking system calls. The below test program shows that a blocking read() is _not_ awakened by pthread_cancel, making the cancelation handling in Leopard no improvement over earlier OS releases and in violation of the SUSv3 standard. The test program starts a worker thread that loops reading from standard input. The main thread then sleeps for a while to ensure that the worker thread has blocked in read(2) and the main thread then cancels the worker thread. This program terminates on Linux and Solaris but not on Intel Leopard 10.5.1.
From a cursory glance at xnu-1228, e.g. sys_generic.c, it looks as if the system calls that SUSv3 requires to be cancelation points are all implemented something like the following template: That is, if a cancel is pending when the system call is entered then the call will be canceled but once the real system call is blocking it will not react to cancel requests. This is with Xcode 3.0. Compiling with -D_APPLE_C_SOURCE does not make a difference. #define CHECK(X) do { if ((X) == 0) { fprintf(stderr, "%s:%d CHECK FAILED\n", __FILE__, (int)__LINE__); } } while (0) exit_status = EXIT_SUCCESS;
/* redundant */ CHECK(pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, &oldstate) == 0); /* Will not terminate unless cancelled or read error */ do { fprintf(stderr, "calling read..");fflush(stderr); res = read(STDIN_FILENO, &buf, 1); fprintf(stderr, ".called read()==%ld\n", (long)res);fflush(stderr); } while (res != -1); So, has Apple and the Open Group conformance testers made a huge mistake or am I missing something? This email sent to site_archiver@lists.apple.com