The ØMQ Guide Chapter 2 #2
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rfarnham
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It is important to set both ends of the self pipe as nonblocking, otherwise the signal handler might block on the `write` call and the process will deadlock. Example: ``` (lldb) bt * thread booksbyus#1, queue = 'com.apple.main-thread', stop reason = signal SIGSTOP * frame #0: 0x00007fff5f09ff46 libsystem_kernel.dylib`write + 10 frame booksbyus#1: 0x000000010f8f9cd2 interrupt`s_signal_handler(signal_value=2) at interrupt.c:20:12 frame booksbyus#2: 0x00007fff5f152b5d libsystem_platform.dylib`_sigtramp + 29 frame booksbyus#3: 0x000000010f8f981a interrupt`burn_cpu at interrupt.c:44:3 frame booksbyus#4: 0x000000010f8f9b60 interrupt`main at interrupt.c:114:23 frame booksbyus#5: 0x00007fff5ef673d5 libdyld.dylib`start + 1 frame booksbyus#6: 0x00007fff5ef673d5 libdyld.dylib`start + 1 (lldb) f 1 frame booksbyus#1: 0x000000010f8f9cd2 interrupt`s_signal_handler(signal_value=2) at interrupt.c:20:12 17 #define S_ERROR_MSG "Error while writing to self-pipe.\n" 18 static int s_fd; 19 static void s_signal_handler(int signal_value) { -> 20 int rc = write(s_fd, S_NOTIFY_MSG, sizeof(S_NOTIFY_MSG)); 21 if (rc != sizeof(S_NOTIFY_MSG)) { 22 write(STDOUT_FILENO, S_ERROR_MSG, sizeof(S_ERROR_MSG) - 1); 23 perror("error writing\n"); ``` Original code was buggy, as it did not use the loop variable `i`: ``` for (int i = 0; i < 2; i++) { int flags = fcntl(pipefds[0], F_GETFL, 0); if (flags < 0) { perror ("fcntl(F_GETFL)"); exit(1); } rc = fcntl (pipefds[0], F_SETFL, flags | O_NONBLOCK); if (rc != 0) { perror ("fcntl(F_SETFL)"); exit(1); } } ``` The loop was erroneously removed in this commit booksbyus@aebc4b5
rfarnham
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May 22, 2020
It is important to set both ends of the self pipe as nonblocking, otherwise the signal handler might block on the `write` call and the process will deadlock. Example: ``` (lldb) bt * thread booksbyus#1, queue = 'com.apple.main-thread', stop reason = signal SIGSTOP * frame #0: 0x00007fff5f09ff46 libsystem_kernel.dylib`write + 10 frame booksbyus#1: 0x000000010f8f9cd2 interrupt`s_signal_handler(signal_value=2) at interrupt.c:20:12 frame booksbyus#2: 0x00007fff5f152b5d libsystem_platform.dylib`_sigtramp + 29 frame booksbyus#3: 0x000000010f8f981a interrupt`burn_cpu at interrupt.c:44:3 frame booksbyus#4: 0x000000010f8f9b60 interrupt`main at interrupt.c:114:23 frame booksbyus#5: 0x00007fff5ef673d5 libdyld.dylib`start + 1 frame booksbyus#6: 0x00007fff5ef673d5 libdyld.dylib`start + 1 (lldb) f 1 frame booksbyus#1: 0x000000010f8f9cd2 interrupt`s_signal_handler(signal_value=2) at interrupt.c:20:12 17 #define S_ERROR_MSG "Error while writing to self-pipe.\n" 18 static int s_fd; 19 static void s_signal_handler(int signal_value) { -> 20 int rc = write(s_fd, S_NOTIFY_MSG, sizeof(S_NOTIFY_MSG)); 21 if (rc != sizeof(S_NOTIFY_MSG)) { 22 write(STDOUT_FILENO, S_ERROR_MSG, sizeof(S_ERROR_MSG) - 1); 23 perror("error writing\n"); ``` Original code was buggy, as it did not use the loop variable `i`: ``` for (int i = 0; i < 2; i++) { int flags = fcntl(pipefds[0], F_GETFL, 0); if (flags < 0) { perror ("fcntl(F_GETFL)"); exit(1); } rc = fcntl (pipefds[0], F_SETFL, flags | O_NONBLOCK); if (rc != 0) { perror ("fcntl(F_SETFL)"); exit(1); } } ``` The loop was erroneously removed in this commit booksbyus@aebc4b5
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// in The Socket API section's first sample code
zmq_msg_t mouse;
mouse = zmq_msg_init (&mouse); // <-- this line must exist bug
zmq_recv (mousetrap, &mouse, 0);
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