| 1 | #include <errno.h>
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| 2 | #include <pthread.h>
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| 3 | #include <stdarg.h>
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| 4 | #include <stdio.h>
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| 5 | #include <stdlib.h>
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| 6 | #include <string.h>
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| 7 | #include <sys/wait.h>
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| 8 | #include <time.h>
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| 9 | #include <unistd.h>
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| 10 |
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| 11 |
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| 12 | static pthread_mutex_t sMutex = PTHREAD_MUTEX_INITIALIZER;
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| 13 | static volatile int* sSync;
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| 14 |
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| 15 |
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| 16 | static void
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| 17 | require_errno(bool condition, const char* message,...)
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| 18 | {
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| 19 | if (condition)
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| 20 | return;
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| 21 |
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| 22 | va_list args;
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| 23 | va_start(args, message);
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| 24 | vfprintf(stderr, message, args);
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| 25 | va_end(args);
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| 26 |
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| 27 | fprintf(stderr, ": %s\n", strerror(errno));
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| 28 | exit(1);
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| 29 | }
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| 30 |
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| 31 |
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| 32 | static void
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| 33 | require_no_error(int error, const char* message,...)
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| 34 | {
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| 35 | if (error >= 0)
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| 36 | return;
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| 37 |
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| 38 | va_list args;
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| 39 | va_start(args, message);
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| 40 | vfprintf(stderr, message, args);
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| 41 | va_end(args);
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| 42 |
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| 43 | fprintf(stderr, ": %s\n", strerror(error));
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| 44 | exit(1);
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| 45 | }
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| 46 |
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| 47 |
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| 48 | static void
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| 49 | sync_threads(volatile int* syncMe, volatile int* syncOther, int value)
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| 50 | {
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| 51 | bool meFirst = syncMe < syncOther;
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| 52 | if (meFirst)
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| 53 | *syncMe = value;
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| 54 |
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| 55 | while (*syncOther != value) {
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| 56 | }
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| 57 |
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| 58 | if (!meFirst)
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| 59 | *syncMe = value;
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| 60 | }
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| 61 |
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| 62 |
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| 63 | static void*
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| 64 | locker_thread(void* arg)
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| 65 | {
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| 66 | volatile int* syncMe = (volatile int*)arg;
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| 67 | volatile int* syncOther = syncMe == sSync ? sSync + 1 : sSync;
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| 68 |
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| 69 | for (;;) {
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| 70 | sync_threads(syncMe, syncOther, 1);
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| 71 |
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| 72 | timespec timeout;
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| 73 | require_errno(clock_gettime(CLOCK_REALTIME, &timeout) == 0,
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| 74 | "failed to get time");
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| 75 | timeout.tv_nsec += 10000;
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| 76 | if (timeout.tv_nsec >= 1000000000) {
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| 77 | timeout.tv_nsec -= 1000000000;
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| 78 | timeout.tv_sec += 1;
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| 79 | }
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| 80 |
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| 81 | pthread_mutex_timedlock(&sMutex, &timeout);
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| 82 |
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| 83 | sync_threads(syncMe, syncOther, 2);
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| 84 | }
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| 85 |
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| 86 | return NULL;
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| 87 | }
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| 88 |
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| 89 |
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| 90 | int
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| 91 | main()
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| 92 | {
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| 93 | sSync = new int[2];
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| 94 | sSync[0] = sSync[1] = 0;
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| 95 |
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| 96 | require_no_error(pthread_mutex_lock(&sMutex), "failed to lock mutex");
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| 97 |
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| 98 | // start the threads
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| 99 | pthread_t thread;
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| 100 | require_no_error(pthread_create(&thread, NULL, &locker_thread,
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| 101 | (void*)sSync), "failed to create thread 1");
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| 102 | require_no_error(pthread_create(&thread, NULL, &locker_thread,
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| 103 | (void*)(sSync + 1)), "failed to create thread 1");
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| 104 |
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| 105 | // wait for the threads to be ready
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| 106 | while (sSync[1] != 1) {
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| 107 | }
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| 108 |
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| 109 | // fork
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| 110 | pid_t pid = fork();
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| 111 | require_errno(pid >= 0, "fork() failed");
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| 112 | if (pid == 0)
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| 113 | usleep(100000);
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| 114 | else
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| 115 | waitpid(pid, NULL, 0);
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| 116 |
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| 117 | return 0;
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| 118 | }
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