1. 子线程循环 10 次，接着主线程循环 100 次，接着又回到子线程循环 10 次，接着再回到主线程又循环 100 次，如此循环50次，试写出代码

    [X] 无法锁住的错误解法

    1. #include "comm.h"
    3. pthread_mutex_t gmux = PTHREAD_MUTEX_INITIALIZER;
    5. #define lock    pthread_mutex_lock
    6. #define unlock  pthread_mutex_unlock
    8. int cnt = 0;
    9. int sta = 1;
    10. volatile int status = 1;
    12. void *child(void *arg) {
    13.         int i = 0;
    15.         while(1) {
    16.                 if (!status) {
    17.                         printf("child over\n");
    18.                         break;
    19.                 }
    21.                 lock(&gmux);
    22.                 for (i = 0; i < 10; i++) {
    23.                         printf("child %d\n", i + 1);
    24.                 }
    25.                 unlock(&gmux);
    26.                 sched_yield();
    27.         }
    29.         return NULL;
    30. }
    32. int main(int argc, char **argv) {
    34.         int i;
    35.         pthread_t tid;
    37.         pthread_create(&tid, NULL, child, NULL);
    39.         while(1) {
    41.                 sched_yield();
    42.                 lock(&gmux);
    44.                 for (i = 0; i < 100; i++) {
    45.                         printf("parent %d\n", i + 1);
    46.                 }
    48.                 if (100 == cnt) {
    49.                         status = 0;
    50.                         unlock(&gmux);
    51.                         break;
    52.                 }
    53.                 unlock(&gmux);
    55.                 cnt++;
    56.         }
    58.         printf("parent over\n");
    59.         return 0;
    60. }

    解法一：

    1. #include "comm.h"
    3. pthread_mutex_t gmux = PTHREAD_MUTEX_INITIALIZER;
    5. #define lock    pthread_mutex_lock
    6. #define unlock  pthread_mutex_unlock
    8. int cnt = 0;
    9. int sta = 1;
    10. volatile int status = 1;
    12. void *child(void *arg) {
    13.         int i = 0;
    15.         while(1) {
    16.                 do {
    17.                         unlock(&gmux);
    18.                         sched_yield();
    19.                         lock(&gmux);
    20.                 } while (0 == sta);
    22.                 if (!status) {
    23.                         printf("child over\n");
    24.                         break;
    25.                 }
    26.                 for (i = 0; i < 10; i++) {
    27.                         printf("child %d\n", i + 1);
    28.                 }
    29.                 sta = 0;
    30.         }
    32.         return NULL;
    33. }
    35. int main(int argc, char **argv) {
    37.         int i;
    38.         pthread_t tid;
    40.         pthread_create(&tid, NULL, child, NULL);
    42.         while(1) {
    44.                 do {
    45.                         unlock(&gmux);
    46.                         sched_yield();
    47.                         lock(&gmux);
    48.                 } while(1 == sta);
    50.                 for (i = 0; i < 100; i++) {
    51.                         printf("parent %d\n", i + 1);
    52.                 }
    54.                 if (100 == cnt) {
    55.                         status = 0;
    56.                         unlock(&gmux);
    57.                         break;
    58.                 }
    59.                 sta = 1;
    60.                 cnt++;
    61.         }
    63.         printf("parent over\n");
    64.         return 0;
    65. }

    解法二：（用条件变量）