并发：线程和锁

#include<stdio.h>
#include<pthread.h>

pthread_mutex_t lock;

//pthread_mutex_lock(&lock);
static volatile int counter = 0;
//pthread_mutex_unlock(&lock);

void* myLockThread(void* arg)
{
        printf("%s: begin\n", (char*) arg);
        int i;
        //for (i=0; i<1e7;++i) ++counter;
        for (i = 0; i < 1e8; ++i)
        {
                pthread_mutex_lock(&lock);
                ++counter;
                pthread_mutex_unlock(&lock);
        }
        printf("%s: done\n", (char*) arg);
        return NULL;
}

void* myThread(void* arg)
{
        printf("%s: begin\n", (char*) arg);
        int i;
        for (i=0; i<1e8; ++i) ++counter;
        printf("%s:done\n", (char*) arg);
        return NULL;
}

int main(int argc, char* argv[])
{
        pthread_t p1, p2;
        printf("main: begin (counter = %d)\n", counter);
        pthread_create(&p1, NULL, myLockThread, "A");
        pthread_create(&p2, NULL, myLockThread, "B");

        // join waits for the threads to finish
        pthread_join(p1, NULL);
        pthread_join(p2, NULL);
        printf("main: done with both (counter = %d)\n", counter);
        return 0;
}

time ./lock
main: begin (counter = 0)
A: begin
B: begin
A: done

B: done
main: done with both (counter = 200000000)

real    0m10.328s
user    0m13.250s
sys     0m6.297s


time ./a.out
main: begin (counter = 0)
A: begin
B: begin
B:done
A:done
main: done with both (counter = 116098548)

real    0m0.605s
user    0m1.172s
sys     0m0.000s

可见对让线程在临界区持有锁的做法，由于循环的存在，大幅度降低了性能，从而不如简单的同步实现