putchar的线程安全

       前面调用的stdio函数一般都是printf和fgets。这些函数都是一次读入或输出一串数据。他们的操作是满足原子性的。ANSI C中同时还有putchar和getchar这样的函数。在新版本的putchar和getchar都是有加锁操作的，用来保护缓冲区不被破坏。我开始觉得这有点不好理解，以为单个字符原子性是可以保证的，其实不然。假设putchar用如下的伪代码实现:

int putchar(char c, FILE *stream){//获取缓冲区的队尾指针char *tail = gettail(stream);//移动指针tail++;//将字符写入缓冲区*(tail) = c;return 0；}

        问题一下就暴露出来了，两个线程都在调用putchar()，缓冲区的信息就会成为互斥资源。比如如下的情形，线程一在tail++执行之前被挂起了，线程二进入运行，它完整的执行了一次putchar。随后线程一又重新开始运行，它并不知道它获得的tail指针已经不是指向缓冲区末端了，它后续的写入操作将会覆盖掉线程二的写入数据。这大概就是传说中的缓冲区破坏了吧！

        现在的getchar和putchar都是可以保护缓冲区不被破坏的，但是保护就意味着加锁和效率降低。为了兼顾效率，ANSI C里面还提供了getchar_unlock和putchar_unlock这些不实现缓冲区保护的快速函数。不过使用这些函数的时候就要注意加flockfile和funlockfile自行对输入输出加锁。下面的代码展示了各函数的区别：

/* * putchar.c * * Demonstrate use of stdio file locking to generate an "atomic" * sequence of character writes (using putchar). If run with an * argument of "1", or no argument, the program uses a sequence * of putchar_unlocked calls within flockfile/funlockfile to * ensure that one threads writes cannot be interleaved with * another's. * * With an argument of "0", the program uses putchar, without * file locks, to show that the writes may be interleaved. * * The putchar[_unlocked] loop is punctuated with sleep(1) calls * to ensure that the desired behavior is demonstrated. Without * some delay, even on a multiprocessor the program may often * fail to display the interleaved output in this simplified * case. * * With file locking, you can expect to see the following output: * *      thread 1 *      thread 2 *      thread 3 * * While without file locking, you can expect to see something * much less predictable, but probably resembling this: * *      ttthhhiiisss   iiisss   ttthhhrrreeeaaaddd   123 * */#include <pthread.h>#include "errors.h"/* * This function writes a string (the function's arg) to stdout, * by locking the file stream and using putchar_unlocked to * write each character individually. */void *lock_routine (void *arg){    char *pointer;    flockfile (stdout);    for (pointer = arg; *pointer != '\0'; pointer++) {        putchar_unlocked (*pointer);        sleep (1);    }    funlockfile (stdout);    return NULL;}/* * This function writes a string (the function's arg) to stdout, * by using putchar to write each character individually. * Although the internal locking of putchar prevents file stream * corruption, the writes of various threads may be interleaved. */void *unlock_routine (void *arg){    char *pointer;    for (pointer = arg; *pointer != '\0'; pointer++) {        putchar (*pointer);        sleep (1);    }    return NULL;}int main (int argc, char *argv[]){    pthread_t thread1, thread2, thread3;    int flock_flag = 1;    void *(*thread_func)(void *);    int status;    if (argc > 1)        flock_flag = atoi (argv[1]);    if (flock_flag)        thread_func = lock_routine;    else        thread_func = unlock_routine;    status = pthread_create (        &thread1, NULL, thread_func, "this is thread 1\n");    if (status != 0)        err_abort (status, "Create thread");    status = pthread_create (        &thread2, NULL, thread_func, "this is thread 2\n");    if (status != 0)        err_abort (status, "Create thread");    status = pthread_create (        &thread3, NULL, thread_func, "this is thread 3\n");    if (status != 0)        err_abort (status, "Create thread");    pthread_exit (NULL);}

        当你选择unlock_routine时，它调用的putchar可以保证缓冲区不被破坏，但很可能会出现字符顺序错误的情况。选用lock_routine时，自己加的文件锁不仅保证了缓冲区不被破坏，还可以保证字符出现的顺序。