：共享内存

Question

下面的代码有问题：

struct {
    int a, b;
    sem_t client;
    sem_t server;
} *shm_acc;

void server() {
    while(1) {
        sem_wait(&shm_acc->server);

        if(shm_acc->a == -1 && shm_acc->b == -1)
           return;

        printf("The product of the two entered numbers is %d\n", shm_acc->a * shm_acc->b)
        sem_post(&shm_acc->client);
    }
}

void client() {
    while(1) {
        printf("Please enter two unsigned integer with whitespace in between. For exit enter -1 -1.\n");
        if(2 != scanf("%d %d\n", &shm_acc->a, &shm_acc->b) {
            perror("An error occured. Please try again\n");

            while(getchar() != '\n');

            continue;
        }

         sem_post(&shm_acc->server);

         if(shm_acc->a == -1 && shm_acc->b == -1)
            return;

         sem_wait(&shm_acc->client);

     }

     exit(EXIT_SUCCES);
}

int main() {
    setbuf(stdout, NULL);

    int shm = 0;
    shm = shmget(IPC_PRIVATE, SHAREDMEM_SIZE, S_IRUSR | S_IWUSR);
    if(shm == -1)
        perror("Failed to create shared memory.\n");

    shm_acc = shmat(shm, NULL, 0);

    sem_init(&shm_acc->client, 1, 0);
    sem_init(&shm_acc->server, 1, 0);

    pid_t pid = fork();

    if(pid == 0) {

        client();
        shmdt(shm_acc);
    } else if(pid >0) {

        server();
        waitpid(pid, NULL, 0);
        sem_destroy(&shm_acc->server);
        sem_destroy(&shm_acc->client);
    } else if(pid == -1) {

        perror("fork");
        sem_destroy(&shm_acc->server);
        sem_destroy(&shm_acc->client);
    }

    return 0;
}

在上面的代码中，客户机和服务器使用POSIX共享内存相互通信。父进程表示服务器，子进程表示客户端。另外，我正在使用POSIX信号量。服务器必须等到客户端输入两个值后才处理输入，客户端必须等到服务器处理完输入并打印输出后才提示输入值。

问题是，代码并没有表现出应有的效果。

它应该是：

Please enter two unsigned integer with whitespace in between. For exit enter -1 -1.
2 2
The product of the two entered values is 4.
Please enter two unsigned integer with whitespace in between. For exit enter -1 -1.
3 3
The product of the two entered values is 9.
Please enter two unsigned integer with whitespace in between. For exit enter -1 -1.
-1 -1
//exit

但它所做的是：

Please enter two unsigned integer with whitespace in between. For exit enter -1 -1.
2 2 // after entering it doesn't calculate the input
2 3 // after entering again it calculates 2x2
The product of the two entered values is 4.
Please enter two unsigned integer with whitespace in between. For exit enter -1 -1.
3 3
The product of the two entered values is 6. // This is the result of input 2x3, see above
Please enter two unsigned integer with whitespace in between. For exit enter -1 -1.
-1 -1
The product of the two entered values is 9
2 4
// Now it terminates.

我不知道哪里出问题了。有人知道为什么密码会这么做吗？

Answer 1

将\n保留在scanf格式字符串的末尾几乎总是一个问题(因为它不是交互式的，所以对fscanf来说不是问题)，因为格式字符串中的任何空格字符实际上都代表"0或更多(可能无限多)空格字符“--它们可以是任何空白字符(空格、制表符或回车返回)。

因此，当您将\n (或<space>或\t)放在格式字符串的末尾时，scanf将不会停止并返回输入，直到它看到一个非空白字符(或遇到某种匹配错误)。从你的角度看，它看起来好像没有打印提示符，或者处理输入，因为它不是--它仍然在同一个位置。如果您只在行上键入一个数字，也会发生同样的情况--程序将暂停，因为scanf仍在等待您键入第二个数字(与以前一样，您键入的enter键“适合”格式中的空格，因此还没有出现匹配错误)。

由于%d (以及除了%c和%[之外的几乎所有格式说明符)无论如何都忽略了前导空格，所以您通常会发现显式地包含它们是不必要的。