c - 如果不操纵它如何改变值(value)？

Question

关闭。此题需要 details or clarity 。目前不接受答案。

---

想要改进这个问题吗？通过 editing this post 添加详细信息并澄清问题.

已关闭5 年前。

Improve this question

服务器:

void send_list(sqlite3 *db, int sock) {

    int Fwrite,Fread;
    int id_order;
    int bet2=0;
    Booking info;
    char query2[200];


    Fread = FullRead(sock,&bet2,sizeof(bet2));

    if(Fread <0)
    {
        perror("Error to Recive \n");
        exit(1);
    }



    Fwrite = FullWrite(sockREP,&bet2,sizeof(bet2));

    if(Fwrite < 0)
    {
        perror("Error to Send \n");
        exit(1);
    }

    printf("bet2: %d\n",bet2 );

    Fread = FullRead(sock,&info.NAME,sizeof(info));

    if(Fread <0)
    {
        perror("Error to Recive \n");
        exit(1);
    }

    Fread = FullRead(sock,&info.SURNAME,sizeof(info));

    if(Fread <0)
    {
        perror("Error to Recive \n");
        exit(1);
    }

    Fread = FullRead(sock,&info.COD_RIC,sizeof(info));

    if(Fread <0)
    {
        perror("Error to Recive \n");
        exit(1);
    }



    struct timeval time;
    gettimeofday(&time, NULL);
    srand((time.tv_sec*1000)+(time.tv_sec/1000));

    id_order = rand() %100 + 1;


    Fwrite = FullWrite(sockREP,&info.NAME,sizeof(info));

    if(Fwrite < 0)
    {
        perror("Error to Send \n");
        exit(1);
    }

    Fwrite = FullWrite(sockREP,&info.SURNAME,sizeof(info));

    if(Fwrite < 0)
    {
        perror("Error to Send \n");
        exit(1);
    }

    Fwrite = FullWrite(sockREP,&info.COD_RIC,sizeof(info));

    if(Fwrite < 0)
    {
        perror("Error to Send \n");
        exit(1);
    }

    Fwrite = FullWrite(sockREP,&id_order,sizeof(id_order));

    if(Fwrite < 0)
    {
        perror("Error to Send \n");
        exit(1);
    }

printf("this bet2: %d\n",bet2 );


    Fwrite = FullWrite(sock,&id_order,sizeof(id_order));

    if(Fwrite < 0)
    {
        perror("Error to Send \n");
        exit(1);
    }
}

服务器2(SockREP):

void recv_list(sqlite3 *db, int sock) {

 int Fwrite,Fread;
 int id_order;
 int bet2=0;
 Booking info;  


    Fread = FullRead(sock,&bet2,sizeof(bet2));

    if(Fread < 0)
    {
       perror("Error to Recive \n");
       exit(1);
    }

    Fread = FullRead(sock,&info.NAME,sizeof(Booking));

    if(Fread < 0)
    {
        perror("Error to Receive \n");
        exit(1);
    }

    Fread = FullRead(sock,&info.SURNAME,sizeof(Booking));

    if(Fread < 0)
    {
        perror("Error to Receive \n");
        exit(1);
    }

    Fread = FullRead(sock,&info.COD_RIC,sizeof(Booking));

    if(Fread < 0)
    {
        perror("Error to Receive \n");
        exit(1);
    }

    Fread = FullRead(sock,&id_order,sizeof(id_order));

    if(Fread < 0)
    {
        perror("Error to Send \n");
        exit(1);
    }
}

客户端( socks ):

void getMenu(int sock)
{
    int Fwrite,Fread;
    int id_order;
    int bet2=0;
    Booking info;


    printf("Select Data: ");
    scanf("%d",&bet2);

    Fwrite = FullWrite (sock,&bet2,sizeof(bet2));

    if(Fwrite < 0)
    {
        perror("Error to Send \n");
        exit(1);
    }


    printf("Name: ");
    scanf("%s",&info.NAME);

    printf("Surname: ");
    scanf("%s",&info.SURNAME);

    printf("Medical Code: ");
    scanf("%s",&info.COD_RIC);


    Fwrite = FullWrite(sock,&info.NAME,sizeof(Booking));

    if(Fwrite <0)
    {
        perror("Error to Recive \n");
        exit(1);
    }

    Fwrite = FullWrite(sock,&info.SURNAME,sizeof(Booking));

    if(Fwrite <0)
    {
        perror("Error to Recive \n");
        exit(1);
    }

    Fwrite = FullWrite(sock,&info.COD_RIC,sizeof(Booking));

    if(Fwrite <0)
    {
        perror("Error to Recive \n");
        exit(1);
    }



    Fread = FullRead(sock,&id_order,sizeof(id_order));

    if(Fread < 0)
    {
        perror("Error to Send \n");
        exit(1);
    }

}

我的问题是:

为什么在服务器中变量bet2在3次完整读取后改变值:

1) Fread = FullRead(sock,&info.NAME,sizeof(info));

2) Fread = FullRead(sock,&info.SURNAME,sizeof(info));

3) Fread = FullRead(sock,&info.COD_RIC,sizeof(info));

我尝试使用read()，但结果是相等的

FullRead函数是这样的:

ssize_t FullRead(int fd, void *buf, size_t count)
{
    size_t nleft = count;
    ssize_t nread;
    while (nleft > 0) 
    {
        if ( (nread = read(fd, buf, nleft)) < 0) 
        {
            if (errno == EINTR)
                continue;
            else 
                exit(nread);
        } 
        else if (nread == 0)
            break;
        nleft -= nread;
        buf += nread;
    }
    buf = 0;
    return (nleft);
}

FullWrite 函数是这样的:

ssize_t FullWrite(int fd, const void *buf, size_t count)
{
    size_t nleft;
    ssize_t nwritten;
    nleft = count;
    while (nleft > 0) {             /* repeat until no left */
        if ( (nwritten = write(fd, buf, nleft)) < 0) {
            if (errno == EINTR) {   /* if interrupted by system call */
                continue;           /* repeat the loop */
            } else {
                return(nwritten);   /* otherwise exit with error */
            }
        }
        nleft -= nwritten;          /* set left to write */
        buf +=nwritten;             /* set pointer */
    }
    return (nleft);
}

Answer 1

如果我正确理解你的问题，你是在问各种变量(info.NAME、info.SURNAME、bet2等)的值如何改变，因为你的代码中没有任何内容实际上使用 = 运算符设置这些值.

答案是指针别名。例如，info.SURNAME 位于计算机内存中的特定位置。可以通过 & 运算符创建指向该位置的指针，例如void * ptr = &info.SURNAME 。

因此，当您像这样调用 FullRead 时:

Fread = FullRead(sock,&info.SURNAME,sizeof(Booking));

...您向 FullRead 函数提供有关您的 info.SURNAME 字符数组在内存中的位置的信息，以及该字符数组的大小(以字节为单位)。

有了这些信息，FullRead() 现在可以将新字节写入 char 数组。例如它可能会做这样的事情:

int FullRead(int sock, char * buf, int numBytesInBuf)
{
   // the real code would read bytes from the network but
   // to keep this example simple I'll just declare some dummy data
   char dummyData[] = "blah blah blah";

   int numBytesToCopy = std::min(numBytesInBuf, sizeof(dummyData));
   for (int i=0; i<numBytesToCopy; i++) buf[i] = dummyData[i];
   return numBytesToCopy;
}

...这会将数据写入 buf 指向的内存，也就是说，写入 info.SURNAME 。

鉴于 bet2 不是 char 数组而是 int，那么这将如何与 bet2 一起使用？

它的工作方式完全相同 - bet2 可能是一个 int，但它仍然在内存中具有一个位置，并且大小以字节为单位(通常为 4 或 8 字节，具体取决于您使用的计算机类型)，所以同样的技巧也适用...FullRead()可以写入bet2所在的内存区域，从而通过更新其字节来改变bet2的值。

 Fread = FullRead(sock,&bet2,sizeof(bet2));  // writes new value into bet2!