c - 使用套接字读取 char* 时出现段错误

Question

所以我正在用 C 语言编写一个应用程序客户端-服务器，客户端向服务器询问时间，服务器用它进行响应。我正在使用 struct 来做到这一点。当我尝试从 client.c 中的服务器读取数据时，出现段错误。 我在 receiveTimeResponse 处遇到段错误并尝试找出原因。我认为这是因为我没有为 char* date 分配内存，但我认为我分配了内存。

结构:

struct timeStructQuer{
    int header[4];
    int requestID;
};

struct timeStructResponse{
    int header[4];
    int requestID;
    int length;
    char* date;
};

服务器:

int main()
{
    //connection here 
    struct queryStruct myStruct;
    read(client_sockfd, &myStruct, sizeof(myStruct));
    getCurrentTime(client_sockfd, myStruct);
}

void getCurrentTime(int client_sockfd, struct queryStruct str){
    int length;
    char* currentTime = malloc(sizeof(char) * 30);

    time_t rawtime;
    struct tm * timeinfo;
    time (&rawtime);
    timeinfo = localtime (&rawtime);
    length = sprintf (currentTime ,"%s", asctime (timeinfo));

    struct timeStructResponse response;
    response.date = malloc(sizeof(char) * length);
    response.header[0] = 1;
    response.header[1] = 0;
    response.header[2] = 0;
    response.header[3] = 2;
    response.requestID = str.requestID;
    response.length = length;
    response.date = currentTime;

    write(client_sockfd, &response, sizeof(response));
}

客户:

int main()
{
    //connection here
    sendTimeQuery(sockfd);
    receiveTimeResponse(sockfd);

    exit(0);
}

void sendTimeQuery(int sockfd){

    struct queryStruct myStruct;

    int randomN = rand();
    myStruct.header[0] = 0;
    myStruct.header[1] = 0;
    myStruct.header[2] = 0;
    myStruct.header[3] = 2;
    myStruct.requestID = randomN;

    write(sockfd, &myStruct, sizeof(myStruct));
}

void receiveTimeResponse(int sockfd){       
    struct timeStructResponse response;
    response.date = malloc(sizeof(char) * 30);
    printf("seg2");
    read(sockfd, &response, sizeof(response));
    printf("Current time: %s", response.date);
}

Answer 1

read(sockfd, &response, sizeof(response)); 将用无效值覆盖先前分配的 response.data 指针。问题是你不能传输指针(实际上你可以，但指针值在另一端将无效)。您需要传输它们指向的数据。所以写起来应该是这样的:

/* TODO check write results... */
write(client_sockfd, &response.header, sizeof(response.header));
write(client_sockfd, &response.requestID, sizeof(response.requestID));
write(client_sockfd, &response.length, sizeof(response.length));
write(client_sockfd, response.date, response.length);

像这样读:

/* TODO check read and malloc results, length, and date is properly null-terminated... */
read(sockfd, &response.header, sizeof(response.header));
read(sockfd, &response.requestID, sizeof(response.requestID));
read(sockfd, &response.length, sizeof(response.length));
response.date = (char *) malloc(response.length);
read(sockfd, response.date, response.length);