Need to print char* data of a structure in server.c program coming from client.c through UDP socket program C

Question

我正在做我的學校作業，這要求我將一個包含字符指針的結構從客戶端程序傳遞到服務器，並在 Server.c 程序中打印結構數據。 I am being able to pass the structure properly and being able to retrieve the integer data from the structure in server.c, but I am getting an error "Segmentation fault" when I try to print the char* data in the server.c program來自 Client.c 程序。 我如何解決它？

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<string.h>
#include<netinet/in.h>
#include<arpa/inet.h>
#include<sys/types.h>
#include<sys/socket.h>

#define PORT 8080

struct StudentInfo
{
    char* name;
    int roll;

}student;


int main()
{
    student.name=(char*)malloc( 50 * sizeof(char));
    student.name="Sara You";
    student.roll=124;


    int network_socket=socket(AF_INET, SOCK_DGRAM, 0);

    struct sockaddr_in serveraddr;
    memset(&serveraddr, 0, sizeof(serveraddr));

    serveraddr.sin_family=AF_INET;
    serveraddr.sin_port=htons(PORT);
    serveraddr.sin_addr.s_addr=INADDR_ANY;

    sendto(network_socket, &student, sizeof(student),0, (struct sockaddr*) &serveraddr, sizeof(serveraddr));

    close(network_socket);

}

服務器.c

#include<stdio.h>
#include<unistd.h>
#include<stdlib.h>
#include<sys/types.h>
#include<arpa/inet.h>
#include<netinet/in.h>
#include<sys/socket.h>
#include<string.h>

#define PORT 8080


struct StudentInfo
{
    char* name;
    int roll;

} student1;

int main()
{
    student1.name=(char*)malloc( 50 * sizeof(char));
    struct sockaddr_in serveraddr, cliaddr;
    int network_socket=socket(AF_INET, SOCK_DGRAM, 0);

    memset(&serveraddr,0,sizeof(serveraddr));
    memset(&cliaddr,0,sizeof(cliaddr));

    serveraddr.sin_family=AF_INET;
    serveraddr.sin_port=htons(PORT);
    serveraddr.sin_addr.s_addr=INADDR_ANY;

    bind(network_socket, (struct sockaddr*) &serveraddr, sizeof(serveraddr));


    int len=sizeof(cliaddr);
    recvfrom(network_socket, &student1, sizeof(student1), 0, (struct sockaddr*) &cliaddr, &len);
    printf("Name %s: ", student1.name);     //Segmentation fault
    printf("Name %d: ", student1.roll);



}

Answer 1

如果您不介意將學生姓名限制為小於 100 個字節，則可以輕松解決此問題。 看起來您正在發送整個結構而沒有考慮字節順序。 因此，如果客戶端和服務器上的字節順序匹配，那么只需更改結構定義即可解決問題。 當然，通過將 memory 作為結構的名稱部分，您不必分配 memory 並且您必須使用strcpy而不是分配指針。 像這樣：

struct StudentInfo {
    char name[100];
    int  roll;
} student;

int main (int argc, char *argv[]) {
    //student.name=(char*)malloc( 50 * sizeof(char));
    //student.name="Sara You";
    strcpy(student.name, "Sara You"); // check length, truncate, or send multiple messages

更新 1

如果您必須序列化您發送的內容並反序列化您收到的內容，那么您可以使用這兩個函數返回可以free的內容：

char * serializeStudent (struct StudentInfo *stu) {
    char *rval;
    if (!stu || !stu->name) {
        // errno = EINVAL;
        return 0;
    }
    rval = malloc(strlen(stu->name) + 21); // never cast malloc
    if (!rval) {
        return 0; // errno will be set by malloc
    }
    sprintf(rval, "%10d%10d%s", stu->roll, strlen(stu->name), stu->name);
    return rval;
}

發送前調用serializeStudent ，發送它返回的字符串（如果不為null），發送成功后free該字符串。

struct StudentInfo * deserializeSerializedStudent (char *ser) {
    struct StudentInfo *rval;
    if (!ser) {
        // errno = EINVAL;
        return 0;
    }
    rval = malloc(sizeof(*rval));
    if (!rval) {
        return 0; // errno will be set by malloc
    }
    int len;
    int n = sscanf(ser, "%10d%10d", rval->roll, &len);
    if (n != 2) {
        // errno = EINVAL;
        free(rval);
        return 0;
    }
    rval->name = malloc(len+1);
    if (!rval->name) {
        free(rval);
        return 0; // errno will be set by malloc
    }
    n = sscanf(ser+20, "%*s", len, rval->name);
    if (n != 1) {
        // errno = EINVAL;
        free(rval);
        return 0;
    }
    return rval;
}

在收到您知道的完整字符串后調用deserializeSerializedStudent 。 然后對結構做你需要的事情，然后在釋放它包含的字符串后釋放它。