Problem: 我有两个c程序，一个发送UDP消息，然后得到一个响应并打印出来 . 另一个接收UDP消息，修改它并将其发回 . 我正确收到消息，正确修改（打印确定）并发送 . 我在另一端收到消息，但recv返回值为0 .

Question: 为什么我收到正确的消息，但是没有返回消息的长度？这很难检查我收到了正确的长度消息，因为我的目标是访问状态消息的第320个字节

我找到this但我没有使用strlen（），加上缓冲区实际上包含正确的消息 . 我也没有断开套接字，因为我正在使用UDP .

这是客户端的代码：

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <netdb.h>
#include <sys/socket.h>
#define SERVICE_PORT    9090

#define BUFLEN 2048
#define MSGS 5

int main(void)
{
    struct sockaddr_in myaddr, remaddr;
    int fd, i, slen=sizeof(remaddr);
    char *server = "127.0.0.1"; 
    char buf[BUFLEN];
    ssize_t msglen;


    if ((fd=socket(AF_INET, SOCK_DGRAM, 0))==-1)
        printf("socket not created\n");


    memset((char *)&myaddr, 0, sizeof(myaddr));
    myaddr.sin_family = AF_INET;
    myaddr.sin_addr.s_addr = htonl(INADDR_ANY);
    myaddr.sin_port = htons(7096);

    if (bind(fd, (struct sockaddr *)&myaddr, sizeof(myaddr)) < 0) {
        perror("bind failed");
        return 0;
    }

    memset((char *) &remaddr, 0, sizeof(remaddr));
    remaddr.sin_family = AF_INET;
    remaddr.sin_port = htons(SERVICE_PORT);
    if (inet_aton(server, &remaddr.sin_addr)==0) {
        fprintf(stderr, "inet_aton() failed\n");
        exit(1);
    }

    char msg[4096];
    memset(msg, 0, 4096);
    for (i=0; i < MSGS; i++) {
        printf("Sending packet %d to %s port %d\n", i, server, SERVICE_PORT);
        sprintf(buf, "This is packet %d", i);
        if (sendto(fd, buf, sizeof(buf), 0, (struct sockaddr *)&remaddr, slen)==-1)
            perror("sendto");
        memset(msg, 0, 4096);
        if(msglen = recv(fd, msg, sizeof(msg), MSG_WAITALL) == -1){
            printf("Bad recv\n");
        }
        printf("recieved %d bytes: %s\n", msglen, msg);
    }
    close(fd);
    return 0;
}

这是服务器的代码：

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

#include <linux/ip.h> /* for ipv4 header */
#include <linux/udp.h> /* for upd header */

#define ADDR_TO_BIND "127.0.0.1"
#define PORT_TO_BIND 9090
#define SERVICE_PORT 7096
#define MSG_SIZE 256
#define HEADER_SIZE (sizeof(struct iphdr) + sizeof(struct udphdr))

int main(void) {
    int raw_socket, fd;
    struct sockaddr_in sockstr, remaddr;
    socklen_t socklen;

    int retval = 0; 

    char msg[MSG_SIZE];
    ssize_t msglen; 


    if ((raw_socket = socket(AF_INET, SOCK_DGRAM, 0)) == -1) {
        perror("socket");
        return 1; 
    }
    if ((fd = socket(AF_INET, SOCK_DGRAM, 0)) == -1) {
        perror("socket");
        return 1;
    }
    sockstr.sin_family = AF_INET;
    sockstr.sin_port = htons(PORT_TO_BIND);
    sockstr.sin_addr.s_addr = inet_addr(ADDR_TO_BIND);
    socklen = (socklen_t) sizeof(sockstr);
    if (bind(raw_socket, (struct sockaddr*) &sockstr, socklen) == -1) {
        perror("bind");
        retval = 1; 
        goto _go_close_socket;
    }
    if(bind(fd, (struct sockaddr*) &remaddr, socklen) == -1){
        perror("bind");
    }
    memset(msg, 0, MSG_SIZE);
while(1){
    if ((msglen = recv(raw_socket, msg, MSG_SIZE, 0)) == -1) {
        perror("recv");
        retval = 1;
        goto _go_close_socket;
    }
    memset((char *) &remaddr, 0, sizeof(remaddr));
    remaddr.sin_family = AF_INET;
    remaddr.sin_port = htons(SERVICE_PORT);
    if (inet_aton(ADDR_TO_BIND, &remaddr.sin_addr)==0) {
        fprintf(stderr, "inet_aton() failed\n");
        exit(1);
    }
    unsigned char buf[4096];
    msg[msglen] = '\0';
    printf("recieve %d bytes: %s\n", msglen, msg);
    sprintf(buf, "Your msg is: %s\n",msg );
    int slen=sizeof(remaddr);
    if (sendto(fd, buf, sizeof(buf), 0, (struct sockaddr *)&remaddr, slen)==-1){
        printf("send failed");    
    }
    printf("sent: %s\n", buf);
}
_go_close_socket:
    close(raw_socket);

    return retval;
}

运行客户端的输出是：

Sending packet 0 to 127.0.0.1 port 9090
recieved 0 bytes: Your msg is: This is packet 0

Sending packet 1 to 127.0.0.1 port 9090
recieved 0 bytes: Your msg is: This is packet 1

Sending packet 2 to 127.0.0.1 port 9090
recieved 0 bytes: Your msg is: This is packet 2

Sending packet 3 to 127.0.0.1 port 9090
recieved 0 bytes: Your msg is: This is packet 3

Sending packet 4 to 127.0.0.1 port 9090
recieved 0 bytes: Your msg is: This is packet 4

运行服务器的输出是：

recieve 256 bytes: This is packet 0
sent: Your msg is: This is packet 0

recieve 256 bytes: This is packet 1
sent: Your msg is: This is packet 1

recieve 256 bytes: This is packet 2
sent: Your msg is: This is packet 2

recieve 256 bytes: This is packet 3
sent: Your msg is: This is packet 3

recieve 256 bytes: This is packet 4
sent: Your msg is: This is packet 4

如您所见，我在服务器上获得的消息被更改为在开头有“您的消息是：”，我在客户端看到了这一消息，但返回值为0表示长度 . 为什么是这样？

这段代码只是略微修改了示例UDP服务器客户端的代码示例，因此如果指出代码质量不好，我不会被冒犯！

EDIT 这已经解决，这是将recv（）调用包装在括号中的问题，因此条件存储在msglen而不是实际的返回值 . 请参阅下面的答案