Question

我正在尝试使用 sendmsg 和 recvmsg 函数（C语言）通过unix域套接字发送和接收邮件。

这个过程非常简单：

服务器使用 sendmsg 向客户端发送字符串（必须使用 struct msghdr ）;
字符串在main（argv [1]）;
客户端使用 recvmsg ;
客户端将字符串打印到标准输出。

我有以下问题：

1 - 当服务器发送字符串“test”时：

$ ./server test

2 - 客户端正确打印：

$ ./client
print data received > test

但是，

1-当字符串更大（> 7个字符）时，如“hello_world”：

$ ./server hello_world

2 - 客户端打印奇怪的字符（好像没有'\ 0'）：

$ ./client
print data received > hello_woh?!#

问题是否与消息的长度有关？

如何定义邮件的长度才能正确打印？

编辑：服务器源代码：

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/un.h>

#define SOCK_PATH "/tmp/usocket"

int send_msg_to_client(int socketfd, char* data) {

  struct msghdr msg;
  struct iovec iov;
  int s;

  memset(&msg, 0, sizeof(msg));
  memset(&iov, 0, sizeof(iov));

  msg.msg_name = NULL;
  msg.msg_namelen = 0;
  iov.iov_base = data;
  // replace sizeof(data) by strlen(data)+1
  iov.iov_len = strlen(data)+1;
  msg.msg_iov = &iov;
  msg.msg_iovlen = 1;
  msg.msg_control = NULL;
  msg.msg_controllen = 0;
  msg.msg_flags = 0;

  printf("msg: %s\n", (char *) iov.iov_base);

  s = sendmsg(socketfd, &msg, 0);

  if(s < 0){
    perror("sendmsg");
    return 0;
  }

  return s;
}


int main(int argc, char* argv[])
{

        if (argc != 2) {
          printf("Usage: $ %s <data>\n",argv[0]);
          return 0;
        }

    int s, s2, len, slen;
    socklen_t t;
    struct sockaddr_un local, remote;
    char* data = argv[1];

    printf("print data: %s\n",data);

    if ((s = socket(AF_UNIX, SOCK_STREAM, 0)) == -1) {
        perror("socket");
        exit(1);
    }

    memset(&local, 0, sizeof(local));
    local.sun_family = AF_UNIX;
    strcpy(local.sun_path, SOCK_PATH);

    unlink(local.sun_path);

    len = strlen(local.sun_path) + sizeof(local.sun_family);
    if (bind(s, (struct sockaddr *)&local, len) == -1) {
        perror("bind");
        exit(1);
    }

    if (listen(s, 5) == -1) {
        perror("listen");
        exit(1);
    }

    printf("Waiting for a connection...\n");

    t = sizeof(remote);
    if ((s2 = accept(s, (struct sockaddr *)&remote, &t)) == -1) {
        perror("accept");
        exit(1);
    }

    printf("Connected.\n");

    slen = send_msg_to_client(s2, data);

    if(slen < 0)
        perror("send");

    printf("sent data length: %d\n", slen);

    close(s2);

    return 0;
}

编辑：客户端源代码：

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/un.h>

#define SOCK_PATH "/tmp/usocket"
#define MAX 100

int recv_msg_from_server(int socketfd, char data[MAX]) {

  struct msghdr msg;
  struct iovec iov;
  int s;

  memset(&msg, 0, sizeof(msg));
  memset(&iov, 0, sizeof(iov));

  msg.msg_name = NULL;
  msg.msg_namelen = 0;
  iov.iov_base = data;
  //replace sizeof(data) by MAX
  iov.iov_len = MAX;
  msg.msg_iov = &iov;
  msg.msg_iovlen = 1;
  msg.msg_control = NULL;
  msg.msg_controllen = 0;
  msg.msg_flags = 0;


  s = recvmsg(socketfd, &msg, 0);

  if(s < 0){
    perror("recvmsg");
    return 0;
  }

  return s;
}


int main(void)
{
    int s, len, slen;
    struct sockaddr_un remote;
    char data[MAX];

    if ((s = socket(AF_UNIX, SOCK_STREAM, 0)) == -1) {
            perror("socket");
            exit(1);
        }

    printf("Trying to connect...\n");

    memset(&remote, 0, sizeof(remote));

    remote.sun_family = AF_UNIX;
    strcpy(remote.sun_path, SOCK_PATH);
    len = strlen(remote.sun_path) + sizeof(remote.sun_family);

    if (connect(s, (struct sockaddr *)&remote, len) == -1) {
            perror("connect");
            exit(1);
        }

    printf("Connected.\n");

    slen = recv_msg_from_server(s, data);

    if (slen < 0) {
        perror("recvmsg");
    }

    //data[MAX] = '\0';
    printf("print data received > %s\n", data);

    close(s);

    return 0;
}

Answer 1

乍一看，看起来您要将有效负载（iov.iov_len）的长度设置为sizeof(data) sizeof(char*)。猜测你是在64位机器上运行它，这个大小是8个字节，这解释了为什么它适用于较短的字符串。

您可能希望将其设为strlen(data) + 1。

在sendmsg和recvmsg（unix套接字）中定义msghdr长度

1 个答案: