Define msghdr length in sendmsg and recvmsg (unix sockets)

Question

I'm trying to send and receive messages through unix domain sockets, using sendmsg and recvmsg functions (C language).

The process is very simple:

• the server sends a string to the client using sendmsg (must use struct msghdr);
• the string is passed as argument in main (argv[1]);
• the client receives the string using recvmsg;
• the client prints the string to the stdout.

I have the following problem:

1 - When the server sends the string "test":

$ ./server test

2 - The client prints it correctly:

$ ./client
print data received > test

But,

1- When the string is bigger (> 7 chars), like "hello_world":

$ ./server hello_world

2 - The client prints strange characters (as if there was no '\0'):

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

Is the problem related to the length of the message?

How can I define the length of the message in order to print it correctly?

EDIT: Server source code:

#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;
}

EDIT: Client source code:

#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

At the first glance, looks like you are setting the length of payload (iov.iov_len) to sizeof(data) which is sizeof(char*). Guessing that you are running it on a 64-bit machine, this size is 8 bytes, which explains why it did work for shorter strings.

You probably want to make it strlen(data) + 1.