Question

I am having a weird problem. I am trying to create a unix socket in a directory but it does not create the one I want. I have cut down the code to the below example.

#include <iostream>
#include <string>
#include <cstdlib>
#include <string.h>
#include <sys/un.h>
#include <sys/socket.h>
#include <errno.h>
#include <limits.h>

int main(int argc, char *argv[])
{
   std::string socketname(argv[1]);

   socketname += "my_socket";

    int  fd;
    int  result;
    struct  sockaddr_un addr;

    fd = socket(AF_UNIX, SOCK_STREAM, 0);

    if (fd == -1)
    {
        std::cerr << "socket returned " << errno << ": " << strerror(errno) << std::endl;
        exit(1);
    }

    memset(&addr, 0, sizeof(struct sockaddr_un));

    addr.sun_family = AF_UNIX;

    strncpy(addr.sun_path, socketname.c_str(), sizeof(addr.sun_path) - 1);

    result = bind(fd, (struct sockaddr *) &addr, sizeof(struct sockaddr_un));

    if (result == -1)
    {
        std::cerr << "bind returned " << errno << ": " << strerror(errno ) << std::endl;
        exit(1);
    }

    return 0;
}

The problem is that when I run program like

./a.out /home/rasterblaster/local/media/video/television/UnitedStates/series/Californication/SeasonOne/xvid-conversions

I do not get “my_socket”. Instead I find some random socket named like “xvid-convers”.

/home/rasterblaster/local/media/video/television/UnitedStates/series/Californication/SeasonOne$ ls -la

drwxrwxrwx 3 rasterblaster rasterblaster  4096 2012-03-08 22:06 .
drwxrwxrwx 3 rasterblaster rasterblaster  4096 2012-03-08 21:39 ..
srwxrwxr-x 1 rasterblaster rasterblaster  0 2012-03-08 22:06:08 xvid-convers
drwxrwxrwx 2 rasterblaster rasterblaster  4096 2012-03-08 21:39 xvid-conversions

What am I doing wrong?

Answer 1

There is nothing really weird going on. If you look at the result of sizeof(addr.sun_path) it should be about 100 or so bytes. When you try to copy more than that size into it with strncpy() you are truncating the path to what fits into sun_path and bind() is just creating a filesystem entry for any name beyond the last valid directory name.

There is no benefit to placing a socket this deep into a directory structure. You can just as easily put it in /tmp or /run or some common directory that just clients and the server have permission if you want a bit more security.

Also, you probably want

socketname += "/my_socket";  // prefix with slash

just in case the path named passed doesn’t end in a slash.