Question

The following unix domain socket client and server example from http://www.thomasstover.com/uds.html does not work as expected on my slackware linux box. I get this output:

$ ./server1 
$

$ ./client1 
MESSAGE FROM SERVER: hello from a client

I expected the server to print the message hello from client and the client to print hello from server.

My OS and compiler are as these:

$ uname -a                                                                                                    
Linux temeraire 2.6.37.6-smp #2 SMP Sat Apr 9 23:39:07 CDT 2011 i686 Intel(R) Xeon(R) CPU           E3110  @ 3.00GHz Ge
nuineIntel GNU/Linux
$ gcc -v
Reading specs from /usr/lib/gcc/i486-slackware-linux/4.5.2/specs
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/i486-slackware-linux/4.5.2/lto-wrapper
Target: i486-slackware-linux
Configured with: ../gcc-4.5.2/configure --prefix=/usr --libdir=/usr/lib --mandir=/usr/man --infodir=/usr/info --enable-
shared --enable-bootstrap --enable-languages=ada,c,c++,fortran,java,objc,lto --enable-threads=posix --enable-checking=r
elease --with-system-zlib --with-python-dir=/lib/python2.6/site-packages --disable-libunwind-exceptions --enable-__cxa_
atexit --enable-libssp --enable-lto --with-gnu-ld --verbose --with-arch=i486 --target=i486-slackware-linux --build=i486
-slackware-linux --host=i486-slackware-linux
Thread model: posix
gcc version 4.5.2 (GCC) 

server.c:

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

int connection_handler(int connection_fd)
{
 int nbytes;
 char buffer[256];

 nbytes = read(connection_fd, buffer, 256);
 buffer[nbytes] = 0;

 printf("MESSAGE FROM CLIENT: %s\n", buffer);
 nbytes = snprintf(buffer, 256, "hello from the server");
 write(connection_fd, buffer, nbytes);

 close(connection_fd);
 return 0;
}

int main(void)
{
 struct sockaddr_un address;
 int socket_fd, connection_fd;
 socklen_t address_length;
 pid_t child;

 socket_fd = socket(PF_UNIX, SOCK_STREAM, 0);
 if(socket_fd < 0)
 {
  printf("socket() failed\n");
  return 1;
 } 

 unlink("./demo_socket");

 /* start with a clean address structure */
 memset(&address, 0, sizeof(struct sockaddr_un));

 address.sun_family = AF_UNIX;
 snprintf(address.sun_path, UNIX_PATH_MAX, "./demo_socket");

 if(bind(socket_fd, 
         (struct sockaddr *) &address, 
         sizeof(struct sockaddr_un)) != 0)
 {
  printf("bind() failed\n");
  return 1;
 }

 if(listen(socket_fd, 5) != 0)
 {
  printf("listen() failed\n");
  return 1;
 }

 while((connection_fd = accept(socket_fd, 
                               (struct sockaddr *) &address,
                               &address_length)) > -1)
 {
  child = fork();
  if(child == 0)
  {
   /* now inside newly created connection handling process */
   return connection_handler(connection_fd);
  }

  /* still inside server process */
  close(connection_fd);
 }

 close(socket_fd);
 unlink("./demo_socket");
 return 0;
}

client.c:

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

int main(void)
{
 struct sockaddr_un address;
 int  socket_fd, nbytes;
 char buffer[256];

 socket_fd = socket(PF_UNIX, SOCK_STREAM, 0);
 if(socket_fd < 0)
 {
  printf("socket() failed\n");
  return 1;
 }

 /* start with a clean address structure */
 memset(&address, 0, sizeof(struct sockaddr_un));

 address.sun_family = AF_UNIX;
 snprintf(address.sun_path, UNIX_PATH_MAX, "./demo_socket");

 if(connect(socket_fd, 
            (struct sockaddr *) &address, 
            sizeof(struct sockaddr_un)) != 0)
 {
  printf("connect() failed\n");
  return 1;
 }

 nbytes = snprintf(buffer, 256, "hello from a client");
 write(socket_fd, buffer, nbytes);

 nbytes = read(socket_fd, buffer, 256);
 buffer[nbytes] = 0;

 printf("MESSAGE FROM SERVER: %s\n", buffer);

 close(socket_fd);

 return 0;
}

Answer 1

As Dietrich Epp mentioned, you’re failing to initialize the address_length parameter that gets passed to accept in the server code. According to the man page, this parameter is both an input and output parameter: on input, you tell it how big your address structure is (so that it knows not to write memory out of bounds when telling you the address of the connecting client), and on output it tells you how big the address actually was.

To fix the problem, initialize address_length to sizeof(address) before calling accept. You also should do this for every call to accept, since the variable could be modified. For example:

while(1)
{
  address_length = sizeof(address);
  if((connection_fd = accept(..., &address_length)) == -1)
  {
    printf("accept error: %s\n", strerror(errno));
    break;
  }
  ...
}

I was able to reproduce your problem be explicitly initializing address_length to an invalid value of (sockaddr_t)-1, which caused accept to fail with the EINVAL error (invalid parameter), though oddly it didn’t fail until after the client tried to connect. When the client tried to connect, it actually succeeded, but then when it attempted to write to the socket, that failed with ECONNRESET (“Connection reset by peer”).

Another key point is that you should always check your return values. When dealing with socket code, there are a lot of points of failure, so it pays to be pedantic. If a system call fails, it’ll return -1 and set errno to an appropriate value. You can use the strerror function to convert error codes into human-readable messages, as I did in the example above.