Today learn gstshmsink plugin,main_socket and shm_area I don't understand

[zhang007z]

in according to gstshmsink.c code snippet as below:
//-------------------------------------------------------------//
ShmPipe *
sp_writer_create (const char *path, size_t size, mode_t perms)
{
  ShmPipe *self = spalloc_new (ShmPipe);
  int flags;
  struct sockaddr_un sock_un;
  int i = 0;

  memset (self, 0, sizeof (ShmPipe));

  self->main_socket = socket (PF_UNIX, SOCK_STREAM, 0);
  self->use_count = 1;

  if (self->main_socket < 0)
    RETURN_ERROR ("Could not create socket (%d): %s\n", errno,
        strerror (errno));

  flags = fcntl (self->main_socket, F_GETFL, 0);
  if (flags < 0)
    RETURN_ERROR ("fcntl(F_GETFL) failed (%d): %s\n", errno, strerror
(errno));

  if (fcntl (self->main_socket, F_SETFL, flags | O_NONBLOCK | FD_CLOEXEC) <
0)
    RETURN_ERROR ("fcntl(F_SETFL) failed (%d): %s\n", errno, strerror
(errno));

  sock_un.sun_family = AF_UNIX;
  strncpy (sock_un.sun_path, path, sizeof (sock_un.sun_path) - 1);

  while (bind (self->main_socket, (struct sockaddr *) &sock_un,
          sizeof (struct sockaddr_un)) < 0) {
    if (errno != EADDRINUSE)
      RETURN_ERROR ("bind() failed (%d): %s\n", errno, strerror (errno));

    if (i > 256)
      RETURN_ERROR ("Could not find a free socket name for %s", path);

    snprintf (sock_un.sun_path, sizeof (sock_un.sun_path), "%s.%d", path,
i);
    i++;
  }

  self->socket_path = strdup (sock_un.sun_path);

  if (chmod (self->socket_path, perms) < 0)
    RETURN_ERROR ("failed to set socket permissions (%d): %s\n", errno,
        strerror (errno));

  if (listen (self->main_socket, LISTEN_BACKLOG) < 0)
    RETURN_ERROR ("listen() failed (%d): %s\n", errno, strerror (errno));

  self->shm_area = sp_open_shm (NULL, ++self->next_area_id, perms, size);

  self->perms = perms;

  if (!self->shm_area)
    RETURN_ERROR ("Could not open shm area (%d): %s", errno, strerror
(errno));

  return self;
}
//-----------------------------------------------------------------------------------//
this function creates two file descriptors:
(1)self->main_socket = socket (PF_UNIX, SOCK_STREAM, 0);
(2)self->shm_area = sp_open_shm (NULL, ++self->next_area_id, perms, size);
I don't understand that what effect there is  between this two file
descriptors?  by this plugsin name I think   it use shared-memory technology
for transforming of data,but I don't understand that socket function create
fd and what purpose it use this fd?
if anybody have any idea for me ,thank you in advance.   



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