Usage of errno and thread-safety

Hi,
I saw in the daily snapshot that errno is only used in _libssh2_send()
and _libssh2_recv().
That's good because it permits to better adapt libssh2 to other
platforms.

But I'm guessing if it is not possible to get rid of errno:
errno is a global error information that contain an error code but not
systematically from the actual socket.
Using this variable makes libssh2 not thread safe.

In fact _libssh2_send()/_libssh2_recv() returns only 3 values:
 1) >=0: number of bytes transmitted/received
 2) -EAGAIN: special error which indicates that the read/write
operation should be re-executed.
 3) <0 (but not -EAGAIN): real error.

Would it not be possible to use select to generate such error code?
This would made the _libssh2_send()/_libssh2_recv() thread safe.

For example:
ssize_t _libssh2_recv(libssh2_socket_t sock, void *buffer, size_t
length, int flags)
{
    ssize_t rc = recv(sock, buffer, length, flags);
    if (rc < 0 )
        return -errno;
    return rc;
}

would be rewritten something like:
ssize_t _libssh2_recv(libssh2_socket_t sock, void *buffer, size_t
length, int flags)
{
    ssize_t rc = recv(sock, buffer, length, flags);
    if (rc < 0 )
    {
      int res;
      fd_set sready;
      struct timeval nowait;

      FD_ZERO(&sready);
      FD_SET((unsigned int)sock,&sready);
      memset((char *)&nowait,0,sizeof(nowait));

      res = select(sock+1,&sready,NULL,NULL,&nowait); /* select without
timeout */
      if (res<0)
        return -EIO; /* Select gives an error */
      else if( FD_ISSET(sock,&sready) )
        return -EAGAIN; /* this should not occurs often: recv does not
receive, but just after it does */
      else
        return -EAGAIN; /* socket not ready */
    }
    return rc;
}

I'm not sure if this code works.

I'm waiting for comments.
Sébastien

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