mbedtls_net_context: make fd public on Unix/POSIX platforms

On platforms with BSD-like sockets, it is useful for applications to have
access to the underlying file descriptor so that they can use functions like
select() and poll().

Do not promise that the field will exist on other platforms such as
Windows (where the type and name of the field are technically wrong because
Windows socket handles are actually not file descriptors).

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

include/mbedtls/net_sockets.h

@@ -94,7 +94,13 @@ extern "C" {
  */
 typedef struct mbedtls_net_context
 {
-    int MBEDTLS_PRIVATE(fd);             /**< The underlying file descriptor                 */
+    /** The underlying file descriptor.
+     *
+     * This field is only guaranteed to be present on POSIX/Unix-like platforms.
+     * On other platforms, it may have a different type, have a different
+     * meaning, or be absent altogether.
+     */
+    int fd;
 }
 mbedtls_net_context;
 

programs/ssl/mini_client.c

@@ -246,13 +246,13 @@ int main( void )
     addr.sin_addr.s_addr = *((char *) &ret) == ret ? ADDR_LE : ADDR_BE;
     ret = 0;
 
-    if( ( server_fd.MBEDTLS_PRIVATE(fd) = socket( AF_INET, SOCK_STREAM, 0 ) ) < 0 )
+    if( ( server_fd.fd = socket( AF_INET, SOCK_STREAM, 0 ) ) < 0 )
     {
         ret = socket_failed;
         goto exit;
     }
 
-    if( connect( server_fd.MBEDTLS_PRIVATE(fd),
+    if( connect( server_fd.fd,
                 (const struct sockaddr *) &addr, sizeof( addr ) ) < 0 )
     {
         ret = connect_failed;