fixes #375 integer types are error prone

This change converts the various integer types like nng_socket
in the public API to opaque structures that are passed by value.
Basically we just wrap the integer ID.  This "hack" give us strong
type checks by the compiler (yay!), at the expense of not being able
to directly use these as numbers (so comparisions for example don't
work, and neither does initialization to zero using the normal
method.

Comparison of disassembly output shows that at least with the optimizer
enabled there is no difference in the compiler output between using
a structure or an integral value.

1 files changed, 5 insertions, 4 deletions

diff --git a/docs/man/nng_socket.5.adoc b/docs/man/nng_socket.5.adoc
index 0dcdfa99..4711495f 100644
--- a/docs/man/nng_socket.5.adoc
+++ b/docs/man/nng_socket.5.adoc
@@ -19,7 +19,7 @@ nng_socket - socket handle
 ----
 #include <nng/nng.h>
 
-typedef uint32_t nng_socket;
+typedef struct nng_socket_s nng_socket;
 ----
 
 == DESCRIPTION
@@ -34,9 +34,10 @@ may be connected to multiple transports at the same time.
 However, a given socket will have exactly one "`protocol`" associated with it,
 and is responsible for any state machines or other protocol-specific logic.
 
-NOTE: Although `nng_socket` is an integer data type, these objects are not
-ordinary file descriptors, and can only be used with the functions that
-explicitly indicate that it safe and appropropate to do so.
+IMPORTANT: The `nng_socket` structure is always passed by value (both
+for input parameters and return values), and should be treated opaquely.
+Passing structures this way ensures gives the compiler a chance to perform
+accurate type checks in functions passing values of this type.
 
 Each `nng_socket` is created by a protocol-specific constructor, such as
 <<nng_rep_open.3#,`nng_rep_open()`>>.