Add ZMQ_MAX_SOCKETS_MAX to zmq_ctx_get()

The new options allows querying the maximum allowed number of sockets.
This is system dependent and cannot be encoded in the include file as a
preprocessor macro: for ZMQ_USE_SELECT, this depends on the FD_SETSIZE
macro at time of library compilation, not at time of include file use.

doc/zmq_ctx_get.txt

@@ -31,6 +31,11 @@ ZMQ_MAX_SOCKETS: Get maximum number of sockets
 The 'ZMQ_MAX_SOCKETS' argument returns the maximum number of sockets
 allowed for this context.
 
+ZMQ_MAX_SOCKETS_MAX: Get largest configurable number of sockets
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The 'ZMQ_MAX_SOCKETS_MAX' argument returns the largest number of sockets
+that linkzmq:zmq_ctx_set[3] will accept.
+
 ZMQ_IPV6: Set IPv6 option
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 The 'ZMQ_IPV6' argument returns the IPv6 option for the context.

doc/zmq_ctx_set.txt

@@ -35,7 +35,8 @@ Default value:: 1
 ZMQ_MAX_SOCKETS: Set maximum number of sockets
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 The 'ZMQ_MAX_SOCKETS' argument sets the maximum number of sockets allowed
-on the context.
+on the context. You can query the maximal allowed value with
+linkzmq:zmq_ctx_get[3] using 'option_name' set to 'ZMQ_MAX_SOCKETS_MAX'.
 
 [horizontal]
 Default value:: 1024

include/zmq.h

@@ -178,6 +178,7 @@ ZMQ_EXPORT void zmq_version (int *major, int *minor, int *patch);
 /*  Context options                                                           */
 #define ZMQ_IO_THREADS  1
 #define ZMQ_MAX_SOCKETS 2
+#define ZMQ_MAX_SOCKETS_MAX 3
 
 /*  Default for new contexts                                                  */
 #define ZMQ_IO_THREADS_DFLT  1

src/ctx.cpp

@@ -24,6 +24,7 @@
 #include <unistd.h>
 #endif
 
+#include <limits>
 #include <new>
 #include <string.h>
 
@@ -204,6 +205,9 @@ int zmq::ctx_t::get (int option_)
     if (option_ == ZMQ_MAX_SOCKETS)
         rc = max_sockets;
     else
+    if (option_ == ZMQ_MAX_SOCKETS_MAX)
+        rc = clipped_maxsocket (std::numeric_limits<int>::max());
+    else
     if (option_ == ZMQ_IO_THREADS)
         rc = io_thread_count;
     else

tests/test_ctx_options.cpp

@@ -17,6 +17,7 @@
     along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
+#include <limits>
 #include "testutil.hpp"
 
 int main (void)
@@ -29,6 +30,13 @@ int main (void)
     assert (ctx);
     
     assert (zmq_ctx_get (ctx, ZMQ_MAX_SOCKETS) == ZMQ_MAX_SOCKETS_DFLT);
+#if defined(ZMQ_USE_SELECT)
+    assert (zmq_ctx_get (ctx, ZMQ_MAX_SOCKETS_MAX) == ZMQ_MAX_SOCKETS_DFLT);
+#elif    defined(ZMQ_USE_POLL) || defined(ZMQ_USE_EPOLL)     \
+      || defined(ZMQ_USE_DEVPOLL) || defined(ZMQ_USE_KQUEUE)
+    assert (zmq_ctx_get (ctx, ZMQ_MAX_SOCKETS_MAX)
+                              == std::numeric_limits<int>::max());
+#endif
     assert (zmq_ctx_get (ctx, ZMQ_IO_THREADS) == ZMQ_IO_THREADS_DFLT);
     assert (zmq_ctx_get (ctx, ZMQ_IPV6) == 0);