Flush stderr buffer before calling zmq_abort in assert macros

On Windows, the written message does not seem to be guaranteed to be
written to stderr, in particular when stderr is redirected to a file. I
suppose this is because RaiseException terminates the process in a way
that does not give the CRT a chance to flush stdio buffers (or if it
does, there might be a problem when more than one CRT instance is linked
into the program and they overwrite each other's exception handler). Either
way, just make sure the assertion message ends up written to stderr to
ease diagnostics.

src/err.hpp

@@ -78,6 +78,7 @@ namespace zmq
             if (errstr != NULL) {\
                 fprintf (stderr, "Assertion failed: %s (%s:%d)\n", errstr, \
                     __FILE__, __LINE__);\
+                fflush (stderr);\
                 zmq::zmq_abort (errstr);\
             }\
         }\
@@ -90,6 +91,7 @@ namespace zmq
         if (errstr != NULL) {\
             fprintf (stderr, "Assertion failed: %s (%s:%d)\n", errstr, \
                 __FILE__, __LINE__);\
+            fflush (stderr);\
             zmq::zmq_abort (errstr);\
         }\
     } while (false)
@@ -102,6 +104,7 @@ namespace zmq
             zmq::win_error (errstr, 256);\
             fprintf (stderr, "Assertion failed: %s (%s:%d)\n", errstr, \
                 __FILE__, __LINE__);\
+            fflush (stderr);\
             zmq::zmq_abort (errstr);\
         }\
     } while (false)
@@ -116,6 +119,7 @@ namespace zmq
         if (unlikely (!(x))) {\
             fprintf (stderr, "Assertion failed: %s (%s:%d)\n", #x, \
                 __FILE__, __LINE__);\
+            fflush (stderr);\
             zmq::zmq_abort (#x);\
         }\
     } while (false)
@@ -126,6 +130,7 @@ namespace zmq
         if (unlikely (!(x))) {\
             const char *errstr = strerror (errno);\
             fprintf (stderr, "%s (%s:%d)\n", errstr, __FILE__, __LINE__);\
+            fflush (stderr);\
             zmq::zmq_abort (errstr);\
         }\
     } while (false)
@@ -136,6 +141,7 @@ namespace zmq
         if (unlikely (x)) {\
             const char *errstr = strerror (x);\
             fprintf (stderr, "%s (%s:%d)\n", errstr, __FILE__, __LINE__);\
+            fflush (stderr);\
             zmq::zmq_abort (errstr);\
         }\
     } while (false)
@@ -146,6 +152,7 @@ namespace zmq
         if (unlikely (x)) {\
             const char *errstr = gai_strerror (x);\
             fprintf (stderr, "%s (%s:%d)\n", errstr, __FILE__, __LINE__);\
+            fflush (stderr);\
             zmq::zmq_abort (errstr);\
         }\
     } while (false)
@@ -156,6 +163,7 @@ namespace zmq
         if (unlikely (!x)) {\
             fprintf (stderr, "FATAL ERROR: OUT OF MEMORY (%s:%d)\n",\
                 __FILE__, __LINE__);\
+            fflush (stderr);\
             zmq::zmq_abort ("FATAL ERROR: OUT OF MEMORY");\
         }\
     } while (false)