Problem: In rare cases, afunix.h doesn't contain a definition for struct sockaddr_un (#4310)

* Problem: In rare cases, afunix.h doesn't contain a definition for struct sockaddr_un

According to https://github.com/microsoft/vcpkg/issues/21623,
struct sockaddr_un might be unavailable on some machines even afunix.h exists.

For example, on some machines, the content of afunix.h looks like this:
typedef struct _SOCKADDR_UN
{
     ADDRESS_FAMILY Family;
     wchar_t Path[63];
} SOCKADDR_UN, *PSOCKADDR_UN;

but on other machines, it may looks like this:
#define UNIX_PATH_MAX 108

typedef struct sockaddr_un
{
    ADDRESS_FAMILY sun_family;
    char sun_path[UNIX_PATH_MAX];
} SOCKADDR_UN, *PSOCKADDR_UN;

Fixes #3949

References:
- [Enable Unix-domain sockets support on Windows](8f3ec75de4)
- [AF_UNIX equivalent for Windows](https://stackoverflow.com/questions/9029174/af-unix-equivalent-for-windows)
- https://devblogs.microsoft.com/commandline/af_unix-comes-to-windows/

CMakeLists.txt

@@ -15,6 +15,7 @@ include(CheckCSourceCompiles)
 include(CheckCSourceRuns)
 include(CMakeDependentOption)
 include(CheckCXXSymbolExists)
+include(CheckStructHasMember)
 include(CheckTypeSize)
 include(FindThreads)
 include(GNUInstallDirs)
@@ -495,8 +496,12 @@ endif()
 
 if(NOT WIN32)
   set(ZMQ_HAVE_IPC 1)
+  set(ZMQ_HAVE_STRUCT_SOCKADDR_UN 1)
 else()
   check_include_files("winsock2.h;afunix.h" ZMQ_HAVE_IPC)
+  if(ZMQ_HAVE_IPC)
+    check_struct_has_member("struct sockaddr_un" sun_path "winsock2.h;afunix.h" ZMQ_HAVE_STRUCT_SOCKADDR_UN)
+  endif()
 endif()
 
 # ##################### BEGIN condition_variable_t selection

builds/cmake/platform.hpp.in

@@ -59,6 +59,7 @@
 #cmakedefine ZMQ_HAVE_LIBBSD
 
 #cmakedefine ZMQ_HAVE_IPC
+#cmakedefine ZMQ_HAVE_STRUCT_SOCKADDR_UN
 
 #cmakedefine ZMQ_USE_BUILTIN_SHA1
 #cmakedefine ZMQ_USE_NSS

src/windows.hpp

@@ -98,3 +98,13 @@ static inline int poll (struct pollfd *pfd, unsigned long nfds, int timeout)
 #define snprintf(buffer_, count_, format_, ...)                                \
     _snprintf_s (buffer_, count_, _TRUNCATE, format_, __VA_ARGS__)
 #endif
+
+//  Workaround missing struct sockaddr_un in afunix.h.
+//  Fix #3949.
+#if defined(ZMQ_HAVE_IPC) && !defined(ZMQ_HAVE_STRUCT_SOCKADDR_UN)
+struct sockaddr_un
+{
+    ADDRESS_FAMILY sun_family; /* AF_UNIX */
+    char sun_path[108];        /* pathname */
+};
+#endif