diff --git a/CMakeLists.txt b/CMakeLists.txt
index c3819214..8f5f59c2 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -331,6 +331,12 @@ get_target_property(LDC_LOC ${LDC_EXE} LOCATION)
 #
 # LDMD
 #
+include(CheckSymbolExists)
+CHECK_SYMBOL_EXISTS(_SC_ARG_MAX "unistd.h" HAVE_SC_ARG_MAX)
+if (HAVE_SC_ARG_MAX)
+   add_definitions(-DHAVE_SC_ARG_MAX)
+endif()
+
 set_source_files_properties(dmd2/root/response.c dmd2/root/man.c PROPERTIES LANGUAGE CXX)
 add_executable(${LDMD_EXE} dmd2/root/response.c dmd2/root/man.c driver/ldmd.cpp)
 set_target_properties(${LDMD_EXE} PROPERTIES
diff --git a/driver/ldmd.cpp b/driver/ldmd.cpp
index 567982b9..a4a6dd9c 100644
--- a/driver/ldmd.cpp
+++ b/driver/ldmd.cpp
@@ -55,6 +55,10 @@
 #include "llvm/Support/SystemUtils.h"
 #include "llvm/Support/raw_ostream.h"
 
+#ifdef HAVE_SC_ARG_MAX
+# include <unistd.h>
+#endif
+
 namespace ls = llvm::sys;
 
 // We reuse DMD's response file parsing routine for maximum compatibilty - it
@@ -784,13 +788,15 @@ void buildCommandLine(std::vector<const char*>& r, const Params& p)
  */
 size_t maxCommandLineLen()
 {
-#ifdef WINDOWS
-    // http://blogs.msdn.com/b/oldnewthing/archive/2003/12/10/56028.aspx
-    return 32767;
-#else
+#if defined(HAVE_SC_ARG_MAX)
     // http://www.in-ulm.de/~mascheck/various/argmax – the factor 2 is just
     // a wild guess to account for the enviroment.
     return sysconf(_SC_ARG_MAX) / 2;
+#elif defined(_WIN32)
+    // http://blogs.msdn.com/b/oldnewthing/archive/2003/12/10/56028.aspx
+    return 32767;
+#else
+# error "Do not know how to determine maximum command line length."
 #endif
 }