diff --git a/programs/platform.h b/programs/platform.h
index 6135cb582..c69dcb2dd 100644
--- a/programs/platform.h
+++ b/programs/platform.h
@@ -49,10 +49,10 @@ extern "C" {
 *  PLATFORM_POSIX_VERSION = 0 for Unix-like non-POSIX
 *  PLATFORM_POSIX_VERSION >= 1 is equal to found _POSIX_VERSION
 ***************************************************************/
-#if !defined(_WIN32) && (defined(__unix__) || defined(__unix) || (defined(__APPLE__) && defined(__MACH__)) || defined(__midipix__))
+#if !defined(_WIN32) && (defined(__unix__) || defined(__unix) || (defined(__APPLE__) && defined(__MACH__)) || defined(__midipix__) || defined(__VMS))
 	/* UNIX-style OS. ------------------------------------------- */
-#  if (defined(__APPLE__) && defined(__MACH__) || defined(__SVR4) || defined(_AIX) || defined(__hpux) \
-     || defined(__DragonFly__) || defined(__FreeBSD__) || defined(__NetBSD__) || defined(__OpenBSD__) \  /* POSIX.1–2001 (SUSv3) conformant */
+#  if (defined(__APPLE__) && defined(__MACH__)) || defined(__SVR4) || defined(_AIX) || defined(__hpux) \
+     || defined(__DragonFly__) || defined(__FreeBSD__) || defined(__NetBSD__) || defined(__OpenBSD__)  /* POSIX.1–2001 (SUSv3) conformant */
 #    define PLATFORM_POSIX_VERSION 200112L
 #  else
 #    if defined(__linux__) || defined(__linux)
diff --git a/programs/util.h b/programs/util.h
index 179d5ca98..8c171c3b1 100644
--- a/programs/util.h
+++ b/programs/util.h
@@ -63,7 +63,7 @@ extern "C" {
 #  define SET_HIGH_PRIORITY SetPriorityClass(GetCurrentProcess(), REALTIME_PRIORITY_CLASS)
 #  define UTIL_sleep(s) Sleep(1000*s)
 #  define UTIL_sleepMilli(milli) Sleep(milli)
-#elif (defined(__unix__) || defined(__unix) || defined(__VMS) || defined(__midipix__) || (defined(__APPLE__) && defined(__MACH__)))
+#elif PLATFORM_POSIX_VERSION >= 0 /* Unix-like operating system */
 #  include <unistd.h>
 #  include <sys/resource.h> /* setpriority */
 #  include <time.h>         /* clock_t, nanosleep, clock, CLOCKS_PER_SEC */
@@ -73,7 +73,7 @@ extern "C" {
 #    define SET_HIGH_PRIORITY /* disabled */
 #  endif
 #  define UTIL_sleep(s) sleep(s)
-#  if PLATFORM_POSIX_VERSION >= 200112L  /* nanosleep requires POSIX.1-2001 */
+#  if (defined(__linux__) && (PLATFORM_POSIX_VERSION >= 199309L)) || (PLATFORM_POSIX_VERSION >= 200112L)  /* nanosleep requires POSIX.1-2001 */
 #      define UTIL_sleepMilli(milli) { struct timespec t; t.tv_sec=0; t.tv_nsec=milli*1000000ULL; nanosleep(&t, NULL); }
 #  else
 #      define UTIL_sleepMilli(milli) /* disabled */
@@ -300,10 +300,7 @@ UTIL_STATIC int UTIL_prepareFileList(const char *dirName, char** bufStart, size_
     return nbFiles;
 }
 
-#elif (defined(_POSIX_C_SOURCE) && (_POSIX_C_SOURCE >= 200112L)) || \
-      (defined(__APPLE__) && defined(__MACH__)) || defined(__SVR4) || defined(_AIX) || defined(__hpux) || \
-       defined(__DragonFly__) || defined(__FreeBSD__) || defined(__NetBSD__) || defined(__OpenBSD__) || \
-      (defined(__linux__) && defined(_POSIX_C_SOURCE)) /* opendir */
+#elif (defined(__linux__) && (PLATFORM_POSIX_VERSION >= 1)) || (PLATFORM_POSIX_VERSION >= 200112L)  /* opendir, readdir require POSIX.1-2001 */
 #  define UTIL_HAS_CREATEFILELIST
 #  include <dirent.h>       /* opendir, readdir */
 
diff --git a/programs/zstdcli.c b/programs/zstdcli.c
index fad237530..197dd3ff9 100644
--- a/programs/zstdcli.c
+++ b/programs/zstdcli.c
@@ -44,7 +44,7 @@
 #if defined(MSDOS) || defined(OS2) || defined(WIN32) || defined(_WIN32) || defined(__CYGWIN__)
 #  include <io.h>       /* _isatty */
 #  define IS_CONSOLE(stdStream) _isatty(_fileno(stdStream))
-#elif PLATFORM_POSIX_VERSION >= 200112L  /* isatty requires POSIX.1-2001 */
+#elif (defined(__linux__) && (PLATFORM_POSIX_VERSION >= 1)) || PLATFORM_POSIX_VERSION >= 200112L  /* isatty requires POSIX.1-2001 */
 #  include <unistd.h>   /* isatty */
 #  define IS_CONSOLE(stdStream) isatty(fileno(stdStream))
 #else
@@ -470,6 +470,15 @@ int main(int argCount, const char* argv[])
 
     /* Welcome message (if verbose) */
     DISPLAYLEVEL(3, WELCOME_MESSAGE);
+#ifdef _POSIX_C_SOURCE
+    DISPLAYLEVEL(4, "_POSIX_C_SOURCE defined: %ldL\n", (long) _POSIX_C_SOURCE);
+#endif
+#ifdef _POSIX_VERSION
+    DISPLAYLEVEL(4, "_POSIX_VERSION defined: %ldL\n", (long) _POSIX_VERSION);
+#endif
+#ifdef PLATFORM_POSIX_VERSION
+    DISPLAYLEVEL(4, "PLATFORM_POSIX_VERSION defined: %ldL\n", (long) PLATFORM_POSIX_VERSION);
+#endif
 
 #ifdef UTIL_HAS_CREATEFILELIST
     if (recursive) {  /* at this stage, filenameTable is a list of paths, which can contain both files and directories */