diff --git a/lib/common/zstd_deps.h b/lib/common/zstd_deps.h
index 4d767ae9b..f41a973c3 100644
--- a/lib/common/zstd_deps.h
+++ b/lib/common/zstd_deps.h
@@ -24,6 +24,18 @@
 #ifndef ZSTD_DEPS_COMMON
 #define ZSTD_DEPS_COMMON
 
+/* Even though we use qsort_r only for the dictionary builder, the macro
+ * _GNU_SOURCE has to be declared *before* the inclusion of any standard
+ * header and the script 'combine.sh' combines the whole zstd source code
+ * in a single file.
+ */
+#if defined(__linux) || defined(__linux__) || defined(linux) || defined(__gnu_linux__) || \
+    defined(__CYGWIN__) || defined(__MSYS__)
+#if !defined(_GNU_SOURCE)
+#define _GNU_SOURCE
+#endif
+#endif
+
 #include <limits.h>
 #include <stddef.h>
 #include <string.h>
diff --git a/lib/dictBuilder/cover.c b/lib/dictBuilder/cover.c
index 44f9029ac..386a4d572 100644
--- a/lib/dictBuilder/cover.c
+++ b/lib/dictBuilder/cover.c
@@ -21,8 +21,17 @@
 /*-*************************************
 *  Dependencies
 ***************************************/
+/* qsort_r is an extension. */
+#if defined(__linux) || defined(__linux__) || defined(linux) || defined(__gnu_linux__) || \
+    defined(__CYGWIN__) || defined(__MSYS__)
+#if !defined(_GNU_SOURCE)
+#define _GNU_SOURCE
+#endif
+#endif
+
 #include <stdio.h>  /* fprintf */
-#include <stdlib.h> /* malloc, free, qsort */
+#include <stdlib.h> /* malloc, free, qsort_r */
+
 #include <string.h> /* memset */
 #include <time.h>   /* clock */
 
@@ -232,8 +241,10 @@ typedef struct {
   unsigned d;
 } COVER_ctx_t;
 
-/* We need a global context for qsort... */
+#if !defined(_GNU_SOURCE) && !defined(__APPLE__) && !defined(_MSC_VER)
+/* C90 only offers qsort() that needs a global context. */
 static COVER_ctx_t *g_coverCtx = NULL;
+#endif
 
 /*-*************************************
 *  Helper functions
@@ -276,11 +287,15 @@ static int COVER_cmp8(COVER_ctx_t *ctx, const void *lp, const void *rp) {
 
 /**
  * Same as COVER_cmp() except ties are broken by pointer value
- * NOTE: g_coverCtx must be set to call this function.  A global is required because
- * qsort doesn't take an opaque pointer.
  */
-static int WIN_CDECL COVER_strict_cmp(const void *lp, const void *rp) {
-  int result = COVER_cmp(g_coverCtx, lp, rp);
+#if (defined(_WIN32) && defined(_MSC_VER)) || defined(__APPLE__)
+static int WIN_CDECL COVER_strict_cmp(void* g_coverCtx, const void* lp, const void* rp) {
+#elif defined(_GNU_SOURCE)
+static int COVER_strict_cmp(const void *lp, const void *rp, void *g_coverCtx) {
+#else /* C90 fallback.*/
+static int COVER_strict_cmp(const void *lp, const void *rp) {
+#endif
+  int result = COVER_cmp((COVER_ctx_t*)g_coverCtx, lp, rp);
   if (result == 0) {
     result = lp < rp ? -1 : 1;
   }
@@ -289,14 +304,50 @@ static int WIN_CDECL COVER_strict_cmp(const void *lp, const void *rp) {
 /**
  * Faster version for d <= 8.
  */
-static int WIN_CDECL COVER_strict_cmp8(const void *lp, const void *rp) {
-  int result = COVER_cmp8(g_coverCtx, lp, rp);
+#if (defined(_WIN32) && defined(_MSC_VER)) || defined(__APPLE__)
+static int WIN_CDECL COVER_strict_cmp8(void* g_coverCtx, const void* lp, const void* rp) {
+#elif defined(_GNU_SOURCE)
+static int COVER_strict_cmp8(const void *lp, const void *rp, void *g_coverCtx) {
+#else /* C90 fallback.*/
+static int COVER_strict_cmp8(const void *lp, const void *rp) {
+#endif
+  int result = COVER_cmp8((COVER_ctx_t*)g_coverCtx, lp, rp);
   if (result == 0) {
     result = lp < rp ? -1 : 1;
   }
   return result;
 }
 
+/**
+ * Abstract away divergence of qsort_r() parameters.
+ * Hopefully when C11 become the norm, we will be able
+ * to clean it up.
+ */
+static void stableSort(COVER_ctx_t *ctx) {
+#if defined(__APPLE__)
+    qsort_r(ctx->suffix, ctx->suffixSize, sizeof(U32),
+            ctx,
+            (ctx->d <= 8 ? &COVER_strict_cmp8 : &COVER_strict_cmp));
+#elif defined(_GNU_SOURCE)
+    qsort_r(ctx->suffix, ctx->suffixSize, sizeof(U32),
+            (ctx->d <= 8 ? &COVER_strict_cmp8 : &COVER_strict_cmp),
+            ctx);
+#elif defined(_WIN32) && defined(_MSC_VER)
+    qsort_s(ctx->suffix, ctx->suffixSize, sizeof(U32),
+            (ctx->d <= 8 ? &COVER_strict_cmp8 : &COVER_strict_cmp),
+            ctx);
+#elif defined(__OpenBSD__)
+    g_coverCtx = ctx;
+    mergesort(ctx->suffix, ctx->suffixSize, sizeof(U32),
+          (ctx->d <= 8 ? &COVER_strict_cmp8 : &COVER_strict_cmp));
+#else /* C90 fallback.*/
+    g_coverCtx = ctx;
+    /* TODO(cavalcanti): implement a reentrant qsort() when is not available. */
+    qsort(ctx->suffix, ctx->suffixSize, sizeof(U32),
+          (ctx->d <= 8 ? &COVER_strict_cmp8 : &COVER_strict_cmp));
+#endif
+}
+
 /**
  * Returns the first pointer in [first, last) whose element does not compare
  * less than value.  If no such element exists it returns last.
@@ -620,17 +671,7 @@ static size_t COVER_ctx_init(COVER_ctx_t *ctx, const void *samplesBuffer,
     for (i = 0; i < ctx->suffixSize; ++i) {
       ctx->suffix[i] = i;
     }
-    /* qsort doesn't take an opaque pointer, so pass as a global.
-     * On OpenBSD qsort() is not guaranteed to be stable, their mergesort() is.
-     */
-    g_coverCtx = ctx;
-#if defined(__OpenBSD__)
-    mergesort(ctx->suffix, ctx->suffixSize, sizeof(U32),
-          (ctx->d <= 8 ? &COVER_strict_cmp8 : &COVER_strict_cmp));
-#else
-    qsort(ctx->suffix, ctx->suffixSize, sizeof(U32),
-          (ctx->d <= 8 ? &COVER_strict_cmp8 : &COVER_strict_cmp));
-#endif
+    stableSort(ctx);
   }
   DISPLAYLEVEL(2, "Computing frequencies\n");
   /* For each dmer group (group of positions with the same first d bytes):