Index: include/stdlib.h
===================================================================
--- include/stdlib.h
+++ include/stdlib.h
@@ -309,7 +309,18 @@
 void	 sranddev(void);
 void	 srandomdev(void);
 long long
-	strtonum(const char *, long long, long long, const char **);
+	 strtonum(const char *, long long, long long, const char **);
+
+/* Linux qsort_r compatibility shim */
+#if __GNUC_PREREQ__(3, 1) && !defined(__cplusplus)
+typedef	int (__linux_qsort_cmp_t)(const void *, const void *, void *);
+void	 __linux_qsort_r(void *, size_t, size_t, __linux_qsort_cmp_t *,
+	    void *);
+#define	qsort_r(base, nmemb, size, A, B) \
+	__builtin_choose_expr( \
+	    __builtin_types_compatible_p(__typeof(A), __linux_qsort_cmp_t), \
+	    __linux_qsort_r, qsort_r)(base, nmemb, size, A, B)
+#endif
 
 /* Deprecated interfaces, to be removed in FreeBSD 6.0. */
 __int64_t
Index: lib/libc/stdlib/Makefile.inc
===================================================================
--- lib/libc/stdlib/Makefile.inc
+++ lib/libc/stdlib/Makefile.inc
@@ -11,8 +11,8 @@
 	getsubopt.c hcreate.c hcreate_r.c hdestroy_r.c heapsort.c heapsort_b.c \
 	hsearch_r.c imaxabs.c imaxdiv.c \
 	insque.c l64a.c labs.c ldiv.c llabs.c lldiv.c lsearch.c \
-	merge.c mergesort_b.c ptsname.c qsort.c qsort_r.c quick_exit.c \
-	radixsort.c rand.c \
+	merge.c mergesort_b.c ptsname.c qsort.c qsort_r.c linux_qsort_r.c \
+	quick_exit.c radixsort.c rand.c \
 	random.c reallocarray.c reallocf.c realpath.c remque.c strfmon.c \
 	strtoimax.c \
 	strtol.c strtoll.c strtoq.c strtoul.c strtonum.c strtoull.c \
Index: lib/libc/stdlib/linux_qsort_r.c
===================================================================
--- /dev/null
+++ lib/libc/stdlib/linux_qsort_r.c
@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2017 Conrad Meyer <cem@FreeBSD.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
+#define I_AM_LINUX_QSORT_R
+#include "qsort.c"
Index: lib/libc/stdlib/qsort.c
===================================================================
--- lib/libc/stdlib/qsort.c
+++ lib/libc/stdlib/qsort.c
@@ -35,8 +35,10 @@
 
 #include <stdlib.h>
 
-#ifdef I_AM_QSORT_R
+#if defined(I_AM_QSORT_R)
 typedef int		 cmp_t(void *, const void *, const void *);
+#elif defined(I_AM_LINUX_QSORT_R)
+typedef int		 cmp_t(const void *, const void *, void *);
 #else
 typedef int		 cmp_t(const void *, const void *);
 #endif
@@ -89,15 +91,17 @@
 #define	vecswap(a, b, n)				\
 	if ((n) > 0) swapfunc(a, b, n, swaptype_long, swaptype_int)
 
-#ifdef I_AM_QSORT_R
+#if defined(I_AM_QSORT_R)
 #define	CMP(t, x, y) (cmp((t), (x), (y)))
+#elif defined(I_AM_LINUX_QSORT_R)
+#define	CMP(t, x, y) (cmp((x), (y), (t)))
 #else
 #define	CMP(t, x, y) (cmp((x), (y)))
 #endif
 
 static inline char *
 med3(char *a, char *b, char *c, cmp_t *cmp, void *thunk
-#ifndef I_AM_QSORT_R
+#if !defined(I_AM_QSORT_R) && !defined(I_AM_LINUX_QSORT_R)
 __unused
 #endif
 )
@@ -107,9 +111,12 @@
 	      :(CMP(thunk, b, c) > 0 ? b : (CMP(thunk, a, c) < 0 ? a : c ));
 }
 
-#ifdef I_AM_QSORT_R
+#if defined(I_AM_QSORT_R)
 void
-qsort_r(void *a, size_t n, size_t es, void *thunk, cmp_t *cmp)
+(qsort_r)(void *a, size_t n, size_t es, void *thunk, cmp_t *cmp)
+#elif defined(I_AM_LINUX_QSORT_R)
+void
+__linux_qsort_r(void *a, size_t n, size_t es, cmp_t *cmp, void *thunk)
 #else
 #define	thunk NULL
 void
@@ -187,8 +194,10 @@
 	r = MIN(pd - pc, pn - pd - es);
 	vecswap(pb, pn - r, r);
 	if ((r = pb - pa) > es)
-#ifdef I_AM_QSORT_R
+#if defined(I_AM_QSORT_R)
 		qsort_r(a, r / es, es, thunk, cmp);
+#elif defined(I_AM_LINUX_QSORT_R)
+		__linux_qsort_r(a, r / es, es, cmp, thunk);
 #else
 		qsort(a, r / es, es, cmp);
 #endif