diff --git a/lib/libc/gen/sched_getaffinity.c b/lib/libc/gen/sched_getaffinity.c
--- a/lib/libc/gen/sched_getaffinity.c
+++ b/lib/libc/gen/sched_getaffinity.c
@@ -26,16 +26,29 @@
  * SUCH DAMAGE.
  */
 
+#define	_WANT_P_OSREL
+#include <sys/param.h>
 #include <errno.h>
 #include <sched.h>
 #include <string.h>
 
+#include "libc_private.h"
+
 int
 sched_getaffinity(pid_t pid, size_t cpusetsz, cpuset_t *cpuset)
 {
+	cpuwhich_t which;
 	int error;
 
-	error = cpuset_getaffinity(CPU_LEVEL_WHICH, CPU_WHICH_TIDPID,
+	if (__getosreldate() < P_OSREL_TIDPID) {
+		if (pid == 0 || pid > _PID_MAX)
+			which = CPU_WHICH_TID;
+		else
+			which = CPU_WHICH_PID;
+	} else
+		which = CPU_WHICH_TIDPID;
+
+	error = cpuset_getaffinity(CPU_LEVEL_WHICH, which,
 	    pid == 0 ? -1 : pid, cpusetsz, cpuset);
 	if (error == -1 && errno == ERANGE)
 		errno = EINVAL;
diff --git a/lib/libc/gen/sched_setaffinity.c b/lib/libc/gen/sched_setaffinity.c
--- a/lib/libc/gen/sched_setaffinity.c
+++ b/lib/libc/gen/sched_setaffinity.c
@@ -26,20 +26,32 @@
  * SUCH DAMAGE.
  */
 
+#define	_WANT_P_OSREL
 #include <sys/param.h>
 #include <sys/sysctl.h>
 #include <errno.h>
 #include <sched.h>
 #include <string.h>
 
+#include "libc_private.h"
+
 int
 sched_setaffinity(pid_t pid, size_t cpusetsz, const cpuset_t *cpuset)
 {
 	static int mp_maxid;
+	cpuwhich_t which;
 	cpuset_t c;
 	int error, lbs, cpu;
 	size_t len, sz;
 
+	if (__getosreldate() < P_OSREL_TIDPID) {
+		if (pid == 0 || pid > _PID_MAX)
+			which = CPU_WHICH_TID;
+		else
+			which = CPU_WHICH_PID;
+	} else
+		which = CPU_WHICH_TIDPID;
+
 	sz = cpusetsz > sizeof(cpuset_t) ? sizeof(cpuset_t) : cpusetsz;
 	memset(&c, 0, sizeof(c));
 	memcpy(&c, cpuset, sz);
@@ -58,7 +70,7 @@
 			if (cpu > mp_maxid)
 				CPU_CLR(cpu, &c);
 	}
-	error = cpuset_setaffinity(CPU_LEVEL_WHICH, CPU_WHICH_TIDPID,
+	error = cpuset_setaffinity(CPU_LEVEL_WHICH, which,
 	    pid == 0 ? -1 : pid, sizeof(cpuset_t), &c);
 	if (error == -1 && errno == EDEADLK)
 		errno = EINVAL;
diff --git a/lib/libc/include/libc_private.h b/lib/libc/include/libc_private.h
--- a/lib/libc/include/libc_private.h
+++ b/lib/libc/include/libc_private.h
@@ -39,6 +39,12 @@
 #include <sys/_types.h>
 #include <sys/_pthreadtypes.h>
 
+/*
+ * The kernel doesn't expose PID_MAX to the user space. Save it here
+ * to allow to run a newer world on a pre-1400079 kernel.
+ */
+#define	_PID_MAX	99999
+
 /*
  * This global flag is non-zero when a process has created one
  * or more threads. It is used to avoid calling locking functions
diff --git a/sys/sys/param.h b/sys/sys/param.h
--- a/sys/sys/param.h
+++ b/sys/sys/param.h
@@ -107,6 +107,7 @@
 #define	P_OSREL_CK_SUPERBLOCK		1300000
 #define	P_OSREL_CK_INODE		1300005
 #define	P_OSREL_POWERPC_NEW_AUX_ARGS	1300070
+#define	P_OSREL_TIDPID			1400079
 
 #define	P_OSREL_MAJOR(x)		((x) / 100000)
 #endif