diff --git a/include/unistd.h b/include/unistd.h
index 7abf307a9c48..099cf53268d5 100644
--- a/include/unistd.h
+++ b/include/unistd.h
@@ -167,6 +167,21 @@ int	 fchown(int, uid_t, gid_t);
 int	 gethostname(char *, int /* socklen_t */);
 int	 setegid(gid_t);
 int	 seteuid(uid_t);
+
+/* X/Open mistake copied by POSIX */
+#define	_CS_POSIX_V6_ILP32_OFF32_CFLAGS		2
+#define	_CS_POSIX_V6_ILP32_OFF32_LDFLAGS	3
+#define	_CS_POSIX_V6_ILP32_OFF32_LIBS		4
+#define	_CS_POSIX_V6_ILP32_OFFBIG_CFLAGS	5
+#define	_CS_POSIX_V6_ILP32_OFFBIG_LDFLAGS	6
+#define	_CS_POSIX_V6_ILP32_OFFBIG_LIBS		7
+#define	_CS_POSIX_V6_LP64_OFF64_CFLAGS		8
+#define	_CS_POSIX_V6_LP64_OFF64_LDFLAGS		9
+#define	_CS_POSIX_V6_LP64_OFF64_LIBS		10
+#define	_CS_POSIX_V6_LPBIG_OFFBIG_CFLAGS	11
+#define	_CS_POSIX_V6_LPBIG_OFFBIG_LDFLAGS	12
+#define	_CS_POSIX_V6_LPBIG_OFFBIG_LIBS		13
+#define	_CS_POSIX_V6_WIDTH_RESTRICTED_ENVS	14
 #endif
 
 /*
diff --git a/lib/libc/gen/confstr.c b/lib/libc/gen/confstr.c
index ad2e5a0959c3..592a3eff2ecf 100644
--- a/lib/libc/gen/confstr.c
+++ b/lib/libc/gen/confstr.c
@@ -40,20 +40,81 @@ __FBSDID("$FreeBSD$");
 #include <sys/param.h>
 
 #include <errno.h>
+#include <limits.h>
 #include <paths.h>
-#include <unistd.h>
 #include <string.h>
+#include <unistd.h>
+
 
 size_t
 confstr(int name, char *buf, size_t len)
 {
 	const char *p;
+	const char UPE[] = "unsupported programming environment";
 
 	switch (name) {
 	case _CS_PATH:
 		p = _PATH_STDPATH;
 		goto docopy;
 
+		/*
+		 * POSIX/SUS ``Programming Environments'' stuff
+		 *
+		 * We don't support more than one programming environment
+		 * on any platform (yet), so we just return the empty
+		 * string for the environment we are compiled for,
+		 * and the string "unsupported programming environment"
+		 * for anything else.  (The Standard says that if these
+		 * values are used on a system which does not support
+		 * this environment -- determined via sysconf() -- then
+		 * the value we return is unspecified.  So, we return
+		 * something which will cause obvious breakage.)
+		 *
+		 * Note that the _V6_LP64_OFF64 is actually an *I*LP64
+		 * environment; FreeBSD's LP64 architectures use the
+		 * _V6_LPBIG_OFFBIG environment instead.
+		 */
+	case _CS_POSIX_V6_ILP32_OFF32_CFLAGS:
+	case _CS_POSIX_V6_ILP32_OFF32_LDFLAGS:
+	case _CS_POSIX_V6_ILP32_OFF32_LIBS:
+	case _CS_POSIX_V6_LP64_OFF64_CFLAGS:
+	case _CS_POSIX_V6_LP64_OFF64_LDFLAGS:
+	case _CS_POSIX_V6_LP64_OFF64_LIBS:
+		/*
+		 * These six environments are never supported.
+		 */
+		p = UPE;
+		goto docopy;
+
+	case _CS_POSIX_V6_ILP32_OFFBIG_CFLAGS:
+	case _CS_POSIX_V6_ILP32_OFFBIG_LDFLAGS:
+	case _CS_POSIX_V6_ILP32_OFFBIG_LIBS:
+		if (sizeof(long) * CHAR_BIT == 32 &&
+		    sizeof(off_t) > sizeof(long))
+			p = "";
+		else
+			p = UPE;
+		goto docopy;
+
+	case _CS_POSIX_V6_LPBIG_OFFBIG_CFLAGS:
+	case _CS_POSIX_V6_LPBIG_OFFBIG_LDFLAGS:
+	case _CS_POSIX_V6_LPBIG_OFFBIG_LIBS:
+		if (sizeof(long) * CHAR_BIT >= 64 &&
+		    sizeof(void *) * CHAR_BIT >= 64 &&
+		    sizeof(int) * CHAR_BIT >= 32 &&
+		    sizeof(off_t) >= sizeof(long))
+			p = "";
+		else
+			p = UPE;
+		goto docopy;
+
+	case _CS_POSIX_V6_WIDTH_RESTRICTED_ENVS:
+		if (sizeof(long) * CHAR_BIT >= 64)
+			p = "_V6_LPBIG_OFFBIG";
+		else
+			p = "_V6_ILP32_OFFBIG";
+		goto docopy;
+
 docopy:
 		if (len != 0 && buf != NULL)
 			strlcpy(buf, p, len);