Index: include/stdlib.h
===================================================================
--- include/stdlib.h
+++ include/stdlib.h
@@ -271,6 +271,9 @@
 	    size_t, int (^ _Nonnull)(const void *, const void *));
 #endif
 char	*getbsize(int *, long *);
+
+int	 clearenv(void);
+
 					/* getcap(3) functions */
 char	*cgetcap(char *, const char *, int);
 int	 cgetclose(void);
Index: lib/libc/stdlib/Symbol.map
===================================================================
--- lib/libc/stdlib/Symbol.map
+++ lib/libc/stdlib/Symbol.map
@@ -126,6 +126,7 @@
 	qsort_s;
 	rand;
 	srand;
+	clearenv;
 };
 
 FBSDprivate_1.0 {
Index: lib/libc/stdlib/getenv.3
===================================================================
--- lib/libc/stdlib/getenv.3
+++ lib/libc/stdlib/getenv.3
@@ -32,10 +32,11 @@
 .\"     @(#)getenv.3	8.2 (Berkeley) 12/11/93
 .\" $FreeBSD$
 .\"
-.Dd June 20, 2007
+.Dd January 17, 2021
 .Dt GETENV 3
 .Os
 .Sh NAME
+.Nm clearenv ,
 .Nm getenv ,
 .Nm putenv ,
 .Nm setenv ,
@@ -45,6 +46,8 @@
 .Lb libc
 .Sh SYNOPSIS
 .In stdlib.h
+.Ft int
+.Fn clearenv "void"
 .Ft char *
 .Fn getenv "const char *name"
 .Ft int
@@ -59,6 +62,14 @@
 .Em environment list .
 .Pp
 The
+.Fn clearenv
+function clears all environment variables.
+New variables can be added using
+.Fn setenv
+and
+.Fn getenv .
+.Pp
+The
 .Fn getenv
 function obtains the current value of the environment variable,
 .Fa name .
@@ -128,7 +139,7 @@
 .Dv NULL
 is returned.
 .Pp
-.Rv -std setenv putenv unsetenv
+.Rv -std clearenv setenv putenv unsetenv
 .Sh ERRORS
 .Bl -tag -width Er
 .It Bq Er EINVAL
@@ -211,6 +222,13 @@
 as the memory location of the ``name=value'' pair to follow the
 .Tn POSIX
 specification.
+.Pp
+The
+.Fn clearenv
+function first appeared in glibc.
+It was never part of any standard.
+Support for it was added in
+.Fx 13 .
 .Sh BUGS
 Successive calls to
 .Fn setenv
Index: lib/libc/stdlib/getenv.c
===================================================================
--- lib/libc/stdlib/getenv.c
+++ lib/libc/stdlib/getenv.c
@@ -691,3 +691,28 @@
 
 	return (0);
 }
+
+/*
+ * Unset all variable by flagging it as inactive.  No variable is
+ * ever freed.
+ */
+int
+clearenv(void)
+{
+	int ndx;
+
+	/* Initialize environment. */
+	if (__merge_environ() == -1 || (envVars == NULL && __build_env() == -1))
+		return (-1);
+
+	/* Remove from the end to not shuffle memory too much. */
+	for (ndx = envVarsTotal - 1; ndx >= 0; ndx--) {
+		envVars[ndx].active = false;
+		if (envVars[ndx].putenv)
+			__remove_putenv(ndx);
+	}
+
+	__rebuild_environ(0);
+
+	return (0);
+}
Index: lib/libc/tests/stdlib/Makefile
===================================================================
--- lib/libc/tests/stdlib/Makefile
+++ lib/libc/tests/stdlib/Makefile
@@ -2,6 +2,7 @@
 
 .include <src.opts.mk>
 
+ATF_TESTS_C+=		clearenv_test
 ATF_TESTS_C+=		dynthr_test
 ATF_TESTS_C+=		heapsort_test
 ATF_TESTS_C+=		mergesort_test