Index: include/stdlib.h
===================================================================
--- include/stdlib.h
+++ include/stdlib.h
@@ -225,6 +225,7 @@
 long	 nrand48(unsigned short[3]);
 int	 posix_openpt(int);
 char	*ptsname(int);
+int	 ptsname_r(int, char *, size_t);
 int	 putenv(char *);
 long	 random(void);
 unsigned short
Index: lib/libc/stdlib/Makefile.inc
===================================================================
--- lib/libc/stdlib/Makefile.inc
+++ lib/libc/stdlib/Makefile.inc
@@ -50,7 +50,7 @@
 MLINKS+=hcreate.3 hcreate_r.3 hcreate.3 hdestroy_r.3 hcreate.3 hsearch_r.3
 MLINKS+=insque.3 remque.3
 MLINKS+=lsearch.3 lfind.3
-MLINKS+=ptsname.3 grantpt.3 ptsname.3 unlockpt.3
+MLINKS+=ptsname.3 grantpt.3 ptsname.3 ptsname_r.3 ptsname.3 unlockpt.3
 MLINKS+=qsort.3 heapsort.3 qsort.3 mergesort.3 qsort.3 qsort_r.3 \
 	qsort.3 qsort_s.3
 MLINKS+=rand.3 rand_r.3 rand.3 srand.3
Index: lib/libc/stdlib/Symbol.map
===================================================================
--- lib/libc/stdlib/Symbol.map
+++ lib/libc/stdlib/Symbol.map
@@ -125,6 +125,7 @@
 	qsort_s;
 	rand;
 	srand;
+	ptsname_r;
 };
 
 FBSDprivate_1.0 {
Index: lib/libc/stdlib/ptsname.3
===================================================================
--- lib/libc/stdlib/ptsname.3
+++ lib/libc/stdlib/ptsname.3
@@ -31,12 +31,13 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd August 20, 2008
+.Dd October 9, 2020
 .Dt PTSNAME 3
 .Os
 .Sh NAME
 .Nm grantpt ,
 .Nm ptsname ,
+.Nm ptsname_r ,
 .Nm unlockpt
 .Nd pseudo-terminal access functions
 .Sh LIBRARY
@@ -47,6 +48,8 @@
 .Fn grantpt "int fildes"
 .Ft "char *"
 .Fn ptsname "int fildes"
+.Ft "int"
+.Fn ptsname_r "int fildes" "char *buffer" "size_t buflen"
 .Ft int
 .Fn unlockpt "int fildes"
 .Sh DESCRIPTION
@@ -87,12 +90,23 @@
 have been called.
 .Pp
 The
+.Fn ptsname_r
+function is the thread-safe version of
+.Fn ptsname .
+The caller must provide storage for the results of the full pathname of
+the slave device in the
+.Fa buffer
+and
+.Fa bufsize
+arguments.
+.Pp
+The
 .Fn unlockpt
 function clears the lock held on the pseudo-terminal pair
 for the master device specified with
 .Fa fildes .
 .Sh RETURN VALUES
-.Rv -std grantpt unlockpt
+.Rv -std grantpt ptsname_r unlockpt
 .Pp
 The
 .Fn ptsname
@@ -103,7 +117,8 @@
 .Sh ERRORS
 The
 .Fn grantpt ,
-.Fn ptsname
+.Fn ptsname ,
+.Fn ptsname_r
 and
 .Fn unlockpt
 functions may fail and set
@@ -119,6 +134,16 @@
 .El
 .Pp
 In addition, the
+.Fn ptsname_r
+function may set
+.Va errno
+to:
+.Bl -tag -width Er
+.It Bq Er ERANGE
+The buffer was too small.
+.El
+.Pp
+In addition, the
 .Fn grantpt
 function may set
 .Va errno
Index: lib/libc/stdlib/ptsname.c
===================================================================
--- lib/libc/stdlib/ptsname.c
+++ lib/libc/stdlib/ptsname.c
@@ -41,6 +41,7 @@
 #include <errno.h>
 #include <paths.h>
 #include <stdlib.h>
+#include <string.h>
 #include "un-namespace.h"
 
 /*
@@ -71,23 +72,46 @@
 __strong_reference(__isptmaster, unlockpt);
 
 /*
- * ptsname():  return the pathname of the slave pseudo-terminal device
- *             associated with the specified master.
+ * ptsname_r(): return the pathname of the slave pseudo-terminal device
+ *              associated with the specified master.
  */
-char *
-ptsname(int fildes)
+int
+ptsname_r(int fildes, char *buffer, size_t buflen)
 {
-	static char pt_slave[sizeof _PATH_DEV + SPECNAMELEN] = _PATH_DEV;
-	char *ret = NULL;
+
+	if (buflen <= sizeof(_PATH_DEV)) {
+		errno = ERANGE;
+		return (-1);
+	}
 
 	/* Make sure fildes points to a master device. */
 	if (__isptmaster(fildes) != 0)
-		goto done;
+		return (-1);
+
+	memcpy(buffer, _PATH_DEV, sizeof(_PATH_DEV));
+	buffer += sizeof(_PATH_DEV) - 1;
+	buflen -= sizeof(_PATH_DEV) - 1;
+
+	if (fdevname_r(fildes, buffer, buflen) == NULL) {
+		if (errno == EINVAL)
+			errno = ERANGE;
+		return (-1);
+	}
+
+	return (0);
+}
 
-	if (fdevname_r(fildes, pt_slave + (sizeof _PATH_DEV - 1),
-	    sizeof pt_slave - (sizeof _PATH_DEV - 1)) != NULL)
-		ret = pt_slave;
+/*
+ * ptsname():  return the pathname of the slave pseudo-terminal device
+ *             associated with the specified master.
+ */
+char *
+ptsname(int fildes)
+{
+	static char pt_slave[sizeof _PATH_DEV + SPECNAMELEN];
 
-done:
-	return (ret);
+	if (ptsname_r(fildes, pt_slave, sizeof(pt_slave)) == 0)
+		return (pt_slave);
+	else
+		return (NULL);
 }