diff --git a/src/lib9/errstr.c b/src/lib9/errstr.c
new file mode 100644
index 0000000..e576b12
--- /dev/null
+++ b/src/lib9/errstr.c
@@ -0,0 +1,68 @@
+/*
+ * We assume there's only one error buffer for the whole system.
+ * If you use ffork, you need to provide a _syserrstr.  Since most
+ * people will use libthread (which provides a _syserrstr), this is
+ * okay.
+ */
+
+#include <errno.h>
+#include <string.h>
+#include <lib9.h>
+
+enum
+{
+	EPLAN9 = 0x19283745,
+};
+
+char *(*_syserrstr)(void);
+static char xsyserr[ERRMAX];
+static char*
+getsyserr(void)
+{
+	char *s;
+
+	s = nil;
+	if(_syserrstr)
+		s = (*_syserrstr)();
+	if(s == nil)
+		s = xsyserr;
+	return s;
+}
+
+int
+errstr(char *err, uint n)
+{
+	char tmp[ERRMAX];
+	char *syserr;
+
+	syserr = getsyserr();
+	if(errno != EPLAN9)
+		strcpy(syserr, strerror(errno));
+
+	strecpy(tmp, tmp+ERRMAX, syserr);
+	strecpy(syserr, syserr+ERRMAX, err);
+	strecpy(err, err+n, tmp);
+	errno = EPLAN9;
+	return 0;
+}
+
+void
+rerrstr(char *err, uint n)
+{
+	char *syserr;
+
+	syserr = getsyserr();
+	if(errno != EPLAN9)
+		strcpy(syserr, strerror(errno));
+	strecpy(err, err+n, syserr);
+}
+
+/* replaces __errfmt in libfmt */
+
+int
+__errfmt(Fmt *f)
+{
+	if(errno == EPLAN9)
+		return fmtstrcpy(f, getsyserr());
+	return fmtstrcpy(f, strerror(errno));
+}