diff --git a/man3/atoi.3 b/man3/atoi.3
index 3c6527a5b..87d1e1efd 100644
--- a/man3/atoi.3
+++ b/man3/atoi.3
@@ -126,6 +126,11 @@ only.
 .\" .BR atoll ()
 .\" function is present in glibc 2 since version 2.0.2, but
 .\" not in libc4 or libc5.
+.SH NOTES
+POSIX.1 leaves the return value of
+.BR atoi ()
+on error unspecified.
+On glibc, musl libc, and uClibc, 0 is returned on error.
 .SH SEE ALSO
 .BR atof (3),
 .BR strtod (3),