If the parent directory of the destination file has its immutable flag set,

symlink(2) returns EPERM.

lib/libc/sys/symlink.2

@@ -32,7 +32,7 @@
 .\"     @(#)symlink.2	8.1 (Berkeley) 6/4/93
 .\" $FreeBSD$
 .\"
-.Dd June 4, 1993
+.Dd December 9, 2006
 .Dt SYMLINK 2
 .Os
 .Sh NAME
@@ -82,6 +82,12 @@ The path name pointed at by the
 .Fa name2
 argument
 already exists.
+.It Bq Er EPERM
+The parent directory of the file named by
+.Fa name2
+has its immutable flag set, see the
+.Xr chflags 2
+manual page for more information.
 .It Bq Er EIO
 An I/O error occurred while making the directory entry for
 .Fa name2 ,
@@ -128,6 +134,7 @@ points outside the process's allocated address space.
 .El
 .Sh SEE ALSO
 .Xr ln 1 ,
+.Xr chflags 2 ,
 .Xr link 2 ,
 .Xr lstat 2 ,
 .Xr readlink 2 ,