diff --git a/io/io_passfd.3 b/io/io_passfd.3
index ce345f9..33b9dfd 100644
--- a/io/io_passfd.3
+++ b/io/io_passfd.3
@@ -15,5 +15,7 @@ system).
 The peer can then use io_receivefd(3) to receive the file descriptor.
 
 Note that the passed descriptor stays open in the sending process.
+.SH "RETURN VALUE"
+io_passfd returns 0 on success, -1 on error (setting errno accordingly).
 .SH "SEE ALSO"
 io_receivefd(3)
diff --git a/io/io_passfd.c b/io/io_passfd.c
index d721dce..ee21900 100644
--- a/io/io_passfd.c
+++ b/io/io_passfd.c
@@ -31,5 +31,5 @@ int io_passfd(int64 sock,int64 fd) {
   cmsg->cmsg_len = CMSG_LEN(sizeof(int));
   *((int*)CMSG_DATA(cmsg))=fd;
   msg.msg_controllen = cmsg->cmsg_len;
-  return sendmsg(sock,&msg,0);
+  return sendmsg(sock,&msg,0)>=0?0:-1;
 }