Add zerowrite test

Makefile

@@ -1,6 +1,6 @@
 CFLAGS=-O0 -g
 
-all: fdpassing multiwrite lostfd xreq
+all: fdpassing multiwrite lostfd zerowrite xreq
 
 fdpassing: fdpassing.o fdpass.o
 	$(CC) $(CFLAGS) -o $@ fdpassing.o fdpass.o
@@ -11,5 +11,8 @@ multiwrite: multiwrite.o fdpass.o
 lostfd: lostfd.o fdpass.o
 	$(CC) $(CFLAGS) -o $@ lostfd.o fdpass.o
 
+zerowrite: zerowrite.o fdpass.o
+	$(CC) $(CFLAGS) -o $@ zerowrite.o fdpass.o
+
 xreq: xreq.o fdpass.o
 	$(CC) $(CFLAGS) -o $@ xreq.o fdpass.o

zerowrite.c

@@ -0,0 +1,80 @@
+/*
+ * Copyright © 2012 Keith Packard <keithp@keithp.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA.
+ */
+
+#include "fdpass.h"
+
+void
+child(int sock)
+{
+	int	fd;
+	char	buf[16];
+	ssize_t	size;
+
+	sleep(1);
+	for (;;) {
+		size = sock_fd_read(sock, buf, sizeof(buf), &fd);
+		if (size <= 0)
+			break;
+		printf ("read %d\n", size);
+		if (fd != -1) {
+			write(fd, "hello, world\n", 13);
+			close(fd);
+		}
+	}
+}
+
+void
+parent(int sock)
+{
+	ssize_t	size;
+	int	i;
+	int	fd;
+
+	fd = 1;
+	size = sock_fd_write(sock, "1", 1, -1);
+	printf ("wrote %d without fd\n", size);
+	size = sock_fd_write(sock, NULL, 0, 1);
+	printf ("wrote %d with fd\n", size);
+	size = sock_fd_write(sock, "1", 1, -1);
+	printf ("wrote %d without fd\n", size);
+}
+
+int
+main(int argc, char **argv)
+{
+	int	sv[2];
+	int	pid;
+
+	if (socketpair(AF_LOCAL, SOCK_STREAM, 0, sv) < 0) {
+		perror("socketpair");
+		exit(1);
+	}
+	switch ((pid = fork())) {
+	case 0:
+		close(sv[0]);
+		child(sv[1]);
+		break;
+	case -1:
+		perror("fork");
+		exit(1);
+	default:
+		close(sv[1]);
+		parent(sv[0]);
+		break;
+	}
+	return 0;
+}