use CMSG_LEN(), this fixes an alert "sendmsg() failed (9: Bad file descriptor)"
on some 64-bit platforms

src/os/unix/ngx_channel.c

diff --git a/src/os/unix/ngx_channel.c b/src/os/unix/ngx_channel.c
index 62ea7cc..f0f337d 100644
--- a/src/os/unix/ngx_channel.c
+++ b/src/os/unix/ngx_channel.c
@@ -33,7 +33,7 @@
         msg.msg_control = (caddr_t) &cmsg;
         msg.msg_controllen = sizeof(cmsg);
 
-        cmsg.cm.cmsg_len = sizeof(cmsg);
+        cmsg.cm.cmsg_len = CMSG_LEN(sizeof(int));
         cmsg.cm.cmsg_level = SOL_SOCKET;
         cmsg.cm.cmsg_type = SCM_RIGHTS;
         *(int *) CMSG_DATA(&cmsg.cm) = ch->fd;
@@ -138,7 +138,7 @@
 
     if (ch->command == NGX_CMD_OPEN_CHANNEL) {
 
-        if (cmsg.cm.cmsg_len < (socklen_t) sizeof(cmsg)) {
+        if (cmsg.cm.cmsg_len < (socklen_t) CMSG_LEN(sizeof(int))) {
             ngx_log_error(NGX_LOG_ALERT, log, 0,
                           "recvmsg() returned too small ancillary data");
             return NGX_ERROR;

src/os/unix/ngx_freebsd_config.h

diff --git a/src/os/unix/ngx_freebsd_config.h b/src/os/unix/ngx_freebsd_config.h
index 818e230..24dcdcb 100644
--- a/src/os/unix/ngx_freebsd_config.h
+++ b/src/os/unix/ngx_freebsd_config.h
@@ -48,11 +48,16 @@
 
 #if __FreeBSD_version < 400017
 
-/* FreeBSD 3.x has no CMSG_SPACE() at all and has the broken CMSG_DATA() */
+/*
+ * FreeBSD 3.x has no CMSG_SPACE() and CMSG_LEN() and has the broken CMSG_DATA()
+ */
 
 #undef  CMSG_SPACE
 #define CMSG_SPACE(l)       (ALIGN(sizeof(struct cmsghdr)) + ALIGN(l))
 
+#undef  CMSG_LEN
+#define CMSG_LEN(l)         (ALIGN(sizeof(struct cmsghdr)) + (l))
+
 #undef  CMSG_DATA
 #define CMSG_DATA(cmsg)     ((u_char *)(cmsg) + ALIGN(sizeof(struct cmsghdr)))
 

src/os/unix/ngx_posix_config.h

diff --git a/src/os/unix/ngx_posix_config.h b/src/os/unix/ngx_posix_config.h
index 49c3c4d..37176e0 100644
--- a/src/os/unix/ngx_posix_config.h
+++ b/src/os/unix/ngx_posix_config.h
@@ -110,11 +110,16 @@
 
 #include <sys/param.h>          /* ALIGN() */
 
-/* FreeBSD 3.x has no CMSG_SPACE() at all and has the broken CMSG_DATA() */
+/* 
+ * FreeBSD 3.x has no CMSG_SPACE() and CMSG_LEN() and has the broken CMSG_DATA()
+ */
 
 #undef  CMSG_SPACE
 #define CMSG_SPACE(l)       (ALIGN(sizeof(struct cmsghdr)) + ALIGN(l))
 
+#undef  CMSG_LEN
+#define CMSG_LEN(l)         (ALIGN(sizeof(struct cmsghdr)) + (l))
+
 #undef  CMSG_DATA
 #define CMSG_DATA(cmsg)     ((u_char *)(cmsg) + ALIGN(sizeof(struct cmsghdr)))