[pulseaudio-discuss] [PATCH] iochannel: don't use variable length array in union

[Tanu Kaskinen]

Clang didn't like the variable length array:

pulsecore/iochannel.c:358:17: error: fields must have a constant size:
'variable length array in structure' extension will never be supported
        uint8_t data[CMSG_SPACE(sizeof(int) * nfd)];
                ^

Commit 451d1d6762 introduced the variable length array in order to have
the correct value in msg_controllen. This patch reverts that commit and
uses a different way to achieve the same goal.

BugLink: https://bugs.freedesktop.org/show_bug.cgi?id=99458
---
 src/pulsecore/iochannel.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/src/pulsecore/iochannel.c b/src/pulsecore/iochannel.c
index 8ace297ff..897337522 100644
--- a/src/pulsecore/iochannel.c
+++ b/src/pulsecore/iochannel.c
@@ -355,7 +355,7 @@ ssize_t pa_iochannel_write_with_fds(pa_iochannel*io, const void*data, size_t l,
     struct iovec iov;
     union {
         struct cmsghdr hdr;
-        uint8_t data[CMSG_SPACE(sizeof(int) * nfd)];
+        uint8_t data[CMSG_SPACE(sizeof(int) * MAX_ANCIL_DATA_FDS)];
     } cmsg;
 
     pa_assert(io);
@@ -382,7 +382,13 @@ ssize_t pa_iochannel_write_with_fds(pa_iochannel*io, const void*data, size_t l,
     mh.msg_iov = &iov;
     mh.msg_iovlen = 1;
     mh.msg_control = &cmsg;
-    mh.msg_controllen = sizeof(cmsg);
+
+    /* If we followed the example on the cmsg man page, we'd use
+     * sizeof(cmsg.data) here, but if nfd < MAX_ANCIL_DATA_FDS, then the data
+     * buffer is larger than needed, and the kernel doesn't like it if we set
+     * msg_controllen to a larger than necessary value. The commit message for
+     * commit 451d1d6762 contains a longer explanation. */
+    mh.msg_controllen = CMSG_SPACE(sizeof(int) * nfd);
 
     if ((r = sendmsg(io->ofd, &mh, MSG_NOSIGNAL)) >= 0) {
         io->writable = io->hungup = false;
-- 
2.11.0