Re: Error sending UDP message: No buffer space available, not reproducible with truss

On Sat, Jan 10, 2026 at 5:18 PM Rodrigo Osorio <[email protected]> wrote:

> Hi,
>
> I have this strange behavior with an application I ported a couple of
> weeks ago
> multimedia/tsduck which can broadcast MPEG transport streams over UDP.
>
> When I run the application as a regular user :
>
> tsp -O ip 239.1.0.38:11112  -I file media.ts I got an error on the
> sendto() call
>
> "No buffer space available"
>

This is normal on FreeBSD: send() to a full UDP send queue causes the
ENOBUFS error.

This can be surprising, because on other operating systems, the behaviour
is different. On Linux, a full send queue causes UDP packets to get
silently dropped - no error is reported back to the caller. Many
applications developed with Linux in mind are thus noisy about ENOBUFS on
FreeBSD, while they have no choice but to be silent about Linux's dropped
packets.

Try calling ioctl() with FIONWRITE and FIONSPACE (see the ioctl(2) man
page) to see the used and free queue space, when you get ENOBUFS, and if
necessary, increase your send buffer size, or improve your network
performance.


> When trying to analize the issue with ktrace or truss, sudenly I got no
> errors and
> the application just works.
>
>   5798: socket(PF_INET,SOCK_DGRAM,IPPROTO_UDP)    = 6 (0x6)
>   5798: setsockopt(6,IPPROTO_IP,IP_RECVDSTADDR,0x8205160a4,4) = 0 (0x0)
>   5798: bind(6,{ AF_INET 0.0.0.0:0 },16)      = 0 (0x0)
>   5798: getsockname(6,{ AF_INET 0.0.0.0:63851 },0x820515ef4) = 0 (0x0)
>   5798: setsockopt(6,IPPROTO_IP,IP_MULTICAST_LOOP,0x820516088,1) = 0 (0x0)
>
> [...]
>
>   5798: sendto(6,"G@\0\^Q\0\0\M-0\r\0\^B\M-A\0\0\0"...,1316,0,{ AF_INET
> 239.1.0.38:11112 },16) = 1316 (0x524)
>
> Since the two calls are performed (without and with truss) are performed
> without any change, I wonder
> what can explain this behavior. calling the application again without
> truss leads to the same buffer error.
>

The use of truss slows down the application, and maybe that causes data to
be sent slower, so the send queue doesn't fill up?


>
> Here is the full trace using truss:
> https://people.freebsd.org/~rodrigo/tsduck_no_buffer_space.txt
>
> Thanks in advance for your help.
>
> -- rodrigo
>
>
>
Regards
Damjan