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

[Rodrigo Osorio]

On 10/01/26 18:52, Damjan Jovanovic wrote:


On Sat, Jan 10, 2026 at 5:18 PM Rodrigo Osorio <rodr...@freebsd.org> 
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 <http://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 <http://0.0.0.0:0> },16)      =
    0 (0x0)
      5798: getsockname(6,{ AF_INET 0.0.0.0:63851
    <http://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 <http://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

Hi Damjan,

Thanks for you explanations who helps me to understand the issue.

I was able to make the application works by increasing the incresing the 
buffer size
or by decreasing the amount of packets sent per second.

For the record this is the way to decrease the packet throughput:
> tsp -r -O ip 239.1.0.38:11112  -P regulate -I file  media.ts

Regards,
-- rodrigo