lsl_have_consumers always returns 'true'

[jseyelink]

Problem

Function lsl_have_consumers( ) from lsl_c.h always returns true. This persists even when there are no longer any consumers.

Environment

Linux display-desktop 6.8.0-101-generic #101~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Wed Feb 11 13:19:54 UTC x86_64 x86_64 x86_64 GNU/Linux

gcc (Ubuntu 11.4.0-1ubuntu1~22.04.3) 11.4.0

cmake version 4.2.3

Test program

CMakeLists.txt
main.c

Steps to replicate

1. Build test program.
2. Run test program. This will now wait for a consumer.
3. Run LabRecorder.
4. From LabRecorder:
   a. Select the Test stream.
   b. Start recording.
5. The test program will now begin printing the return value that it receives from repeated calls to lsl_have_consumers( ).
6. From LabRecorder:
   a. Stop recording.
   b. Deselect the Test stream.
   c. Close the program.
7. Use Ctrl-C to end the test program, which is still running.

Example session

From terminal with test program,

$ ./build/lsl-haveconsumer-test 
2026-03-05 14:42:27.815 (   0.000s) [lsl-haveconsume ]      netinterfaces.cpp:89    INFO| netif 'lo' (status: 1, multicast: 0, broadcast: 0)
2026-03-05 14:42:27.815 (   0.000s) [lsl-haveconsume ]      netinterfaces.cpp:89    INFO| netif 'enp3s0' (status: 1, multicast: 4096, broadcast: 2)
2026-03-05 14:42:27.815 (   0.000s) [lsl-haveconsume ]      netinterfaces.cpp:89    INFO| netif 'enp0s31f6' (status: 1, multicast: 4096, broadcast: 2)
2026-03-05 14:42:27.815 (   0.000s) [lsl-haveconsume ]      netinterfaces.cpp:89    INFO| netif 'lo' (status: 1, multicast: 0, broadcast: 0)
2026-03-05 14:42:27.815 (   0.000s) [lsl-haveconsume ]      netinterfaces.cpp:89    INFO| netif 'enp3s0' (status: 1, multicast: 4096, broadcast: 2)
2026-03-05 14:42:27.815 (   0.000s) [lsl-haveconsume ]      netinterfaces.cpp:102   INFO| 	IPv4 addr: 64010102
2026-03-05 14:42:27.815 (   0.000s) [lsl-haveconsume ]      netinterfaces.cpp:89    INFO| netif 'enp0s31f6' (status: 1, multicast: 4096, broadcast: 2)
2026-03-05 14:42:27.815 (   0.000s) [lsl-haveconsume ]      netinterfaces.cpp:102   INFO| 	IPv4 addr: a23074a
2026-03-05 14:42:27.815 (   0.000s) [lsl-haveconsume ]      netinterfaces.cpp:89    INFO| netif 'lo' (status: 1, multicast: 0, broadcast: 0)
2026-03-05 14:42:27.815 (   0.000s) [lsl-haveconsume ]      netinterfaces.cpp:89    INFO| netif 'enp3s0' (status: 1, multicast: 4096, broadcast: 2)
2026-03-05 14:42:27.815 (   0.000s) [lsl-haveconsume ]      netinterfaces.cpp:105   INFO| 	IPv6 addr: fe80::bab3:e1a5:6cb1:8254%enp3s0
2026-03-05 14:42:27.815 (   0.000s) [lsl-haveconsume ]         api_config.cpp:270   INFO| Loaded default config
LSL version information
    library   1.16
    protocol  1.10
    lib info  git:6ca188c266c21f7228dc67077303fa6abaf2e8be/branch:refs/tags/v1.16.2/build:Release/compiler:GNU-11.3.0/link:SHARED
2026-03-05 14:42:27.815 (   0.000s) [lsl-haveconsume ]             common.cpp:65    INFO| git:6ca188c266c21f7228dc67077303fa6abaf2e8be/branch:refs/tags/v1.16.2/build:Release/compiler:GNU-11.3.0/link:SHARED
lsl_have_consumers 0 return 1
lsl_have_consumers 1 return 1
lsl_have_consumers 2 return 1
lsl_have_consumers 3 return 1
lsl_have_consumers 4 return 1
lsl_have_consumers 5 return 1
lsl_have_consumers 6 return 1
lsl_have_consumers 7 return 1
lsl_have_consumers 8 return 1
lsl_have_consumers 9 return 1
lsl_have_consumers 10 return 1
lsl_have_consumers 11 return 1
lsl_have_consumers 12 return 1
lsl_have_consumers 13 return 1
lsl_have_consumers 14 return 1
lsl_have_consumers 15 return 1
lsl_have_consumers 16 return 1
lsl_have_consumers 17 return 1
lsl_have_consumers 18 return 1
lsl_have_consumers 19 return 1
lsl_have_consumers 20 return 1
lsl_have_consumers 21 return 1
lsl_have_consumers 22 return 1
lsl_have_consumers 23 return 1
lsl_have_consumers 24 return 1
lsl_have_consumers 25 return 1
lsl_have_consumers 26 return 1
lsl_have_consumers 27 return 1
lsl_have_consumers 28 return 1
lsl_have_consumers 29 return 1
lsl_have_consumers 30 return 1
lsl_have_consumers 31 return 1
lsl_have_consumers 32 return 1
^CHandled signal 2
lsl_have_consumers final return 1

From terminal with LabRecorder,

$ LabRecorder
"LabRecorder.cfg"
"/home/display/Documents/Jira/API-3375/LabRecorder.cfg"
"/home/display/.config/LabRecorder/LabRecorder.cfg"
"/etc/xdg/xdg-ubuntu/LabRecorder/LabRecorder.cfg"
"/etc/xdg/LabRecorder/LabRecorder.cfg"
"/home/display/.local/share/LabRecorder/LabRecorder.cfg"
"/usr/share/ubuntu/LabRecorder/LabRecorder.cfg"
"/usr/share/gnome/LabRecorder/LabRecorder.cfg"
"/usr/local/share/LabRecorder/LabRecorder.cfg"
"/usr/share/LabRecorder/LabRecorder.cfg"
loading config file  "/usr/share/LabRecorder/LabRecorder.cfg"
2026-03-05 14:42:30.315 (   0.055s) [LabRecorder     ]      netinterfaces.cpp:89    INFO| netif 'lo' (status: 1, multicast: 0, broadcast: 0)
2026-03-05 14:42:30.315 (   0.055s) [LabRecorder     ]      netinterfaces.cpp:89    INFO| netif 'enp3s0' (status: 1, multicast: 4096, broadcast: 2)
2026-03-05 14:42:30.315 (   0.055s) [LabRecorder     ]      netinterfaces.cpp:89    INFO| netif 'enp0s31f6' (status: 1, multicast: 4096, broadcast: 2)
2026-03-05 14:42:30.315 (   0.055s) [LabRecorder     ]      netinterfaces.cpp:89    INFO| netif 'lo' (status: 1, multicast: 0, broadcast: 0)
2026-03-05 14:42:30.315 (   0.055s) [LabRecorder     ]      netinterfaces.cpp:89    INFO| netif 'enp3s0' (status: 1, multicast: 4096, broadcast: 2)
2026-03-05 14:42:30.315 (   0.055s) [LabRecorder     ]      netinterfaces.cpp:102   INFO| 	IPv4 addr: 64010102
2026-03-05 14:42:30.315 (   0.055s) [LabRecorder     ]      netinterfaces.cpp:89    INFO| netif 'enp0s31f6' (status: 1, multicast: 4096, broadcast: 2)
2026-03-05 14:42:30.315 (   0.055s) [LabRecorder     ]      netinterfaces.cpp:102   INFO| 	IPv4 addr: a23074a
2026-03-05 14:42:30.315 (   0.055s) [LabRecorder     ]      netinterfaces.cpp:89    INFO| netif 'lo' (status: 1, multicast: 0, broadcast: 0)
2026-03-05 14:42:30.315 (   0.055s) [LabRecorder     ]      netinterfaces.cpp:89    INFO| netif 'enp3s0' (status: 1, multicast: 4096, broadcast: 2)
2026-03-05 14:42:30.315 (   0.055s) [LabRecorder     ]      netinterfaces.cpp:105   INFO| 	IPv6 addr: fe80::bab3:e1a5:6cb1:8254%enp3s0
2026-03-05 14:42:30.315 (   0.055s) [LabRecorder     ]         api_config.cpp:270   INFO| Loaded default config
Missing:  QSet()
2026-03-05 14:42:39.046 (   8.787s) [LabRecorder     ]             common.cpp:65    INFO| git:6ca188c266c21f7228dc67077303fa6abaf2e8be/branch:refs/tags/v1.16.2/build:Release/compiler:GNU-11.3.0/link:SHARED
Opened the stream Test.
Received header for stream Test.
Started data collection for stream Test.
Offsets thread is finished
Wrote footer for stream Test.
Closing the file.

[daverlon]

Hi @jseyelink

I was able to reproduce this.

lsl_have_consumers() does not check whether a consumer is still connected on the network. It only checks whether the outlet still has an internal queue registered.
Normally the queue is not unregistered until a sample is pushed and fails to write. Until a sample is pushed the transfer thread waits on pop_sample() and lsl_have_consumers() remains true.

I have a small fix that watches the socket for the peer connection closing, so the consumer is unregistered without waiting for a sample push.

Here is output of your included test program. I ran it alongside LabRecorder app in the same way you described.

2026-08-19 00:09:58.492 (   0.002s) [          3D35C6]         api_config.cpp:126   INFO| Loaded default config
LSL version information
    library   1.17
    protocol  1.10
    lib info  git:/branch:dev/build:Release/compiler:AppleClang-21.0.0.21000101/link:SHARED
2026-08-19 00:09:58.494 (   0.002s) [          3D35C6]             common.cpp:78    INFO| git:/branch:dev/build:Release/compiler:AppleClang-21.0.0.21000101/link:SHARED
lsl_have_consumers 0 return 1
lsl_have_consumers 1 return 1
lsl_have_consumers 2 return 1
lsl_have_consumers 3 return 1
lsl_have_consumers 4 return 1
lsl_have_consumers 5 return 1
lsl_have_consumers 6 return 1
lsl_have_consumers 7 return 1
lsl_have_consumers 8 return 1
lsl_have_consumers 9 return 1
lsl_have_consumers 10 return 1
lsl_have_consumers 11 return 1
lsl_have_consumers final return 0