ondemandcam.c not waiting for frames to be consumed

[allsey87]

System configuration

version: v4l2loopback driver version 0.12.2 loaded
kernel version: Linux ToshibaLaptop 5.2.11-arch1-1-ARCH #1 SMP PREEMPT Thu Aug 29 08:09:36 UTC 2019 x86_64 GNU/Linux

Problem

The example ondemandcam.c is producing frames as fast as possible and does not wait for the other end of the V4L2 pipe to be opened.

Steps to reproduce:

1. modprobe the driver
2. add a fprintf(stderr, "."); to sendvid in ondemandcam.c
3. compile and link ondemandcam.c against pthread and run it

Observed Results:

The ondemandcam is printing dots to the screen at a rapid rate even though the device is not open for reading. Opening the device in ffplay shows that the code is working (cycling through 4 colors, changing every second), but reports an error when trying to set the framerate:

[video4linux2,v4l2 @ 0x7f61b0000b80] The driver does not permit changing the time per frame

Expected Results:

1. Frames should not be produced unless a program has opened /dev/videoX for reading.
2. ffplay should be able to control the framerate and consume X FPS

[copiltembel]

More than one year later, I'm having the same problem.

@allsey87 Did you manage to fix this?

[copiltembel]

This works and it is a more elegant solution: #109 (comment)