Connecting a webcam
Many people have problems reading out a webcam on a RaspBerry Pi. I
encountered it too. One of the first error messages logged is this:
libv4l2: error allocating conversion
I traced the problem, by tracing the 'streamer' binary with strace.
The problem is this line:
mmap2(NULL, 268435456, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = -1 ENOMEM (Cannot allocate
Apparently, libv4l2 tries to allocate a conversion buffer length
268435456 bytes, or 256 MiB. Or, in the case of a first release
RaspberryPi, all of it's memory, which obviously is never
available. So this call will never succeed. On the other hand, it is
hardly conceivable that libv4l2 will really need all that memory.
Luckily, the kernel designers have a solution for problems caused by
applications claiming too much memory. A solution is to add this
line to /etc/sysctl.conf:
Now all memory allocations succeed, no matter how little memory is
present. Obviously, this has it's problems. E.g. if an application
tries to find out how much memory there is, by allocating it, this
fails. Also, it is now much easier for an application to crash the
system. And, an application reserving memory, then assuming it can
use it, can unexpectedly fail (and crash) later.
Another option is to temporarily use overcommit, e.g.:
streamer -t 0:30 -o video.avi
The best way, of course, is to repair libv4l2..