Dump QImage raw pixel data into `std::vector<char>`
-
Thread 6 "mirvncserver" received signal SIGBUS, Bus error. [Switching to Thread 0xb0db8bc0 (LWP 3495)] 0xb6e433b2 in CheckSolidTile32 (cl=0xb1700470, needSameColor=0 '\000', colorPtr=0xb0db8438, h=16, w=16, y=-1231936575, x=20) at tight.c:611 611 tight.c: No such file or directory. (gdb) thread apply all bt Thread 6 (Thread 0xb0db8bc0 (LWP 3495)): #0 0xb6e433b2 in CheckSolidTile32 (cl=0xb1700470, needSameColor=0 '\000', colorPtr=0xb0db8438, h=16, w=16, y=-1231936575, x=20) at tight.c:611 #1 CheckSolidTile (cl=cl@entry=0xb1700470, x=x@entry=0, y=y@entry=465, w=w@entry=16, h=h@entry=16, colorPtr=colorPtr@entry=0xb0db8438, needSameColor=needSameColor@entry=0 '\000') at tight.c:570 #2 0xb6e45aae in SendRectEncodingTight (cl=cl@entry=0xb1700470, x=x@entry=0, y=433, w=w@entry=432, h=h@entry=335) at tight.c:388 #3 0xb6e45fba in SendRectEncodingTight (cl=cl@entry=0xb1700470, x=0, y=0, w=432, h=768) at tight.c:450 #4 0xb6e460f6 in rfbSendRectEncodingTight (cl=cl@entry=0xb1700470, x=<optimized out>, y=<optimized out>, w=<optimized out>, h=h@entry=768) at tight.c:275 #5 0xb6e2cf7c in rfbSendFramebufferUpdate (cl=cl@entry=0xb1700470, givenUpdateRegion=givenUpdateRegion@entry=0xb0200470) at rfbserver.c:3165 #6 0xb6e27e0c in clientOutput (data=0xb1700470) at main.c:497 #7 0xb68b65b4 in start_thread (arg=0x0) at pthread_create.c:335 #8 0xb6967c5c in ?? () at ../sysdeps/unix/sysv/linux/arm/clone.S:89 from /lib/arm-linux-gnueabihf/libc.so.6 Backtrace stopped: previous frame identical to this frame (corrupt stack?)
-
@abmyii said in Dump QImage raw pixel data into `std::vector<char>`:
std::vector<char> &fb
Is this used in another thread?
Otherwise I don't see anything obvious.
-
@Christian-Ehrlicher I think the while loop is in another thread and VNC is accessing
fb.data()
throughout execution. -
@abmyii So you have your answer. Concurrent access to a memory region from two different threads.
-
@Christian-Ehrlicher Ah I see, that makes perfect sense now! Is there a simple solution to this? I tried the following which didn't work:
std::vector<char> temp_fb(frame_size_bytes, 0); std::memcpy(temp_fb.data(), (const char *)image.constBits(), image.byteCount()); fb = temp_fb
-
Use mutexes or copy the data - I would suggest reading a little bit on how threading works. E.g. https://doc.qt.io/qt-5/threads-synchronizing.html - c++ uses the same mechanisms
-
@abmyii said in Dump QImage raw pixel data into `std::vector<char>`:
I never asked if I could use
malloc
instead ofmemcpy
[sic.]in fact the actual function I'm using is
memcpy
... Does that change things and/or can I usemalloc
instead? [*sic.]If you say so! I'll leave others to untangle, who clearly understand what you are saying better than I :) Don't worry about explaining to me.
-
@abmyii said in Dump QImage raw pixel data into `std::vector<char>`:
std::vector<unsigned char> vectorBuffer(charBuffer, charBuffer + length);
fb = vectorBuffer;This does two memcpy for no good reason. Since I don't know what libvncserver function you use you're on your own. Read the documentation of them and also read what's needed for threadsafe access.
-
@JonB Haha, you think you are the only one?!
I am planning to try the mutex idea - I haven't had a chance to look it up and figure out how to do it yet. A basic
QMutex
lock/unlock didn't fix it (though I probably applied it wrong)...@Christian-Ehrlicher Ah, I see. What "libvncserver function" do you mean?
-
@abmyii said in Dump QImage raw pixel data into `std::vector<char>`:
What "libvncserver function" do you mean?
You are using libvncserver, not me.
Giving up here now.
-
Going to mark this thread as closed since it seems the issue lies with VNC, not Qt. Thank you all for your suggestions and helping me to isolate the problem.
@Christian-Ehrlicher Sorry for stressing you out - I thought you were referring to a specific function for some reason. I really need to slow down when reading and comprehend rather than just having a quick look and then replying which is causing confusion on all sides. Thank you again for your patience and assistance throughout.
-
Hi I've done something recently I've needed so my approach was:
total_len = qimg.width() * qimg.height(); std::vector<unsigned int> data; data.reserve(total_len); for(int i=0; i < qimg.height(); i++) for(int j=0; j < qimg.width(); j++){ data.push_back(qimg.pixel(j, i)); }
I needed it as an rgb pixels in order to do some image manipulations, but I believe you can also take the uchar data.