如何从单个文件描述符中分配多个MMAP？(How would I assign multiple MMAP's from single file descriptor?)

 因此，对于我的最后一年项目，我使用Video4Linux2从相机中提取YUV420图像，将它们解析为x264（本机使用这些图像），然后通过Live555将编码流发送到符合RTP / RTCP的视频播放器通过无线网络在客户端上。 所有这些我都试图实时进行，所以会有一个控制算法，但这不是这个问题的范围。 所有这些 - 除了Live555 - 都是用C语言编写的。目前，我已接近编码视频的结尾，但希望提高性能。 
 
 至少可以说，我遇到了麻烦......我正在尝试避免使用V4L2的用户空间指针并使用mmap（）。 我正在对视频进行编码，但由于它是YUV420，我一直在使用新的存储器将Y'，U和V平面保存在三个不同的变量中，以供x264读取。 我想将这些变量作为指向mmap内存的指针。 
 
 但是，V4L2设备有一个用于缓冲流的单个文件描述符，我需要将流分成三个符合YUV420标准的mmap变量，就像这样...... 
 
buffers[n_buffers].y_plane = mmap(NULL, (2 * width * height) / 3,
                                    PROT_READ | PROT_WRITE, MAP_SHARED,
                                    fd, buf.m.offset);
buffers[n_buffers].u_plane = mmap(NULL, width * height / 6,
                                    PROT_READ | PROT_WRITE, MAP_SHARED,
                                    fd, buf.m.offset +
                                    ((2 * width * height) / 3 + 1) /
                                    sysconf(_SC_PAGE_SIZE));
buffers[n_buffers].v_plane = mmap(NULL, width * height / 6,
                                    PROT_READ | PROT_WRITE, MAP_SHARED,
                                    fd, buf.m.offset +
                                    ((2 * width * height) / 3 + 
                                    width * height / 6 + 1) / 
                                    sysconf(_SC_PAGE_SIZE));
 
 其中“宽度”和“高度”是视频的分辨率（例如640x480）。 
 
 根据我的理解...... MMAP寻找一个文件，有点像这样（伪代码）： 
 
fd = v4l2_open(...);
lseek(fd, buf.m.offset + (2 * width * height) / 3);
read(fd, buffers[n_buffers].u_plane, width * height / 6);
 
 我的代码位于Launchpad Repo（更多背景信息）： http ： //bazaar.launchpad.net/~alex-stevens/+junk/spyPanda/files （Revision 11） 
 
 从这个Wiki插图中可以清楚地看到YUV420格式： http ： //en.wikipedia.org/wiki/File ： Yuv420.svg （我基本上想要将Y，U和V字节分成每个mmap'ed记忆） 
 
 有人想解释一种方法，从一个文件描述符中将三个变量映射到内存，或者为什么我出错？ 或者甚至暗示将YUV420缓冲区解析为x264更好的主意？ ：P 
 
 干杯! ^^ 

So, for my final year project, I'm using Video4Linux2 to pull YUV420 images from a camera, parse them through to x264 (which uses these images natively), and then send the encoded stream via Live555 to an RTP/RTCP compliant video player on a client over a wireless network. All of this I'm trying to do in real-time, so there'll be a control algorithm, but that's not the scope of this question. All of this - except Live555 - is being written in C. Currently, I'm near the end of encoding the video, but want to improve performance.
 
To say the least, I've hit a snag... I'm trying to avoid User Space Pointers for V4L2 and use mmap(). I'm encoding video, but since it's YUV420, I've been malloc'ing new memory to hold the Y', U and V planes in three different variables for x264 to read upon. I would like to keep these variables as pointers to an mmap'ed piece of memory.
 
However, the V4L2 device has one single file descriptor for the buffered stream, and I need to split the stream into three mmap'ed variables adhering to the YUV420 standard, like so...
 
buffers[n_buffers].y_plane = mmap(NULL, (2 * width * height) / 3,
                                    PROT_READ | PROT_WRITE, MAP_SHARED,
                                    fd, buf.m.offset);
buffers[n_buffers].u_plane = mmap(NULL, width * height / 6,
                                    PROT_READ | PROT_WRITE, MAP_SHARED,
                                    fd, buf.m.offset +
                                    ((2 * width * height) / 3 + 1) /
                                    sysconf(_SC_PAGE_SIZE));
buffers[n_buffers].v_plane = mmap(NULL, width * height / 6,
                                    PROT_READ | PROT_WRITE, MAP_SHARED,
                                    fd, buf.m.offset +
                                    ((2 * width * height) / 3 + 
                                    width * height / 6 + 1) / 
                                    sysconf(_SC_PAGE_SIZE));
 
Where "width" and "height" is the resolution of the video (eg. 640x480).
 
From what I understand... MMAP seeks through a file, kind of like this (pseudoish-code):
 
fd = v4l2_open(...);
lseek(fd, buf.m.offset + (2 * width * height) / 3);
read(fd, buffers[n_buffers].u_plane, width * height / 6);
 
My code is located in a Launchpad Repo here (for more background): http://bazaar.launchpad.net/~alex-stevens/+junk/spyPanda/files (Revision 11)
 
And the YUV420 format can be seen clearly from this Wiki illustration: http://en.wikipedia.org/wiki/File:Yuv420.svg (I essentially want to split up the Y, U, and V bytes into each mmap'ed memory)
 
Anyone care to explain a way to mmap three variables to memory from the one file descriptor, or why I went wrong? Or even hint at a better idea to parse the YUV420 buffer to x264? :P
 
Cheers! ^^

原文：https://stackoverflow.com/questions/6290147