mmap

. The driver uses multi-buffering to divide a large DMA buffer into multiple small buffers, which are called fragment. They are of the same size. When the driver starts, you only need to wait for the two fragment to start playing. In this way, you can increase the buffer size by increasing the number of fragment, but each fragment is limited to an appropriate size without affecting the latency. Generally, the multi-buffer mechanism in the audio driver uses the scatter-gather function of the base

Linux c development-Memory Manager ptmallocMemory Layout To understand the ptmalloc Memory Manager, you must first understand the memory layout of the operating system. The following figure shows the Memory Distribution of stacks and stacks. X86 LINUX Process Memory layout: Is the memory layout of the linux operating system. The memory distribution of each module in the operating system is displayed from low to high. Test Segment: stores program code. It is read-only and determined during compi

1. What is video4linux?Video4linux (v4l for short) is the kernel driver for video devices in Linux.Video4linux2 is now available and has not been added to the Linux kernel. You need to download the patch by yourself.In Linux, a video device is a device file that can be read and written just like a common file.The camera is under/dev/video0. 2. Video programming process in video4linux1. Enable the video device:2. Read Device Information3. Change the current device settings (if necessary)4. Two Me

use these system APIs. Ii. File Operations in Linux File Operations on Linux systems do not belong to this document. However, you still need to understand the functions and usage of related system calls. These include open (), read (), close (), IOCTL (), and MMAP (). Detailed usage is not described. In Linux systems, various devices (including video devices) are also used in the form of files. They exist in the dev directory, so essentially, the use

Series of articles directory: http://blog.csdn.net/wylblq/article/details/51841684 4.3 Storage mappingsPOSIX provides related calls that allow us to map files into memory, which allows us to easily read file data from memory, modify in-memory data to change the contents of a file, or implement inter-parent interprocess communication.4.3.1 mmap ()#include void *mmap(voidlengthintintint fd, off_t offset)

Video4linux Chinese parsing favorites • Video4linux (v4l for short) is the kernel driver for video devices in Linux. • Video4linux2 is now available and has not been added to the Linux kernel. You need to download the patch by yourself. • In Linux, a video device is a device file that can be read and written just like a common file. • Camera under/dev/video0 1. Enable the video device: 2. Read Device Information 3. Change the current device settings (if necessary) 4. Two Methods for video collec

Introduction to Shared memory shared memory allows two or more processes to share a given store. This is the fastest IPC because the data does not need to replicate between the client process and the server process. There are two ways to share memory: mmap () system calls and System V shared memory. The mmap () system call MMAP () system call enables shared memor

green) are using the PA17 and PL10 pins respectively, if you control both pins using the second method, you will be prompted with "Device or resource Busy" error by the following command:Cat /sys/kernel/debug/gpioYou can see that the system has occupied the following IO ports:The first GPIO17 is PA17, which is configured as output mode by default and is set to low level. Here need to explain the system in the GPIO port numbering method, the system is in accordance with PA~PL a total of 12 group

Linux kernel State and user state shared memory 1. Mmap system call (function)void* mmap (void * addr, size_t len, int prot, int flags, int fd, off_t offset)Memory-mapped function mmap, which is responsible for mapping the contents of the file to the virtual memory space of the process, by reading and modifying the memory, to realize the reading and modification

Heap heap allocation at the user level: malloc function for heap memory allocationvoid malloc (size_t size); Virtual memory address layout for the process:For users, the primary focus is on user space. After you enlarge the user space, you can see that it is mainly divided into the following sections: Code: This is the lowest address part of the entire user space, which is the instruction (the executable machine code compiled by the program) Data: The initialized global v

print the result is not the same ah, that is because the reason for judging the conditions, the above list is only the process of code and instructions, and variables ah.After the fork is executed, the variable for process 1 is count=0,fpid! =0 (parent process). The variable for process 2 is count=0,fpid=0 (child process), the variables of both processes are independent, there are different addresses, not shared, and this should be noted. We can say that we are using fpid to identify and manipu

_ v4ldevice { Int FD; // device number Struct video_capability capability; Struct video_channel [10]; Struct video_picture picture; Struct video_clip clip; Struct video_window window; Struct video_capture capture; Struct video_buffer buffer; Struct video_mmap MMAP; Struct video_mbuf mbuf; Struct video_unit unit; Unsigned char * map; // The first address of the data when the MMAP method is used to obtain da

large, you can tune mmap_time = mmap. mmap (-1, 1024, access = mmap. ACCESS_WRITE, tagname = 'share _ Time') # Read the number of valid bits, excluding empty bits cnt = mmap_time.read_byte () if cnt = 0: print ("Load time to memory") mmap_time = mmap. mmap (0, 1024, access

the two cache systems, you can understand what they can do. What is page cache? Page cache is mainly used as the cache of file data on the file system, especially when the process has read/write operations on the file. If you think about it carefully, it can be called as a system that can map files to the memory: Does mmap naturally need to use page cache? In the current system implementation, page cache is also used as a cache device for other file

of shared memory. Note that the shared memory itself does not provide any synchronization functionality. That is, before the first process finishes writing on shared memory, there is no automatic function to prevent the second process from starting to read it. The problem of access synchronization for shared memory must be the responsibility of the programmer. Optional synchronization methods include mutual exclusion lock, conditional variable, read-write lock, record lock, Semaphore. Second,