mmap file

In the driver code, we often need to map the hardware address to the virtual memory, which is why we need to learn MMAP! The so-called cutting-edge without mistake, let's calm down and learn this very useful tool. Prototype: void * MMAP (void * Start, size_t length, int Prot, int flags,Int FD, off_t offset ); Parameter: Start address of the ing area. (It is generally recommended to be null so that the kern

A process can system invoke mmap () to map the contents of an open file to its user space with the following user interface:Mmap (void *start, size_t length, int prot, int flags, int fd, off_t offset).The parameter fd represents an open file, offset is the starting point in the file, and start is mapped to the start ad

); Mapfd (FD, size); // obtain a piece of memory by using the MMAP method of the FD. // Do not understand. Check man MMAP. After mapfd is complete, the mbase variable points to the starting position of the memory. msize indicates the size of the allocated memory, and MFD indicates File descriptor returned by ashmem_create_region } Memoryheapbase provides severa

Use MMAP/dev/MEM to read and write Linux memory Category: Linux Kernel 114 reading comments (1) Favorites Report Use hexedit/dev/MEM to display information in all physical memory. Use MMAP to map/dev/MEM, and then directly read and write it to implement kernel operations in the user space. Below is a sample I wroteView plain # Include # Include # Include # Include # Include # Include Int ma

1. Application space void *virt_mem = Kzalloc (page_size, Gfp_kernel); 2. Drive Layer----->> implement File_operation interface. Mmap method int Led_drv_mmap (struct file *filp, struct vm_area_struct *vma) { VMA is an information body, the user provides MMAP request//Parameter 1, VMA information carrier//parameter 2, User space virtual start position//Pa

First, it is necessary to state that the new kernel has already adopted Vb2_queue and Vb2_buffer Replace the Videobuf_queue and videobuf_buffer that are often used in older versions of the kernel. The two are mainly used when applying vidioc_reqbuf to the user layer. From the user layer request of the type of memory, the typical two are: V4l2_memory_userptr and V4l2_memory_mmap, the former is located in the user layer, the video output memory address of the driver is provided by the user la

communication) , and it is also described in the framebuffer display driver that mmap can map framebuffer memory to user space. This allows each AP of an Android device to manipulate the mapped memory to change what is displayed.Mmap () system calls enable shared memory between processes by mapping the same common file. After the normal file is mapped to the pro

Mmap: a problem occurs when the file is mapped to the memory and the corresponding method on the window. Our program occupies too much memory and crashes when running on the target machine. Some memory can be mapped to files to save some memory. Make a record for future reference. On linux, use mmap: www.2cto.com 1 int dumpFileDescriptor = open (mmFileName, O_CRE

This should be considered to be an IPC, although efficiency may not be as high as other IPC methods.It's easy to see maps that map. Indeed, Mmap is a mapping that maps open files to a contiguous set of memory. The memory can be manipulated to read and write to the file, in turn, that is, it is possible to achieve process communication in this way.The MMAP series

MMAP () and munmap ()-Unix system functions are closer to the underlying void* mmap(void* addr,size_t size,int prot,int flags, int fd, off_t offset) The ADDR parameter can specify the first address of the ing. Generally, the value 0 is assigned to the kernel. Size is the size of the allocated memory, in the unit of page for ing. Prot is the permission to allocate memory. It is gener

that multithreading only has a read rate of 380 Mb/s.Why is fread less efficient? Read some information to learn how to access the hard disk in fread/fwrite mode. You must specify the number of data to be read to the kernel, and then copy the obtained content from the kernel buffer pool to the user space; writing also requires a process like this. In this way, when I/O is accessed, the buffer of such a kernel is often used, resulting in speed restrictions. One solution is

1. Memory mapping the so-called memory mapping is to map physical memory into the process's address space, these applications can directly use the input and output of the address space, so as to improve the efficiency of reading and writing. Linux provides the mmap () function to map physical memory. In the driver, the application takes a device file as an object, invokes the

Description void *mmap (void *start,size_t length,int prot,int flags,int fd,off_t); Specific parameter meaningStart: Point to the memory start address to map, usually set to NULL, to allow the system to automatically select the address, and the mapping succeeds to return the address.Length: Represents how large portions of a file are mapped to memory.Prot: The protection method of the mapped area. There ar

, this will improve performance, but will reduce performance. Let's put this problem in the first place.Some people say that I do not want to fwrite+fflush this combination, I want to write directly to page cache. This is our common file IO call read/write function.These functions are basically a function that corresponds to a system call, such as Sys_read/sys_write. The write function is called to copy data from the application layer to the kernel la

In this example, a character device is used. The device applies for a piece of memory, and file_operations has the MMAP function. In test. C, the MMAP memory can be modified by operating the user memory. Driver code mmap_demo.c #include Makefile KERNELDIR=/lib/modules/$(shell uname -r)/buildPWD=$(shell pwd)obj-m = mmap_demo.omodules:$(MAKE) -C $(KERNELDIR) M=$(PWD) modulestest: test.cgcc $ Test code #inc

Normal through the file system access to the value of Ain serial port, you can use Fread and other file IO read/sys/bus/iio/devices/iio:device0/in_voltage*_raw content. But this requires high frequency access to the ADC is very inefficient, the data needs to be copied from the disk to the kernel process space, and then copied to the user process space, consuming unnecessary resources. This problem can be so

Access to files on disk, you must consolidate and move files into memory according to certain rules, and then access this file. The order in which the pages of the files to be accessed are arranged intoRadix tree. Assuming that the file is small and there is no memory writeback and access to the entire file, the logically linear organization of this

Dynamically passes the physical address and size of the kernel space to user space. This article also demonstrates two common methods that can be used for communication between kernel space and user space: Proc file system and Mmap shared memory.The entire kernel module, when the module inserts, establishes the proc file, allocates the memory. When the module is

# Added some notes # Rockie Cheng # Include # Include # Include # Include # Include # Include # Include # Include # Include # Include # Include # Include # Include # Include # Include # Include # Define clear (x) memset ( (x), 0, sizeof (x )) Struct buffer {Void * start;Size_t length;}; Static char * dev_name = "/dev/video0"; // The Name Of The camera device.Static int FD =-1;Struct buffer * buffers = NULL;Static unsigned int n_buffers = 0; File *

"Dynamic MMap has no space. Please increase the resolution of APT: Cache-Limit... ". when updating the software in javastun, the following error occurs: Java code jiyj @ ubuntu :~ $ Sudo apt-get install firefox [sudo] password for jiyj: Sorry, try again. [sudo] password for jiyj: www.2cto.com is reading the package list... An error occurred! E: Dynamic MMap has no space. Increase the size of APT: Cache-Limi