mmap tutorial

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

Now, in Linux often can be seen in the user space to write the driver, such as x server, some vendors private drive, etc., which means that user space has access to the hardware, this is usually through the MMAP device memory map to the user process space, This allows the user to gain access to the hardware by reading and writing these memory. The kernel typically buffers I/O operations for better performance, but also provides direct I/O and asynchro

Today Mayuyu encounters two more interesting functions, namely the mmap () and the fmemeopen () functions.First look at the mmap () function, the header file for this function is #include and #include function prototypes are as followsReturn value: If the mapping succeeds, it returns the memory start address of the mapping area, otherwise map_failed (-1) is returned, and the cause of the error is stored in

Original URL: http://www.cnblogs.com/geneil/archive/2011/12/08/2281222.html1.mmap system Callvoid *mmap (void *addr, size_t len, int prot, int flags, int fd, off_t offset);Function: Responsible for mapping the contents of the file to the virtual address space of the process, by reading and modifying the memory to realize the reading and modification of the file, without having to call read and write again;P

At the operating system level, the virtual address space seen by each process is independent and there is no intersection between them. Therefore, you need to map different virtual addresses of multiple associated processes to the same physical address space through an intermediate Association. MMAP is such a function. it treats a file (also... information nbs) as the virtual address space seen by each process is independent at the operating system l

First, IntroductionLinux provides a memory-mapped function mmap, which maps the contents of a file to a piece of memory (accurate virtual memory), through the memory read and modify, to realize the file read and modify, first look at the MMAP function declaration: Header file: Prototype: void *mmap (void *addr, size_t length,

mmap where Linux is. what is mmap. On the illustrated, Mmap is a way to operate these devices, the so-called operating equipment, such as IO port (light an LED), LCD controller, disk controller, is actually to the physical address of the device read and write data. However, because the application cannot directly manipulate the device hardware address, the ope

I. Overview memory mapping, in short, is to map a section of memory area of user space to the kernel space, after the successful mapping, user changes to this area of memory can be directly reflected to the kernel space, the same, the kernel space for this section of the changes also directly reflect the user space. The efficiency is very high for both kernel space The following is a schematic diagram of an area of memory that maps universal files to user space. figure I: second, the basi

How the kernel guarantees that each process addresses the memory page of the same shared memory area1. Page cache and swap cache pages: a physical page of an accessed file resides in the page cache or swap cache, and all the information for a page is described by a struct page. There is a field in the struct page that is pointer mapping, which points to a struct ADDRESS_SPACE type structure. All pages in the page cache or swap cache are differentiated according to the address_space structure and

MMAP function is a system call in Unix/Linux. Let's take a look at the introduction to MMAP in section 12.2 of UNIX netword programming: The MMAP function maps either a file or a POSIX shared memory object into the address space of a process. We use this function for three purposes: 1. With a regular file to provide memory-mapped I/O 2. With special files to prov

Environment: Ubuntu9.04 the precautions for using mmap () in 2.6.28.10 kernel (as shown in kernel 2.6.25 +) are not mentioned here. In the new kernel, there are two options related to mmap () ing memory/dev/mem: CONFIG_X86_PAT and CONFIG_STRICT_DEVMEM. The kernel contains the following: CONFIG_STRICT_DEVMEM: Ifthisopti Environment: Ubuntu 9.04 Let's talk about the precautions for using

Original: http://weibo.com/p/1001603830912709174661Write mmap memory and file generates hundreds of MS latency reasonApril 12, 2015 21:10 reading 4274 Recently saw a bug introduction, the author spent 4 months tracking location, found that the JVM statistics will cause the garbage collection process to pause good hundreds of Ms. The JVM statistics are written to a memory area that mmap the file to/tmp.For m

Function Prototypes:void *mmap (void *addr, size_t length, int prot, int flags, int fd, off_t offset);Function: Maps a file to a process memory space with addr as the starting address, and when called, typically passes the addr parameter to NULL, and the kernel chooses the starting virtual address spaceThe corresponding Munmap (2) system call is de-mapped, and the in-memory data is written to the file. As you can see, after calling Munmap, the pointer

Mmap Implementation Analysis Mmap Implementation Analysis This article does not introduce how to use mmap functions, but analyzes its kernel implementation. There is a lot of information available on the Internet. The essence of Mmap is to assign (or find) a suitable vma for the current process, and then set the cor

(the data structure is used here because the data in the shared memory area is usually in a fixed format, which is determined by the communication processes, the structure is generally representative ). Map_normfile1 first open or create a file, and set the file length to 5 people structure sizes. Then, 10 people structures are set starting from the return address of MMAP. Then, the process sleep for 10 seconds, wait for other processes to map the sa

Comparison between MMAP and read/write on File Access We know that files accessed through MMAP or read/write must be cached in the kernel. When you need to read and write content from the file, the files are copied to the cache in the kernel for communication. 1. in read/write mode, the user must specify the number of reads to the kernel, and then copy the obtained content from the kernel cache to the use

At the operating system level, the virtual address space seen by each process is independent and there is no intersection between them. Therefore, you need to map different virtual addresses of multiple associated processes to the same physical address space through an intermediate Association.MMAP is a function that maps a file (that is, the physical address space in the memory) to the address space of different processes.I. WriteIt is worth noting that MMA