mmap example

: MAP_SHARED: write data to the ing area will be copied back to the file, and other processes that map the file can share. MAP_PRIVATE: write operations on the ing area will generate a copy-on-write operation. modifications made to this area will not be written back to the original file. In addition, several other flags are not frequently used. For details, refer to the linux C function description. Fd: file descriptor returned by open, representing the file to be m

From: http://blog.csdn.net/scorpio16/article/details/2059623 The second volume of UNIX Network Programming inter-process communication describes MMAP functions. This function has three main purposes:1. Map a common file to the memory, which is usually used when files need to be read and written frequently. In this way, I/O reading and writing is replaced with memory reading and writing to achieve high performance;2. Anonymous memory ing of special fil

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

MMAP can map a part of the disk file directly to the memory, so that the location in the file has a corresponding memory address, you can directly use pointers to read and write files without the need for the read/write function. Prototype: # include Void * MMAP (void * ADDR, size_t Len, int Prot, int flag, int filedes, off_t off ); Int munmap (void * ADDR, size_t Len ); The parameters are expla

) content ); } Close (FD ); Munmap (map_base, 0xff ); Return (1 ); } In the preceding example, the starting address 0x20000 (physical address) is mapped to 0xff. Then you can operate the memory like an ordinary array. The output result is as follows: Address: 0x7f3f95391000 content 0x0 updated address: 0x7f3f95391000 content 0x0Address: 0x7f3f95391001 content 0x0 updated address: 0x7f3f95391001 content 0x1Address: 0x7f3f95391002 content 0x

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

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

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,

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

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

not find * (volatile unsigned int *) (virt_addr+8) =0xf0; //Reset 1 off printf ("3value is 0x%x\n", * (unsigned int *) (virt_addr+8)); printf ("4value is 0x%x\n", * (unsigned int *) VIRT_ADDR); Sleep (1); //must have or not soon, the human eye can not find } #endif /* We' ll never get here on this example * unmapped. */ if (Munmap (map_base, map_size) = =-1) { printf ("Memory unmap failed.\n"); } Close (FD); } The a

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