mmap系统调用中MAP_ANONYMOUS标志的作用是什么? [英] What is the purpose of MAP_ANONYMOUS flag in mmap system call?
问题描述
在man页面上，
MAP_ANONYMOUS
The mapping is not backed by any file; its contents are initialized to zero. The fd and offset arguments are ignored; however, some implementations require
fd to be -1 if MAP_ANONYMOUS (or MAP_ANON) is specified, and portable applications should ensure this. The use of MAP_ANONYMOUS in conjunction with
MAP_SHARED is only supported on Linux since kernel 2.4.
使用MAP_ANONYMOUS的目的是什么?任何例子都是好的.还要从哪里映射内存?
What is the purpose of using MAP_ANONYMOUS? Any example would be good. Also From where the memory will be mapped?
The use of MAP_ANONYMOUS in conjunction with MAP_SHARED is only supported on Linux since kernel 2.4.写在man页面上
如何与其他进程共享与MAP_ANONYMOUS映射的内存?
It is written on man page that The use of MAP_ANONYMOUS in conjunction with MAP_SHARED is only supported on Linux since kernel 2.4.
How can i share the memory mapped with MAP_ANONYMOUS with other process?
推荐答案
匿名映射可以显示为归零虚拟文件. 匿名映射只是准备使用的大的零填充内存块. 这些映射位于堆外部，因此不会导致数据段碎片化.
Anonymous mappings can be pictured as a zeroized virtual file. Anonymous mappings are simply large, zero-filled blocks of memory ready for use. These mappings reside outside of the heap, thus do not contribute to data segment fragmentation.
MAP_ANONYMOUS + MAP_PRIVATE:
- 每次通话都会创建一个独特的映射
- 孩子继承了父母的映射
- 儿童在继承映射上的文字以写时复制的方式提供
- 使用这种映射的主要目的是分配一个新的归零内存
- malloc使用匿名私有映射来服务大于MMAP_THRESHOLD字节的内存分配请求.
通常，MMAP_THRESHOLD为128kB.
- every call creates a distinct mapping
- children inherit parent's mappings
- childrens' writes on the inherited mapping are catered in copy-on-write manner
- the main purpose of using this kind of mapping is to allocate a new zeroized memory
- malloc employs anonymous private mappings to serve memory allocation requests larger than MMAP_THRESHOLD bytes.
typically, MMAP_THRESHOLD is 128kB.
MAP_ANONYMOUS + MAP_SHARED:
- 每次调用都会创建一个独特的映射，该映射不会与任何其他映射共享页面
- 孩子继承了父母的映射
- 在共享映射的其他人写在共享映射上时，没有写时复制
- 共享的匿名映射以类似于System V内存段的方式允许IPC，但仅在相关进程之间
- each call creates a distinct mapping that doesn't share pages with any other mapping
- children inherit parent's mappings
- no copy-on-write when someone else sharing the mapping writes on the shared mapping
- shared anonymous mappings allow IPC in a manner similar to System V memory segments, but only between related processes
在Linux上，有两种创建匿名映射的方法:
On Linux, there are two ways to create anonymous mappings:
-
指定MAP_ANONYMOUS标志并为fd传递-1
specify MAP_ANONYMOUS flag and pass -1 for fd
addr = mmap(NULL, length, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
if (addr == MAP_FAILED)
exit(EXIT_FAILURE);
打开/dev/zero并传递此打开的fd
open /dev/zero and pass this opened fd
fd = open("/dev/zero", O_RDWR);
addr = mmap(NULL, length, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
(此方法通常在没有MAP_ANONYMOUS标志的BSD等系统上使用)
(this method is typically used on systems like BSD, that do not have MAP_ANONYMOUS flag)
匿名映射的优点:
-没有虚拟地址空间碎片；取消映射后，内存将立即返回系统.
-它们可以在分配大小，权限方面进行修改，并且还可以像正常映射一样接收建议
-每个分配都是独立的映射，与全局堆分开
Advantages of anonymous mappings:
- no virtual address space fragmentation; after unmapping, the memory is immediately returned to the system
- they are modifiable in terms of allocation size, permissions and they can also receive advice just like normal mappings
- each allocation is a distinct mapping, separate from global heap
匿名映射的缺点:
-每个映射的大小是系统页面大小的整数倍，因此可能导致地址空间的浪费
-与从预先分配的堆
Disadvantages of anonymous mappings:
- size of each mapping is an integer multiple of system's page size, thus it can lead to wastage of address space
- creating and returning mappings incur more overhead than that of from the pre-allocated heap
如果包含此类映射的程序派生了一个进程，则子级将继承该映射. 下面的程序演示了这种继承:
if a program containing such mapping, forks a process, the child inherits the mapping. The following program demonstrates this kinda inheritance:
#ifdef USE_MAP_ANON
#define _BSD_SOURCE
#endif
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/wait.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>
int main(int argc, char *argv[])
{
/*Pointer to shared memory region*/
int *addr;
#ifdef USE_MAP_ANON /*Use MAP_ANONYMOUS*/
addr = mmap(NULL, sizeof(int), PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
if (addr == MAP_FAILED) {
fprintf(stderr, "mmap() failed\n");
exit(EXIT_FAILURE);
}
#else /*Map /dev/zero*/
int fd;
fd = open("/dev/zero", O_RDWR);
if (fd == -1) {
fprintf(stderr, "open() failed\n");
exit(EXIT_FAILURE);
}
addr = mmap(NULL, sizeof(int), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
if (addr == MAP_FAILED) {
fprintf(stderr, "mmap() failed\n");
exit(EXIT_FAILURE);
}
if (close(fd) == -1) { /*No longer needed*/
fprintf(stderr, "close() failed\n");
exit(EXIT_FAILURE);
}
#endif
*addr = 1; /*Initialize integer in mapped region*/
switch(fork()) { /*Parent and child share mapping*/
case -1:
fprintf(stderr, "fork() failed\n");
exit(EXIT_FAILURE);
case 0: /*Child: increment shared integer and exit*/
printf("Child started, value = %d\n", *addr);
(*addr)++;
if (munmap(addr, sizeof(int)) == -1) {
fprintf(stderr, "munmap()() failed\n");
exit(EXIT_FAILURE);
}
exit(EXIT_SUCCESS);
default: /*Parent: wait for child to terminate*/
if (wait(NULL) == -1) {
fprintf(stderr, "wait() failed\n");
exit(EXIT_FAILURE);
}
printf("In parent, value = %d\n", *addr);
if (munmap(addr, sizeof(int)) == -1) {
fprintf(stderr, "munmap()() failed\n");
exit(EXIT_FAILURE);
}
exit(EXIT_SUCCESS);
}
来源:
Linux编程接口
第49章:内存映射，
作者:Michael Kerrisk
Sources:
The Linux Programming Interface
Chapter 49: Memory Mappings,
Author: Michael Kerrisk
Linux系统编程(第3版)
第8章:内存管理，
作者:罗伯特·洛夫(Robert Love)
Linux System Programming (3rd edition)
Chapter 8: Memory Management,
Author: Robert Love
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