Question

所以我有一个我用mmap（）分配的内存区域，类似于下面的代码：

void * ptr = mmap(NULL, size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);

这里的关键是我使用MAP_SHARED标志。与this相关的问题不同，只需再次调用mmap（）来获取MAP_PRIVATE和Copy-on-Write语义就足够了，我不能让内核为我分配不同范围的虚拟地址。另外，在我再次调用mmap（）之前，我不想调用munmap（）并冒险让内核将部分/全部地址范围提供给进程中的其他内容。

是否存在将mmap＆＃t;内存区域从MAP_SHARED切换到MAP_PRIVATE以获取写时复制语义而不取消映射块的现有机制？

Answer 1

再次使用mmap()致电MAP_PRIVATE | MAP_FIXED即可。 MMAP（2）手册页说明使用MAP_FIXED时：

如果无法使用指定的地址，则mmap（）将失败。

因此，只需使用临时指针来存储mmap()结果。如果mmap()失败，则不会造成任何伤害。如果mmap()成功，则表示您已成功将内存映射区域从MAP_SHARED切换为MAP_PRIVATE。（见例）

#include <stdio.h>
#include <sys/mman.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int main()
{
    int fd;
    void *shared_0, *shared_1;
    void *private_0;
    struct stat st;

    if((fd = open("filename", O_RDWR, S_IRUSR | S_IWUSR)) < 0) {
        fprintf(stderr, "Failed to open(): %s\n", strerror(errno));
    }
    else if(fstat(fd, &st) < 0) {
        fprintf(stderr, "Failed fstat(): %s\n", strerror(errno));
    }
    else if((shared_0 = mmap(NULL, st.st_size, PROT_READ | PROT_WRITE,
            MAP_SHARED, fd, 0)) == MAP_FAILED) {
        fprintf(stderr, "Failed to mmap(): %s\n", strerror(errno));
    }
    else if((shared_1 = mmap(NULL, st.st_size, PROT_READ | PROT_WRITE,
            MAP_SHARED, fd, 0)) == MAP_FAILED) {
        fprintf(stderr, "Failed to mmap(): %s\n", strerror(errno));
    }
    else if((private_0 = mmap(shared_0, st.st_size, PROT_READ | PROT_WRITE,
            MAP_FIXED | MAP_PRIVATE, fd, 0)) == MAP_FAILED) {
        fprintf(stderr, "Failed to mmap(): %s\n", strerror(errno));
    }
    else if(shared_0 != private_0) {
        fprintf(stderr, "Error: mmap() didn't map to the same region");
    }
    else {
        printf("shared_0: %p == private_0: %p\n", shared_0, private_0);
        printf("shared_1: %p\n", shared_1);

        printf("Shared mapping before write: %d\n", (*(char *)shared_1));
        printf("Private mapping before write: %d\n", (*(char *)private_0));

        /* write to the private COW mapping and sync changes */
        (*(char*)private_0) = 42;
        if(msync(private_0, 1, MS_SYNC | MS_INVALIDATE) < 0) {
            fprintf(stderr, "Failed msync(): %s\n", strerror(errno));
            return(1);
        }

        printf("Shared mapping after write: %d\n", (*(char *)shared_1));
        printf("Private mapping after write: %d\n", (*(char *)private_0));
    }

    return(0);
}

将mmap＆＃d内存区域从MAP_SHARED更改为MAP_PRIVATE

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