abstract

问：对同一个文件多次mmap，返回的地址相同吗?

答：不相同

code

#ifdef __linux__

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

// 问：对同一个文件多次mmap，返回的地址相同吗?
// 答：不相同
void linux_mmap_test1(void)
{
    const char* file = "/userdata/lxz/test/Makefile";
    struct stat fileInfo = {0};
    int fd = -1;
    std::vector<int> fds;
    std::vector<uint8_t*> ptrs;
    uint8_t* ptr = nullptr;

    if (stat(file, &fileInfo) != 0) {
        fprintf(stderr, "stat error,%s\n", strerror(errno));
        return;
    }

    for (int i = 0; i < 5; ++i) {
        fd = open(file, O_RDONLY);
        if (fd < 0) {
            fprintf(stderr, "i:%d,open error,%s\n", i, strerror(errno));
            break;
        }

        fds.push_back(fd);

        ptr = (uint8_t*)mmap(nullptr, (size_t)fileInfo.st_size, PROT_READ, MAP_SHARED, fd, 0);
        if (!ptr) {
            fprintf(stderr, "i:%d,mmap error,%s\n", i, strerror(errno));
            break;
        }

        fprintf(stderr, "%d <=> %llu\n", fd, (size_t)ptr);
        ptrs.push_back(ptr);
    }

    for (size_t n = 0; n < ptrs.size(); ++n) {
        if (munmap(ptrs[n], size_t(fileInfo.st_size)) != 0) {
            fprintf(stderr, "n:%llu,munmap error,%s\n", n, strerror(errno));
        }
    }

    for (size_t n = 0; n < fds.size(); ++n) {
        close(fds[n]);
    }
}


#endif

performance