c++ mmap读写示例

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分3种情况测试文件读写情况下，page cache 等使用情况，也算是一个使用mmap的例子。文件大小为1GB。

  1. 直接读文件到内存中，内存中开辟文件大小的buffer进行读；
    • mem_total_uesed = 2GB
    • page_cache = 1GB
    • 程序堆栈占用= 1GB
  2. 使用mmap进行文件映射:
    • 仅仅映射，并不把其内容拷贝到内存缓冲区：mem_total_uesed = page_cache = 1GB，程序堆栈占用=0；
    • 将其内容拷贝到内存缓冲区；mem_total_uesed = 2GB；page_cache = 1GB；程序堆栈占用= 1GB
  3. mmap开辟一块1GB匿名内存进行写入：
    • mem_total_uesed = page_cache = 1GB;程序堆栈占用=0； 
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/**
 * @Author  yilan
 * @create  2019/10/23 2:39 PM
 * @brief   测试mmap内存使用情况
 */

#include <iostream>
#include <fstream>
#include <string>
#include <cstring>
#include <sys/mman.h>
#include <sys/types.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/stat.h>
#include <unistd.h>

using namespace std;

/*直接读文件到内存中，内存中开辟文件大小的buffer进行读*/
void read_file_to_buffer(const char* file_name) {
    ifstream fl(file_name);
    fl.seekg(0, ios::end );
    size_t len = fl.tellg();
    cout << len << endl;
    char *ret = new char[len];
    fl.seekg(0, ios::beg);
    fl.read(ret, len);
    fl.close();
//    cout << ret << endl;
}


size_t get_file_size(const char* file_name) {
    struct stat st;
    stat(file_name, &st);
    return st.st_size;
}

/*使用mmap进行文件映射*/
void read_file_from_mmap(const char* file_name) {
    size_t file_size = get_file_size(file_name);
    cout << file_size << endl;
    //Open file
    int fd = open(file_name, O_RDONLY, 0);
    if (fd == -1){
        cout << "Error open file for read" << endl;
        return;
    }
    //Execute mmap
    void* mmapped_data = mmap(NULL, file_size, PROT_READ, MAP_PRIVATE | MAP_POPULATE, fd, 0);
    if (mmapped_data == MAP_FAILED) {
        close(fd);
        cout << "Error mmapping the file" << endl;
        return;
    }

    //Write the mmapped data buffer
    /*将其内容拷贝到内存缓冲区*/
    char *ret = new char[file_size];
    memcpy(ret, mmapped_data, file_size);

    //Cleanup
    int rc = munmap(mmapped_data, file_size);
    if (rc != 0) {
        close(fd);
        cout << "Error un-mmapping the file" << endl;
        return;
    }

//    cout << ret << endl;

    close(fd);
}

/*map开辟一块1GB匿名内存进行写入*/
void write_data_to_anoymous_mmap(){
    cout << "come in the write_data_to_anoymous_mmap" << endl;
    long file_size = 1 * 1024 * 1024 * 1024;
    char* mmapped_data = (char*)mmap(NULL, file_size, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
    if (mmapped_data == MAP_FAILED) {
        cout << "mmap failed: " <<  stderr << endl;
        return;
    }
    cout << "start to write" << endl;
    for (long i = 0; i < file_size; i++){
        mmapped_data[i] = '\0';
    }
    cout << "end of write" << endl;
}

int main(int argc, char **argv) {
    if (argc != 2) {
        printf("wrong params! ./memory_consume_test [file_name]\n");
        return -1;
    }
    char *file_name = argv[1];

    read_file_to_buffer(file_name);
    sleep(600);

    read_file_from_mmap(file_name);
    sleep(600);

    write_data_to_anoymous_mmap();
    sleep(600);

    return 0;
}

#参考链接
A simple mmap() readonly example
mmap and read/write string to file
What is the purpose of MAP_ANONYMOUS flag in mmap system call?