为什么mmap文件会导致使用比文件大小更多的内存？(Why mmap a file result in using more memory than file's size?)

 我正在尝试使用mmap并附带以下示例代码： 
 
    int main() {

    int fd;
    char *filename = "/home/manu/file";
    struct stat statbuf;
    int i = 0;
    char c = *(filename);

    // Get file descriptor and file length
    fd = open(filename, O_RDONLY);
    if (fd == -1) {
        perror("fopen error");
    }
    if (fstat(fd, &statbuf) < 0) {
        perror("fstat error");
    }
    printf("File size is %ld\n", statbuf.st_size);

    // Map the file
    char* mmapA = (char*) mmap(NULL, statbuf.st_size, PROT_READ, MAP_PRIVATE,
            fd, 0);
    if (mmapA == MAP_FAILED) {
        perror("mmap error");
        return 1;
    }

    // Touch all the mapped pages
    while (i < statbuf.st_size) {
        c = mmapA[i];
        i++;
    }
    c++;

    // Close file descriptor
    if (close(fd) == -1) {
        perror("close");
        return 1;
    }

    //Unmap file
    munmap(mmapA, statbuf.st_size);

    return EXIT_SUCCESS;
}
 
 文件大小为137948字节= 134,7千字节。 为了检查程序的内存，我使用top，主要是RES和VIRT列。 我在三个不同的地方寻找这些价值观： 
 
 
 
 就在mmap调用之前 
 
 
 就在mmap调用之后 
 
 
 读完所有映射的内存后，将文件有效地加载到主内存中（页面错误后） 
 
 
 top报告的值是 
 
 
 
 VIRT = 1828 RES = 244 
 
 
 VIRT = 1964 RES = 248 
 
 
 VIRT = 1964 RES = 508 
 
 
 1964 - 1828 = 136，我猜以千字节为单位，因此完全匹配文件的大小。 
 
 但我无法理解508 - 248 = 260的RES差异。为什么它与虚拟内存大小和文件大小不同？ 

I am experimenting with mmap and came with the following sample code:
 
    int main() {

    int fd;
    char *filename = "/home/manu/file";
    struct stat statbuf;
    int i = 0;
    char c = *(filename);

    // Get file descriptor and file length
    fd = open(filename, O_RDONLY);
    if (fd == -1) {
        perror("fopen error");
    }
    if (fstat(fd, &statbuf) < 0) {
        perror("fstat error");
    }
    printf("File size is %ld\n", statbuf.st_size);

    // Map the file
    char* mmapA = (char*) mmap(NULL, statbuf.st_size, PROT_READ, MAP_PRIVATE,
            fd, 0);
    if (mmapA == MAP_FAILED) {
        perror("mmap error");
        return 1;
    }

    // Touch all the mapped pages
    while (i < statbuf.st_size) {
        c = mmapA[i];
        i++;
    }
    c++;

    // Close file descriptor
    if (close(fd) == -1) {
        perror("close");
        return 1;
    }

    //Unmap file
    munmap(mmapA, statbuf.st_size);

    return EXIT_SUCCESS;
}
 
The file size is 137948 bytes = 134,7 kilobytes. To inspect program's memory I am using top, mainly the RES and VIRT columns. I am looking for these values at three different places: 
 
 
 
just before the mmap call
 
 
just after the mmap call
 
 
after having read all the mapped memory to have the file's effectively loaded into main memory (after page faults)
 
 
The value reported by top are
 
 
 
VIRT = 1828 RES = 244
 
 
VIRT = 1964 RES = 248
 
 
VIRT = 1964 RES = 508
 
 
1964 - 1828 = 136, I guess in kilobytes and thus perfectly match the file's size.
 
But I can't understand the RES difference of 508 - 248 = 260 .. Why is it different from virtual memory size and file size ?

原文：https://stackoverflow.com/questions/10216433