c - 基本 mmap(2) 调用失败

Question

我已经为这段非常基本的代码绞尽脑汁好几个小时了，但我一直没能理解为什么 mmap(2) 会失败。

#include <sys/mman.h>
#include <sys/user.h>
#include <err.h>
#include <stdio.h>
#include <stdlib.h>

#define NPAGES      50
#define SLABSIZE    (PAGE_SIZE * NPAGES)      // 200 kB

int
main(int argc, char *argv[])
{
        char *slab;

        printf("DEBUG: mmap(%#x, %u, %#x, %#x, %d, %u)\n",
            NULL, SLABSIZE, PROT_READ | PROT_WRITE, MAP_ANON, -1, 0);
        slab = mmap(NULL, SLABSIZE, PROT_READ | PROT_WRITE, MAP_ANON, -1, 0);
        if (slab == MAP_FAILED)
                err(1, "mmap");
}

但是当我运行它时:

$ make mmap
cc     mmap.c   -o mmap
$ ./mmap 
DEBUG: mmap(0, 204800, 0x3, 0x20, -1, 0)
mmap: mmap: Invalid argument

我检查并重新检查了 mmap(2) 的联机帮助页 [1]，似乎所有要求都没有问题，但我一定遗漏了一些东西。我正在运行 Linux 内核 4.8.13。

谢谢。——杰瑞米

[1] http://man7.org/linux/man-pages/man2/mmap.2.html

Answer 1

当 strace 执行您的程序时，我看到:

mmap(NULL, 204800, PROT_READ|PROT_WRITE, MAP_FILE|MAP_ANONYMOUS, -1, 0) = -1 EINVAL (Invalid argument)

您忘记了 MAP_SHARED 标志(或 MAP_PRIVATE 标志)。有了它(MAP_SHARED 或 MAP_PRIVATE，但您需要其中之一)您的程序可以运行:

    slab = mmap(NULL, SLABSIZE, PROT_READ | PROT_WRITE, 
                MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);

引用mmap(2)手册页:

This behavior is determined by including exactly one of the following values in flags:

^{(重点是我的)}

  MAP_SHARED
          Share this mapping.  Updates to the mapping are visible to
          other processes mapping the same region, and (in the case of
          file-backed mappings) are carried through to the underlying
          file.  (To precisely control when updates are carried through
          to the underlying file requires the use of msync(2).)

  MAP_PRIVATE
          Create a private copy-on-write mapping.  Updates to the
          mapping are not visible to other processes mapping the same
          file, and are not carried through to the underlying file.  It
          is unspecified whether changes made to the file after the
          mmap() call are visible in the mapped region.

因此，在紧张不安之前的一般建议是:再次阅读文档；小睡一下;再次阅读文档并思考您错过了什么。

另一个提示是使用 strace(1)在某些(或多个)现有 可执行文件上。你会学到很多东西。