c - mmap 中的无效参数

Question

#include <stdio.h>      /* fprintf */
#include <stdlib.h>     /* exit */
#include <string.h>     /* memset */
#include <sys/mman.h>       /* mmap */
#include <sys/types.h>      /* pthread types */
#include <sys/stat.h>       /* fchmod */
#include <pthread.h>        /* thread primitives */
#include <fcntl.h>      /* open */
#include <unistd.h>     /* ftruncate */
#include <errno.h>      /* errno */

#define LIB_ADDR   0xaabbccdd   /* memorable random address */

#define UNMAP_FILE "unmapfile"
#define PAGE_SIZE 1024
#define DIE(msg)                \
    printf("-----\nDIE:%s\n-----\n", msg)

int
main(int argc, char **argv)
{
  int err;
  int unmap_fd;

  unmap_fd = open(UNMAP_FILE, O_RDWR | O_CREAT);
  if (unmap_fd < 0) DIE("open of unmap file failed");

  err = ftruncate(unmap_fd, PAGE_SIZE);
  if (err) DIE("ftruncate unmap file to page size failed");

  err = mmap((void *)LIB_ADDR + PAGE_SIZE, PAGE_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_FIXED, unmap_fd, 0);
  if (err = MAP_FAILED) DIE("mmap of to-be-unmapped page failed");

  return 0;
}

我做的很简单。我只想将文件 mmap 到固定地址，但出现错误 mmap of to-be-unmapped page failed。我已经检查了所有内容，但仍然不知道。

Answer 1

这个

  if (err = MAP_FAILED) DIE("mmap of to-be-unmapped page failed");

应该是

  if (err == MAP_FAILED) DIE("mmap of to-be-unmapped page failed"); 
//Notice the ==

你只是在这里赋值，它使条件为真。

mmap(2) 返回一个 void*。所以 err 的类型也是错误的。声明一个新的 void 指针并使用它:

void *mptr;
....
....

 mptr = mmap((void *)LIB_ADDR + PAGE_SIZE, PAGE_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_FIXED, unmap_fd, 0);
 if (mptr == MAP_FAILED) DIE("mmap of to-be-unmapped page failed");