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;
}

What I am doing is quite simple. I just want to mmap a file onto a fixed address, but I get the error mmap of to-be-unmapped page failed . I have checked everything but still have no idea.

Answer 1

This

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

should be

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

You are just assigning here and it makes the condition true.

mmap(2) returns a void* . So type of err is also wrong. Declare a new void pointer and use it:

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");