How to map memory to an userspace program using mmap()

This page provides an example of how to map kernel memory in a userspace application using a mmap() driver. The driver (for Linux 2.6) and userspace program should be looked upon as example code, you will most probably have to change the code to fit your needs. To gain more information about mmap() read the man page: 'man mmap' in a shell or in your browser by clicking here. Additional information on memory mapping can be found in chapter 15 “Memory Mapping and DMA” in the Linux Device Drivers book, Third Edition.

The code below can be downloaded as a tar.gz file: mmap.tar.gz.

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/init.h>
#include <linux/fs.h>
#include <linux/mm.h>
 
#define MAP_MAJOR 126
 
static int
map_mmap(struct file *filp, struct vm_area_struct *vma);
 
struct file_operations map_fops =
{
  .open    = nonseekable_open,
  .mmap    = map_mmap  
};
 
static int
map_mmap(struct file *filp, struct vm_area_struct *vma)
{
	if (remap_pfn_range(vma, vma->vm_start, vma->vm_pgoff,
	                    vma->vm_end - vma->vm_start, vma->vm_page_prot)) {
                return -EAGAIN;
	}
	return 0;
}
 
static int __init 
map_init(void)
{
  if (register_chrdev(MAP_MAJOR,"map", &map_fops) <0 ) 
  {
	printk("unable to get major for map module\n");
	return -EBUSY;
  }
 
  return 0;
}
 
void __exit
map_cleanup(void)
{
  unregister_chrdev(MAP_MAJOR,"map");
}
 
MODULE_LICENSE("GPL");
 
module_init(map_init);
module_exit(map_cleanup);

MODULE    = map.o

LINUX_2_6 = $(shell grep '^\#define UTS_RELEASE' \
		              $(AXIS_KERNEL_DIR)/include/linux/version.h \
			| grep "2.6")

objects-y +=  map_main.o

ifneq ($(LINUX_2_6),)
map-y = $(objects-y)
else
obj-y = $(objects-y)
endif

PREVENT_RECURSIVE_INCLUDE = 1
include $(AXIS_TOP_DIR)/modules/rules.build_modules

install: local_install

local_install:
	$(MKNOD) -m 0644 $(prefix)/dev/map c 126 0

This userspace program maps 3 MB (uncached) SDRAM and writes to it.

#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
 
int main(void)
{
  char* data;
  int fd = open("/dev/map", O_RDWR);
  data = mmap(0, 0x00300000, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0xc0000000);
  close(fd);
  data[100] = 3;
  munmap(data, 0x00300000);
  return 0;
}