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• Accessing specific memory address from user code
  Posted by Ian on 26/08/09

  This code uses the /dev/mem interface to connect from user space (code running with root privilege) to any physical memory address.

   

  You can use it when prototyping to test out memory-mapped peripherals.  It's been tested on the PC and on ARM linux, and works great.  This compiles into a standalone little application to talk to memory, but you can strip out the 'guts' and use it in your own code.

  #include <sys/mman.h>
  #include <stdlib.h>
  #include <stdio.h>
  #include <fcntl.h>
  
  #define MAP_SIZE 48UL
  #define MAP_MASK (MAP_SIZE -1)
  
  int main(int argc, char **argv){
  	int fd, read_result, writeval;
  	void *map_base, *virt_addr;
  	off_t target=0xB5000000;
  
  	if(argc==2)
  	{
  		target=strtoul(argv[1], 0, 0);
  		printf("Reading from address %p.n", target);
  	}
  	else
  	{
  		if(argc==3)
  		{
  	           	target=strtoul(argv[1], 0, 0);
  			writeval=strtoul(argv[2], 0, 0);
         		  	printf("Writing %p to address %p.n", writeval, target);		}
  		else
  		{
  			printf("Error: wrong number of arguments given.n");
  			printf("Usage: %s 0x10009876        will read from address 0x10009876n");
  			printf("       %s 0x12340000 0xff   will write 0xff to address 0x12340000n");
  			exit(0);
  		}
  	}
  
  	if((fd=open("/dev/mem", O_RDWR | O_SYNC)) == -1){
  		printf("/dev/mem could not be opened.n");
  		exit(1);
  	} else
  		printf("/dev/mem opened.n");
  	
  	//
  map_base=mmap(0,MAP_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED,fd,target & ~MAP_MASK);
  	if(map_base==(void *)-1)
  		printf("Mapping failed.n");
  	else
  		printf("Mapped to addr %p.n",map_base);
  
  	virt_addr=map_base + (target & MAP_MASK);
  
  	if(argc==2)
  	{
  		read_result=*((volatile unsigned long *)virt_addr);
  		printf("We read back the value %pn",read_result);
  	}	
  	else
  	{
  		*((volatile unsigned int *)virt_addr)=writeval;
  	}
  
  	if(munmap(map_base,MAP_SIZE)==-1){
  		printf("Unmapping failed.n");
  	}
  	close(fd);
  }