main.c

#include<stdio.h>
#include<stddef.h>
#include<sys/mman.h>
#include<fcntl.h>

#define SHMADDR 0x3F000000

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

    fd = open("/dev/mem", O_RDWR);

    if (fd == -1)
    {
        printf("can not access /dev/mem\n" );
        return -1;
    }

    memaddr =
    (int*)mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, fd, SHMADDR);
    if (memaddr == MAP_FAILED)
    {
        printf("mmap failed\n" );
        return -1;
    }
    printf("value=%d\n", *memaddr);
    

}