Map physical memory from /dev/mem to user space and print value of memroy location specified in command line

mmap_test

#include <cinttypes>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <unistd.h>

int main(int argc, char *argv[]) {
    if (argc < 3) {
        printf("Usage: %s <phys_addr> <offset>\n", argv[0]);
        return 0;
    }

    off_t offset = strtoul(argv[1], NULL, 0);
    size_t len = strtoul(argv[2], NULL, 0);

    // Truncate offset to a multiple of the page size, or mmap will fail.
    size_t pagesize = sysconf(_SC_PAGE_SIZE);
    off_t page_base = (offset / pagesize) * pagesize;
    off_t page_offset = offset - page_base;

    int fd = open("/dev/iomem", O_SYNC);
    uint8_t* mem = static_cast<uint8_t*>(mmap(NULL, page_offset + len, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, page_base));
    if (mem == MAP_FAILED) {
        perror("Can't map memory");
        return -1;
    }

    size_t i;
    for (i = 0; i < len; ++i)
        printf("%02x ", (int)mem[page_offset + i]);

    printf("\n");
    return 0;
}