Read /dev/kmem failed

["openbsd shen" <openbsd.shen@gmail.com>]

struct descriptor_idt {
        unsigned short offset_low, seg_selector;
        unsigned char reserved, flag;
        unsigned short offset_high;
};

       .......

        struct descriptor_idt *descriptor;
       .......

        fd_kmem = open("/dev/kmem", O_RDWR);
        ptr_idt = get_addr_idt();
        descriptor = (struct descriptor_idt *) malloc(sizeof(struct
descriptor_idt));
        ......
        readkmem(descriptor, ptr_idt + 8 * x, sizeof(struct descriptor_idt));

        ......

void readkmem(void *m, unsigned off, int size)
{
        int i;
        if (lseek(fd_kmem, off, SEEK_SET) != off) {
                fprintf(stderr, "Error lseek. Are you root? \n");
                exit(-1);
        }
        if ((i = read(fd_kmem, m, size)) != size) {
                fprintf(stderr, "Error read kmem, only read %d bytes\n",i);
                perror("read");
                exit(-1);
        }
}

unsigned long get_addr_idt(void)
{
        unsigned char idtr[6];
        unsigned long idt;
        __asm__ volatile ("sidt %0":"=m" (idtr));
        idt = *((unsigned long *) &idtr[2]);
        return (idt);
}
----------------------------------------------------------------------
When run it, the output is:

Error read kmem, only read 0 bytes
read: Success


I don't know why read error?