Question

I have written a small program which was actually extracted from the linux source code. Please find my sample code.

#include <stdio.h>

#define CMOS_READ(addr) ({ \
outb_p(0x80|addr,0x70); \
inb_p(0x71); \
})

#define outb_p(value,port) \
__asm__ ("outb %%al,%%dx\n" \
                "\tjmp 1f\n" \
                "1:\tjmp 1f\n" \
                "1:"::"a" (value),"d" (port))

#define inb_p(port) ({ \
unsigned char _v; \
__asm__ volatile ("inb %%dx,%%al\n" \
        "\tjmp 1f\n" \
        "1:\tjmp 1f\n" \
        "1:":"=a" (_v):"d" (port)); \
_v; \
})

#define outb(value,port) \
__asm__ ("outb %%al,%%dx"::"a" (value),"d" (port))


#define inb(port) ({ \
unsigned char _v; \
__asm__ volatile ("inb %%dx,%%al":"=a" (_v):"d" (port)); \
_v; \
})

int main(void)
{
        int val = 0;
        val  = CMOS_READ(0);
        printf("val = %d", val);
        return 0;
}

It compiles fine but on execution it is giving a seg fault.
Could somebody guide me on this?

Answer 1

You don’t have permissions to access the CMOS I/O ports. You would need to be running with CAP_SYS_RAWIO (usually as root, though you could also be using file capabilities), and call ioperm(2) to request access to the CMOS I/O ports.

Also, <sys/io.h> defines I/O access macros, so you don’t need to define them yourself.