What causes a segmentation fault on a call to inb_p()? - c

Question

I am getting a segmentation fault when trying to read a port with inb_p( ). I'm compiling this on a Debian system running 2.6.6 kernel on an Intel D525 dual-core system (Advantech PCM 9389 SBC). Here is a sample program which illustrates the segfault.
What is the probable cause? How do I fix this?
Currently, I don't have any devices hooked up. Could this cause the segfault? I would have expected to get either a zero or some random byte, but not a segfault.
Other things I tried:
1) Declared the input variable as int instead of char.
2) Used iopl() instead of ioperm()
/*
* ioexample.c: very simple ioexample of port I/O
* very simple port i/o
* Compile with `gcc -O2 -o ioexample ioexample.c',
* and run as root with `./ioexample'.
*/
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/io.h>
#define BASEPORT 0x0100 /* iobase for sample system */
#define FLIPC 0x01
#define FLIPST 0x0
#define DIPSWITCH 0x25
int main()
{
char cinput;
cinput = 0xff;
setuid(0);
printf("begin\n");
/* Get access to the ports */
if (ioperm(BASEPORT+DIPSWITCH, 10, 1))
{
perror("ioperm");
exit(EXIT_FAILURE);
}
printf("read the dipswitch with pause\n");
cinput = inb_p(BASEPORT+DIPSWITCH); // <=====SEGFAULT HERE
/* We don't need the ports anymore */
if (ioperm(BASEPORT+DIPSWITCH, 10, 0))
{
perror("ioperm");
exit(EXIT_FAILURE);
}
printf("Dipswitch setting: 0x%X", cinput);
exit(EXIT_SUCCESS);
}
/* end of ioexample.c */
Output:
root#debian:/home/howard/sources# ./ioexample
begin
read the dipswitch with pause
Segmentation fault
Edit: /proc/ioports did not list anything at address 0x100, so I tried several other port addresses that were listed, with the same result. Then I decided to try an output to a known parallel port location (0x0378), and outb did not cause a segfault. However, trying to read either 0x378 or 0x379 did cause a segfault. I am beginning to suspect that the problem is hardware related.

Answer 1

I found the problem. The call to inb_p() requires access to port 0x80 in addition to the port to be read.
Apparently, when I tried iopl(), I didn't call it correctly, because that should have worked.
The following code eliminated the segfault:
/* Get access to the ports */
if (ioperm(0x80, 1, 1))
{
perror("ioperm");
exit(EXIT_FAILURE);
}
if (ioperm(BASEPORT+DIPSWITCH, 10, 1))
{
perror("ioperm");
exit(EXIT_FAILURE);
}