Welcome to OGeek Q&A Community for programmer and developer-Open, Learning and Share
Welcome To Ask or Share your Answers For Others

Categories

0 votes
953 views
in Technique[技术] by (71.8m points)

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

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
");
  /* Get access to the ports */
  if (ioperm(BASEPORT+DIPSWITCH, 10, 1)) 
  {
     perror("ioperm");
     exit(EXIT_FAILURE);
  }

  printf("read the dipswitch with pause
");
  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.

See Question&Answers more detail:os

与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
Welcome To Ask or Share your Answers For Others

1 Reply

0 votes
by (71.8m points)

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);
  }

与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
OGeek|极客中国-欢迎来到极客的世界,一个免费开放的程序员编程交流平台!开放,进步,分享!让技术改变生活,让极客改变未来! Welcome to OGeek Q&A Community for programmer and developer-Open, Learning and Share
Click Here to Ask a Question

1.4m articles

1.4m replys

5 comments

56.8k users

...