在调用inb_p（）时导致分段错误的原因是什么？ [英] What causes a segmentation fault on a call to inb_p()?

问题描述

尝试使用inb_p（）读取端口时出现分段错误。我正在英特尔D525双核系统（Advantech PCM 9389 SBC）上运行2.6.6内核的Debian系统上编译。这是一个示例程序，用于说明段错误。

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.

可能的原因是什么？我该如何解决这个问题？

What is the probable cause? How do I fix this?

目前，我没有连接任何设备。这会导致段错吗？我本来希望得到零或一些随机字节，但不是段错误。

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.

我试过的其他事情：
1）声明输入变量为int而不是char。
2）使用iopl（）而不是ioperm（）

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 */

输出：

root@debian:/home/howard/sources# ./ioexample
begin
read the dipswitch with pause
Segmentation fault

编辑：/ proc / ioports没有列出任何内容在地址0x100，所以我尝试了几个列出的其他端口地址，结果相同。然后我决定尝试输出到已知的并行端口位置（0x0378），并且outb不会导致段错误。但是，尝试读取0x378或0x379 确实会导致段错误。我开始怀疑问题与硬件有关。

/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.

推荐答案

我发现了问题。除了要读取的端口之外，对inb_p（）的调用还需要访问端口0x80。

I found the problem. The call to inb_p() requires access to port 0x80 in addition to the port to be read.

显然，当我尝试使用iopl（）时，我没有调用它正确的，因为它应该有效。

Apparently, when I tried iopl(), I didn't call it correctly, because that should have worked.

以下代码消除了段错误：

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);
  }

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