梦幻屋

 

#include
#include
#include
#include

void dump(int signo)
{
char buf[1024];
char cmd[1024];
FILE *fh;

snprintf(buf, sizeof(buf), "/proc/%d/cmdline", getpid());
if(!(fh = fopen(buf, "r")))
exit(0);
if(!fgets(buf, sizeof(buf), fh))
exit(0);
fclose(fh);
if(buf[strlen(buf) - 1] == 'n')
buf[strlen(buf) - 1] = '';
snprintf(cmd, sizeof(cmd), "gdb %s %d", buf, getpid());
system(cmd);

exit(0);
}

void
dummy_function (void)
{
unsigned char *ptr = 0x00;
*ptr = 0x00;
}

int
main (void)
{
signal(SIGSEGV, &dump);
dummy_function ();

return 0;
}

 

　　编译运行效果如下：

 

xiaosuo@gentux test $ gcc -g -rdynamic f.c
xiaosuo@gentux test $ ./a.out
GNU gdb 6.5
Copyright (C) 2006 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "i686-pc-linux-gnu"...Using host libthread_db library "/lib/libthread_db.so.1".

Attaching to program: /home/xiaosuo/test/a.out, process 9563
Reading symbols from /lib/libc.so.6...done.
Loaded symbols for /lib/libc.so.6
Reading symbols from /lib/ld-linux.so.2...done.
Loaded symbols for /lib/ld-linux.so.2
0xffffe410 in __kernel_vsyscall ()
(gdb) bt
#0 0xffffe410 in __kernel_vsyscall ()
#1 0xb7ee4b53 in waitpid () from /lib/libc.so.6
#2 0xb7e925c9 in strtold_l () from /lib/libc.so.6
#3 0x08048830 in dump (signo=11) at f.c:22
#4
#5 0x0804884c in dummy_function () at f.c:31
#6 0x08048886 in main () at f.c:38

 

　　怎么样？是不是依旧很酷？

 

　　以上方法都是在系统上有gdb的前提下进行的，如果没有呢？其实glibc为我们提供了此类能够dump栈内容的函数簇，详见/usr/include/execinfo.h（这些函数都没有提供man page，难怪我们找不到），另外你也可以通过gnu的手册进行学习。

 

 

　　重写的代码如下:

#include
#include
#include
#include

/* A dummy function to make the backtrace more interesting. */
void
dummy_function (void)
{
unsigned char *ptr = 0x00;
*ptr = 0x00;
}

void dump(int signo)
{
void *array[10];
size_t size;
char **strings;
size_t i;

size = backtrace (array, 10);
strings = backtrace_symbols (array, size);

printf ("Obtained %zd stack frames.n", size);

for (i = 0; i < size; i++)
printf ("%sn", strings[i]);

free (strings);

exit(0);
}

int
main (void)
{
signal(SIGSEGV, &dump);
dummy_function ();

return 0;
}

 

　　编译运行结果如下：

xiaosuo@gentux test $ gcc -g -rdynamic g.c
xiaosuo@gentux test $ ./a.out
Obtained 5 stack frames.
./a.out(dump+0x19) [0x80486c2]
[0xffffe420]
./a.out(main+0x35) [0x804876f]
/lib/libc.so.6(__libc_start_main+0xe6) [0xb7e02866]
./a.out [0x8048601]

　　这次你可能有些失望,似乎没能给出足够的信息来标示错误,不急,先看看能分析出来什么吧,用objdump反汇编程序,找到地址0x804876f对应的代码位置:

xiaosuo@gentux test $ objdump -d a.out

8048765: e8 02 fe ff ff call 804856c
804876a: e8 25 ff ff ff call 8048694
804876f: b8 00 00 00 00 mov $0x0,%eax
8048774: c9 leave

　　我们还是找到了在哪个函数(dummy_function)中出错的,信息已然不是很完整,不过有总比没有好的啊!

 

 

　　本文给出了分析"段错误"的几种方法,不要认为这是与孔乙己先生的"回"字四种写法一样的哦,因为每种方法都有其自身的适用范围和适用环境,请酌情使用,或遵医嘱。