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meglory — Fri, 24 Jun 2011 10:57:00 GMT

function afunc
{
    echo in fnction: $0 $1 $2
    var1="in function"
    echo var1: $var1
}

var1="outside function"
echo var1: $var1
echo $0: $1 $2
afunc funcarg1 funcarg2
echo var1: $var1
echo $0: $1 $2

OUTPUT:

./ascript: arg1 arg2
in fnction: ./ascript funcarg1 funcarg2
var1: in function
var1: in function
./ascript: arg1 arg2

说明var1在afunc内部被改变了。shell的局部变量跟c语言有些差别�Q�这里默认所有外部定义的变量�Q�在函数内部可以讉K��q�且可以改变。即外部定义的变量默认�ؓ全局变量�?br />若想在afunc内部定义一个局部变量，则需要显式的加上local var1.

待箋

meglory 2011-06-24 18:57 发表评论

meglory — Mon, 14 Mar 2011 00:24:00 GMT

UVA457是个水题�Q�但是我�q�是��到了一些小问题�?br>
1.开了一个state[50][42]数组在main函数里面�Q�结果提交之后发现runtime error�Q�觉着是堆栈溢��Z��。突然想��h��50*42>2000�Q�这�U�数�l�一定是开在main外面的，不然必然堆栈溢出�?br>2.是读题不仔细�Q�当成了多重输入�Q�搞了一个while(scanf(...))�Q�结果超时了。第一�ơ在UVA OJ上面��时�?br>
但是�q�篇文章重点不是讲这个，而是讲我在运行过�E�中�l�常遇到的core dumped现象。我在网上找��C��点资料，贴在下面�Q?br>
原帖�Q?a >http://blogold.chinaunix.net/u3/98822/showart_2093542.html

什么是Core Dump?
Core的意思是内存, Dump的意思是扔出�? 堆出�?
开发和使用Unix�E�序�? 有时�E�序莫名其妙的down�? 却没有�Q何的提示(有时候会提示core dumped). �q�时候可以查看一下有没有形如core.�q�程��L��文�g生成, �q�个文�g便是操作�pȝ��把程序down掉时的内存内�Ҏ��出来生成�? 它可以做��试程序的参�?
core dump又叫核心转储, 当程序运行过�E�中发生异常, �E�序异常退出时, 由操作系�l�把�E�序当前的内存状况存储在一个core文�g�? 叫core dump.

如何使用core文�g?
gdb -c core文�g路径 [应用�E�序的�\径]
�q�去后输入where回�R, ��可以显�C�程序在哪一行当掉的, 在哪个函��C��.

��Z��么没有core文�g生成�?
有时候程序down�? 但是core文�g却没有生�? core文�g的生成跟你当前系�l�的环境讄��有关�p? 可以用下面的语句讄��一�? 然后再运行程序便成生成core文�g.
ulimit -c unlimited
�?span style="TEXT-DECORATION: underline">没有扑ֈ�core文�g�Q�我们改改ulimit的设�|�，让它产生�?024是随便取的，要是core文�g大于1024个块�Q�就产生不出来了。）
$ ulimit -c 1024 �Q��{者注: 使用-c unlimited不限制core文�g大小�?br>
core文�g生成的位�|�一般于�q�行�E�序的�\径相�? 文�g名一般�ؓcore.�q�程�?br>
4. 用gdb查看core文�g:
下面我们可以在发生运行时信号引�v的错误时发生core dump�?
发生core dump之后, 用gdb�q�行查看core文�g的内�? 以定位文件中引发core dump的行.
gdb [exec file] [core file]
�?
gdb ./test test.core
在进入gdb�? 用bt命��o查看backtrace以检查发生程序运行到哪里, 来定位core dump的文�?>�?

===========================================================================

造成�E�序core dump的原因很多，�q�里�Ҏ��以往的经验�ȝ��一下：

1 内存讉K��界

a) �׃��使用错误的下标，��D��数组讉K��界

b) 搜烦字符串时�Q�依靠字�W�串�l�束�W�来判断字符串是否结束，但是字符串没有正常的使用�l�束�W?/p>

c) 使用strcpy, strcat, sprintf, strcmp, strcasecmp�{�字�W�串操作函数�Q�将目标字符串读/写爆。应该��用strncpy, strlcpy, strncat, strlcat, snprintf, strncmp, strncasecmp�{�函数防止读写越界�?/p>

2 多线�E�程序��用了�U�程不安全的函数�?/p>

应该使用下面�q�些可重入的函数�Q�尤其注意红色标�C�出来的函数�Q�它们很�Ҏ��被用错：

asctime_r(3c) gethostbyname_r(3n) getservbyname_r(3n) ctermid_r(3s) gethostent_r(3n) getservbyport_r(3n) ctime_r(3c) getlogin_r(3c) getservent_r(3n) fgetgrent_r(3c) getnetbyaddr_r(3n) getspent_r(3c) fgetpwent_r(3c) getnetbyname_r(3n) getspnam_r(3c) fgetspent_r(3c) getnetent_r(3n) gmtime_r(3c) gamma_r(3m) getnetgrent_r(3n) lgamma_r(3m) getauclassent_r(3) getprotobyname_r(3n) localtime_r(3c) getauclassnam_r(3) etprotobynumber_r(3n) nis_sperror_r(3n) getauevent_r(3) getprotoent_r(3n) rand_r(3c) getauevnam_r(3) getpwent_r(3c) readdir_r(3c) getauevnum_r(3) getpwnam_r(3c) strtok_r(3c) getgrent_r(3c) getpwuid_r(3c) tmpnam_r(3s) getgrgid_r(3c) getrpcbyname_r(3n) ttyname_r(3c) getgrnam_r(3c) getrpcbynumber_r(3n) gethostbyaddr_r(3n) getrpcent_r(3n)

3 多线�E�读写的数据未加锁保护�?/p>

对于会被多个�U�程同时讉K��的全局数据�Q�应该注意加锁保护，否则很容易造成core dump

4 非法指针

a) 使用�I�指�?/p>

b) 随意使用指针转换。一个指向一�D�内存的指针�Q�除非确定这�D�内存原先就分配为某�U�结构或�c�d��Q�或者这�U�结构或�c�d��的数�l�，否则不要��它转换��U�结构或�c�d��的指针，而应该将�q�段内存拯��C��个这�U�结构或�c�d��中，再访问这个结构或�c�d��。这是因为如果这�D�内存的开始地址不是按照�q�种�l�构或类型对齐的�Q�那么访问它时就很容易因为bus error而core dump.

5 堆栈溢出

不要使用大的局部变量（因�ؓ局部变量都分配在栈上）�Q�这样容易造成堆栈溢出�Q�破坏系�l�的栈和堆结构，��D��出现莫名其妙的错误�?

-------
我自��q��序core dumped��是因�ؓ�W?个原因，堆栈溢出。我的局部数�l�开的过大，而局部变量分配在栈上�Q�导致堆栈溢出�?/p>

meglory 2011-03-14 08:24 发表评论

meglory — Wed, 22 Dec 2010 08:12:00 GMT

文章转自http://www.ugrad.cs.ubc.ca/~cs219/CourseNotes/Unix/commands-links.html
Hard links and Soft links

Links

As was mentioned in the section on file system structure, every file has a data structure (record) known as an i-node that stores information about the file, and the filename is simply used as a reference to that data structure. A link is simply a way to refer to the contents of a file. There are two types of links:

Hard links: a hard link is a pointer to the file's i-node. For example, suppose that we have a file a-file.txt that contains the string "The file a-file.txt":
```
% cat a-file.txt
The file a-file.txt
%
```
Now we use the ln command to create a link to a-file.txt called b-file.txt:
```
% ls
./  ../  a-file.txt
% ln a-file.txt b-file.txt
% ls
./  ../  a-file.txt  b-file.txt
```
The two names a-file.txt and b-file.txt now refer to the same data:
```
% cat b-file.txt
The file a-file.txt
%
```
If we modify the contents of file b-file.txt, then we also modify the contents of file a-file.txt:
```
% vi b-file.txt
...
% cat b-file.txt
The file a-file.txt has been modified.
% cat a-file.txt
The file a-file.txt has been modified.
%
```
and vice versa:
```
% vi a-file.txt
...
% cat a-file.txt
The file a-file.txt has been modified again!
% cat b-file.txt
The file a-file.txt has been modified again!
%
```
Soft links (symbolic links): a soft link, also called symbolic link, is a file that contains the name of another file. We can then access the contents of the other file through that name. That is, a symbolic link is like a pointer to the pointer to the file's contents. For instance, supposed that in the previous example, we had used the -s option of the ln to create a soft link:
```
% ln -s a-file.txt b-file.txt
```
On disk, the file system would look like the following picture:

But what are the differences between the two types of links, in practice? Let us look at an example that highlights these differences. The directory currently looks like this (let us assume that a-file.txt b-file.txt are both hard links to the same file):

% ls
./  ../  a-file.txt  b-file.txt

Let us first add another symbolic link using the -s option:

% ln -s a-file.txt Symbolicb-file.txt
% ls -F
./  ../  a-file.txt  b-file.txt  Symbolicb-file.txt@

A symbolic link, that ls -F displays with a @ symbol, has been added to the directory. Let us examine the contents of the file:

% cat Symbolicb-file.txt 
The file a-file.txt has been modified again!

If we change the file Symbolicb-file.txt, then the file a-file.txt is also modified.

% vi Symbolicb-file.txt
...
% cat Symbolicb-file.txt
The file a-file.txt has been modified a third time!
% cat a-file.txt
The file a-file.txt has been modified a third time!
% cat b-file.txt
The file a-file.txt has been modified a third time!
%

If we remove the file a-file.txt, we can no longer access the data through the symbolic link Symbolicb-file.txt:

% ls -F
./  ../  a-file.txt  b-file.txt  Symbolicb-file.txt@
% rm a-file.txt
rm: remove `a-file.txt'? y
% ls -F
./  ../  b-file.txt  Symbolicb-file.txt@
% cat Symbolicb-file.txt
cat: Symbolicb-file.txt: No such file or directory

The link Symbolicb-file.txt contains the name a-file.txt, and there no longer is a file with that name. On the other hand, b-file.txt has its own pointer to the contents of the file we called a-file.txt, and hence we can still use it to access the data.

% cat b-file.txt
The file a-file.txt has been modified a third time!

Although it may seem like symbolic links are not particularly useful, hard links have their drawbacks. The most significant drawback is that hard links cannot be created to link a file from one file system to another file on another file system. A Unix file structure hierarchy can consist of several different file systems (possibly on several physical disks). Each file system maintains its own information regarding the internal structure of the system and the individual files on the system. Hard links only know this system-specific information, which make hard links unable to span file systems. Soft links, on the other hand, know the name of the file, which is more general, and are able to span file systems.

For a concrete analogy, suppose that our friend Joel User is a student at both UBC and SFU. Both universities assign him a student number. If he tries to use his UBC student number at SFU, he will not meet with any success. He will also fail if he tries to use his SFU student number at UBC. But if he uses his legal name, Joel User, he will probably be successful. The student numbers are system-specific (like hard links), while his legal name spans both of the systems (like soft links).

Here is an example that demonstrates a situation where a hard link cannot be used and a symbolic link is needed. Suppose that we try to create a hard link from the current working directory to the C header stdio.h.

% ln /usr/include/stdio.h stdio.h
ln: creating hard link `stdio.h' to `/usr/include/stdio.h': Invalid cross-device link
%

The ln command fails because stdio.h is stored on a different file system. If we want to create a link to it, we will have to use a symbolic link:

% ln -s /usr/include/stdio.h stdio.h
% ls -l
lrwxrwxrwx    1 a1a1 guest          20 Apr 20 11:58 stdio.h -> /usr/include/stdio.h
% ls
./  ../  stdio.h@
%

Now we can view the file stdio.h just as if it was located in the working directory. For example:

% cat stdio.h 
/* Copyright (c) 1988 AT&T */ 
/* All Rights Reserved */ 

/* THIS IS UNPUBLISHED PROPRIETARY SOURCE CODE OF AT&T */ 
/* The copyright notice above does not evidence any */ 
/* actual or intended publication of such source code. */ 

/* 
* User-visible pieces of the ANSI C standard I/O package. 
*/ 

#ifndef _STDIO_H 
#define _STDIO_H 
... 
%

The entire output of the cat command was not included to save space.

Note that the long listing (ls -l) of a soft link does not accurately reflect its associated permissions. To view the permissions of the file or directory that the symbolic link references, the -L options of the ls command can be used. For example:

% ln -s /usr/include/stdio.h stdio.h

% ls -l stdio.h
lrwxrwxrwx   1 a1a1     undergrad     20 May 10 15:13 stdio.h -> /usr/include/stdio.h

% ls -l /usr/include/stdio.h
-rw-r--r--   1 root     bin        11066 Jan  5  2000 /usr/include/stdio.h

% ls -lL stdio.h
-rw-r--r--   1 root     bin        11066 Jan  5  2000 stdio.h

meglory 2010-12-22 16:12 发表评论