Tag: linux

Linux下打开文件后没有关闭的后果分析

这两天测试文件操作的性能，发现了有的地方打开文件后没有关闭。不关闭文件的后果是比较严重的，尤其是对服务器端程序来说更为严重。那这样会有什么问题呢？
1.不能再打开新文件.
打开大量文件并且不关闭, 很快会达到进程最大允许打开的文件数限制，这样就不能再打开文件。
在Linux上，可以通过ulimit -n 来查看和更改当前session的限制数，比如在我的机器上是：

$ ulimit -n
7168
$ ulimit -n 10000
10000

也可以通过修改/etc/security/limits.conf来永久性的修改限制数

2. 硬盘空间被占满。
如果文件被打开后，再被删除，在文件不被关闭的情况下， Continue reading “Linux下打开文件后没有关闭的后果分析”

Notes for playing with ptrace on 64 bits Ubuntu 12.10

This blog is the notes during I learning the “Playing with ptrace”(http://www.linuxjournal.com/article/6100).

The original examples was using 32 bits machine, which doesn’t work on my 64 bits Ubuntu 12.10.

Let’s start from the first ptrace example:

#include <sys/ptrace.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <linux/user.h>   /* For constants
                                   ORIG_EAX etc */
int main()
{   pid_t child;
    long orig_eax;
    child = fork();
    if(child == 0) {
        ptrace(PTRACE_TRACEME, 0, NULL, NULL);
        execl("/bin/ls", "ls", NULL);
    }
    else {
        wait(NULL);
        orig_eax = ptrace(PTRACE_PEEKUSER,
                          child, 4 * ORIG_EAX,
                          NULL);
        printf("The child made a "
               "system call %ldn", orig_eax);
        ptrace(PTRACE_CONT, child, NULL, NULL);
    }
    return 0;
}

The compiler shows the following error:

fatal error: 'linux/user.h' file not found
#include <linux/user.h>

Something need to change because of:

  1. The ‘linux/user.h’ no longer exists
  2. The 64 bits register is R*X, so EAX changed to RAX

There are two solutions to fix this: Continue reading “Notes for playing with ptrace on 64 bits Ubuntu 12.10”

HOW TO: Create ssh tunnel at boot time under Ubuntu

Create ssh tunnel

The simplest command to create a ssh tunnel is:

#The following command will create a sock5 proxy on port 7070, and then you can use it in your browser
ssh -ND 7070 HOSTNAME

Use Autossh instead of ssh

I prefer to use autossh instead of ssh because it will auto reconnect if the connection lost, Continue reading “HOW TO: Create ssh tunnel at boot time under Ubuntu”