Neo

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Posts by Neo

How to enable/disable/start/stop a service in Fedora 16

Dec 28th

Posted by Neo in Fedora

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If you are looking for managing system services in earlier versions of Fedora, read this article.

For all those who are using earlier versions of Fedora, the standard chkconfig commands appears to be doing nothing. All the services in Fedora are controlled using a new command called “systemctl“.

The “systemctl” command, if run without any arguments, prints all the system services (units) as shown below.

[neo@techpulp ~]# systemctl
UNIT                      LOAD   ACTIVE SUB       JOB DESCRIPTION
proc-sys...misc.automount loaded active running       Arbitrary Executable File Formats File System Automount Point
...

sys-devi...y-tty10.device loaded active plugged       /sys/devices/virtual/tty/tty10
sys-devi...y-tty11.device loaded active plugged       /sys/devices/virtual/tty/tty11
...

abrt-ccpp.service         loaded active exited        Install More >

How to switch between threads in GDB

Sep 16th

Posted by Neo in C/C++

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The GNU debugger (GDB) provides greater flexibility in debugging multi-threaded applications. Following are some of the facilities that can be used.

List information about all threads

(gdb) info threads

Set break point specific to a thread.

You can mention thread ID after the break point.

(gdb) break Myfunction thread 1

Switch GDB control to a given thread

Following command swicthes GDB control to thread ID 3. The thread ID is derived from the output of “info threads” command.

(gdb) thread 3

Listing Backtrace of all threads at once

(gdb) thread apply all bt

How to improve performance of Linux application

Aug 12th

Posted by Neo in Linux

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Following are some of the tips to improve overall performance of Linux application. It also includes tips for scalability of multi-threaded Linux application.

Reduce number of system calls

Any system call in user space application requires context switching to kernel mode and back to user mode. So the number system calls made in your most frequently executed code may adversely affect your application’s performance.

Use Futex

Unlike Mutex, Futex library attempts to avoid system calls whenever possible. This will improve the performance of the application.

Use Compare and Swap (CAS) and Lockless data structures

This is tricky and getting it right is difficult. However this will More >

How to find size of code and data segments of a program

Aug 4th

Posted by Neo in Command Line

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The Linux provides a command “size” which can shows the sizes of various segments of an executable program.

The following example shows how it can be used:

[neo@techpulp ~]# size /usr/lib/libopensync.so.1.0.0
text       data        bss        dec        hex    filename
387800       3516         28     391344      5f8b0    /usr/lib/libopensync.so.1.0.0
[neo@techpulp ~]#

As you can see the “size” command displays size of “text”, “data” and “bss” segments. The above example used a shared library. However the same can be run against a program as well.

[neo@techpulp ~]# size /bin/bash
text       data        bss        dec        hex    filename
729668      19416   More >

How to catch signals in a bash script

Jul 29th

Posted by Neo in Bash

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POSIX compatible UNIX systems like Linux supports standard signals as part of inter process communication. While some signals like SIGINT can be ignored, SIGKILL and SIGSTOP cannot be caught, blocked or ignored.

Bash provides a command “trap” to assign a signal handler. The following example script catches SIGINT signal that is generally received when user presses Ctrl+C key combination. A bash script will generally terminate on pressing Ctrl+C but it can chose to ignore the signal or do clean up before terminating using signal handler.

#!/bin/bash

trap mysighandler INT

mysighandler()
{
    echo 'You have pressed Ctrl+C.. Ignoring...'
}

for ((i=0;i<10;i++))
do
   echo More >

How to print all lines of a data file in reverse order

Jul 14th

Posted by Neo in Command Line

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The command “tac” (reverse of cat command) which prints contents of a file in reverse order in terms of lines. It can also concatenate multiple files while printing in reverse order.

Sample data file:

[neo@techpulp ~]# cat data.txt
line1: Mango
line2: Apple
line3: Banana
line4: Papaya
[neo@techpulp ~]#

Output of “tac” command on the same data file:

[neo@techpulp ~]# tac data.txt
line4: Papaya
line3: Banana
line2: Apple
line1: Mango
[neo@techpulp ~]#

How to find Ethernet MAC address of my PC in various operating systems

Feb 7th

Posted by Neo in FreeBSD

1 comment

This article explains how to find MAC address of the PC in Linux, Windows, FreeBSP, NetBSP, OpenBSD, Caldera/SCO, IRIX, HP-UX, NeXTStep, AIX, Tru64 UNIX etc.Typically each Ethernet adapter in the world is programmed with an unique MAC address by the manufacturer. In other words, no two Ethernet adapters will have same MAC address.

In Linux (Fedora/Ubuntu/Other flavours)

[neo@techpulp ~]# ifconfig eth0
eth0      Link encap:Ethernet  HWaddr 00:A3:B2:01:5E:4B
inet addr:192.168.1.2  Bcast:192.168.1.255  Mask:255.255.255.0
...

If you have more than one Ethernet adapters in the system, you can use “ifconfig -a” command.

In Microsoft Windows NT/2000/2003/XP Open Run Command Window by pressing R while holding Windows Start Key. Otherwise Click More >

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How to change MAC address of Ethernet interface in Linux or Windows or Mac OS X

Feb 5th

Posted by Neo in Linux

1 comment

The physical MAC address will be permanently programmed to Ethernet adapter by the manufacturer. In other words, each Ethernet adapter carries an unique MAC address in the world.

However the operating systems like Windows or Linux can logically change the MAC address while the adapter in operation. If a MAC address is changed from the original physical MAC address to an another, it is called MAC address spoofing.

The following explains how a MAC address can be modified in various operating systems.

In Linux

[root@techpulp ~]# ifconfig eth0 down
[root@techpulp ~]# ifconfig eth0 hw ether 11:22:33:44:55:66

In FreeBSD,

[root@techpulp ~]# ifconfig nc0 ether 11:22:33:44:55:66

In More >

How to find MAC address of a linux network interface using SIOCGIFHWADDR ioctl

Jan 28th

Posted by Neo in C/C++

1 comment

The following C programming example retrieves all network interfaces in a Linux system and their IP addresses and MAC addresses. The name of network interface and IP address are retrieved using SIOCGIFCONF ioctl while the hardware address of each individual interface is found using SIOCGIFHWADDR ioctl.

/* iflist.c */
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <netinet/in.h>
#include <net/if.h>
#include <arpa/inet.h>
#include <string.h>
#include <unistd.h>

int get_iface_list(struct ifconf *ifconf)
{
  int sock, rval;

  sock = socket(AF_INET,SOCK_DGRAM,0);
  if(sock < 0)
  {
    perror("socket");
    return (-1);
  }

  if((rval = ioctl(sock, SIOCGIFCONF , (char*) ifconf  )) < 0 )
  perror("ioctl(SIOGIFCONF)");

  close(sock);

  return rval; More >

How to print a bash variable along with other text

Jan 10th

Posted by Neo in Bash

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A general programmer tends to use a simple dollar sign ($) before each variable. However its gets difficult if a variable hash to be echoed along with some other as shown below.

An attempt to print “redtape” fails using the following method as bash attempts print value of “COLORtape” variable.

bash# COLOR=red
bash# echo $COLORtape
bash#

In such cases, braces should be used to protect the bash variable. The following example does that.

bash# COLOR=red
bash# echo ${COLOR}tape
redtape
bash#

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