Given a singly linked list find the n-th node from the back.

Solution 1: Reverse the linked list and select the n-th node from the head of the linked list.

Solution 2: Maintain 2 pointers n nodes apart. When the 1st pointer reaches the tail, the second pointer will be pointing to the desired node.

Notes: Both solutions take O(n) time but Solution 2 is more elegant.

Code:

//define the list node
typedef struct _node
{
 int i;
 struct _node *next;
} Node;

Node * FindNthFromBack(Node *listHead, int n)
{
    Node *ptr1, *ptr2;  // we need 2 pointers
    ptr1 = ptr2 = listHead; // set the pointers to point to the list head initially

    while(ptr1->next != NULL) // keep looping until we reach the tail (next will be NULL for the last node)
    {
        if(n > 0)
        {
            ptr1 = ptr1->next; //increment only the 1st pointer
            n--;
        }
        else
        {
            ptr1 = ptr1->next; //increment both pointers
            ptr2 = ptr2->next;
        }
    }
    return ptr2;    //now return the ptr2 which points to the nth node from the tail
}

Where singly linked lists are merged?

There are 2 singly linked lists (list1 and list2) that merge at one of the nodes and share the nodes there onwards. The goal is to find the node where these linked lists merge or join.

Solution:

   1. Find length of both linked lists, say len1 and len2.
   2. Make current pointers for both lists equidistant from the common node. If one of the lists is longer we need to advance its current pointer by len1 - len2 (assuming list1 is longer). Now both current pointers should be equidistant from the common node. If both lists are same len, do nothing in step 2.
   3. Traverse both lists simultaneously while comparing the current pointers for equality. At ant point if they are equal we have found the first common node. If we reach the end of the lists without find the common node then the lists do not overlap.

Loop detection in a singly linked list.

Observation: A linked list with a loop will have a node that is being pointed from 2 different node. 

Solution: This problem was solved in the late 1960s by Robert W. Floyd. The solution is aptly named as Floyd's cycle finding algorithm a.k.a the Tortoise and Hare algorithm. It uses 2 pointers moving at different speeds to walk the linked list. Once they enter the loop they are expected to meet, which denotes that there is a loop. This works because the only way a faster moving pointer would point to the same location as a slower moving pointer is if somehow the entire list or a part of it is circular. Think of a tortoise and a hare running on a track. The faster running hare will catch up with the tortoise if they are running on circular track instead of a straight strip. 

Code: //returns true if the linked list contains a loop

bool ListContainsLoop(Node * head)
{
Node * slowPtr = head;
Node * fastPtr = head;

while(slowPtr  && fastPtr)
{
 fastPtr = fastPtr->next; // advance the fast pointer
 if(fastPtr == slowPtr)   // and check if its equal to the slow pointer
     return true;         // loop detected

 if(fastPtr == NULL)
 {
     return false;        // since fastPtr is NULL we reached the tail
 }

 fastPtr = fastPtr->next; //advance and check again
 if(fastPtr == slowPtr)
     return true;

 slowPtr = slowPtr-next;  // advance the slow pointer only once
}
return false;                // we reach here if we reach the tail
}

Git commands

Create local repository:
-----------------------
mkdir <repository name>
cd <repository name>
git init --bare

Create working directory:
------------------------
mkdir <directory name>
cd <directory name>
git init

Committing file:
---------------
1. git add <filename>
If it is a new folder(my_project) and its contents:
a) git add my_project
b) git add my_project/*
2. git commit -m "<mesage>"
a) git commit -a [To commit all modified and deleted and newly added files]
3. git remote add origin <remote repository URL>
git remote -v
4. git push <filename>
a) git push <origin> master

See all the files in repository:
-------------------------------
git ls-tree --full-tree -r HEAD

Cloning a repository:
--------------------
git clone <repository URL>

Linux commands

tar:

Create a new tar archive:
$ tar cvf archive_name.tar dirname/

Extract from an existing tar archive:
$ tar xvf archive_name.tar

View an existing tar archive:
$ tar tvf archive_name.tar


ssh:

Login to remote host:
ssh -l jsmith remotehost.example.com

Debug ssh client:
ssh -v -l jsmith remotehost.example.com

Display ssh client version:
$ ssh -V
OpenSSH_3.9p1, OpenSSL 0.9.7a Feb 19 2003


sed: When you copy a DOS file to Unix, you could find \r\n in the end of each line. This example converts the DOS file format to Unix file format using sed command.

$sed 's/.$//' filename
Print file content in reverse order

$ sed -n '1!G;h;$p' filename.txt
Add line number for all non-empty-lines in a file

$ sed '/./=' filename.txt | sed 'N; s/\n/ /'

awk:

Remove duplicate lines using awk:
$ awk '!($0 in array) { array[$0]; print }' temp

Print all lines from /etc/passwd that has the same uid and gid
$awk -F ':' '$3==$4' passwd.txt

Print only specific field from a file.
$ awk '{print $2,$5;}' employee.txt
8 Powerful Awk Built-in Variables – FS, OFS, RS, ORS, NR, NF, FILENAME, FNR

vim:

Go to the 143rd line of file
$ vim +143 filename.txt

Go to the first match of the specified
$ vim +/search-term filename.txt

Open the file in read only mode.
$ vim -R /etc/passwd

diff:

Ignore white space while comparing.
# diff -w name_list.txt name_list_new.txt
2c2,3
< John Doe --- > John M Doe
> Jason Bourne
Note: Top 4 File Difference Tools on UNIX / Linux – Diff, Colordiff, Wdiff, Vimdiff

sort:

Sort a file in ascending order
$ sort names.txt

Sort a file in descending order
$ sort -r names.txt

Sort passwd file by 3rd field.
$ sort -t: -k 3n /etc/passwd | more

export:

To view oracle related environment variables.
$ export | grep ORACLE
declare -x ORACLE_BASE="/u01/app/oracle"
declare -x ORACLE_HOME="/u01/app/oracle/product/10.2.0"
declare -x ORACLE_SID="med"
declare -x ORACLE_TERM="xterm"

To export an environment variable:
$ export ORACLE_HOME=/u01/app/oracle/product/10.2.0

xargs:

Copy all images to external hard-drive
# ls *.jpg | xargs -n1 -i cp {} /external-hard-drive/directory

Search all jpg images in the system and archive it.
# find / -name *.jpg -type f -print | xargs tar -cvzf images.tar.gz

Download all the URLs mentioned in the url-list.txt file
# cat url-list.txt | xargs wget –c

ls:

Display filesize in human readable format (e.g. KB, MB etc.,)

$ ls -lh
-rw-r----- 1 ramesh team-dev 8.9M Jun 12 15:27 arch-linux.txt.gz

Order Files Based on Last Modified Time (In Reverse Order) Using ls -ltr
$ ls -ltr

Visual Classification of Files With Special Characters Using ls -F
$ ls -F

pwd:

pwd is Print working directory. What else can be said about the good old pwd who has been printing the current directory name for ages.

cd:

Use “cd -” to toggle between the last two directories

Use “shopt -s cdspell” to automatically correct mistyped directory names on cd

gzip:

To create a *.gz compressed file:
$ gzip test.txt

To uncompress a *.gz file:
$ gzip -d test.txt.gz

Display compression ratio of the compressed file using gzip -l
$ gzip -l *.gz
         compressed        uncompressed  ratio uncompressed_name
              23709               97975  75.8% asp-patch-rpms.txt

bzip2:

To create a *.bz2 compressed file:
$ bzip2 test.txt

To uncompress a *.bz2 file:
bzip2 -d test.txt.bz2

unzip:

To extract a *.zip compressed file:
$ unzip test.zip

View the contents of *.zip file (Without unzipping it):
$ unzip -l jasper.zip
Archive:  jasper.zip
  Length     Date   Time    Name
 --------    ----   ----    ----
    40995  11-30-98 23:50   META-INF/MANIFEST.MF
    32169  08-25-98 21:07   classes_
    15964  08-25-98 21:07   classes_names
    10542  08-25-98 21:07   classes_ncomp

shutdown:

Shutdown the system and turn the power off immediately.
# shutdown -h now

Shutdown the system after 10 minutes.
# shutdown -h +10

Reboot the system using shutdown command.
# shutdown -r now

Force the filesystem check during reboot.
# shutdown -Fr now

ftp:

Both ftp and secure ftp (sftp) has similar commands. To connect to a remote server and download multiple files, do the following.
$ ftp IP/hostname
ftp> mget *.html

To view the file names located on the remote server before downloading, mls ftp command as shown below.
ftp> mls *.html -
/ftptest/features.html
/ftptest/index.html
/ftptest/othertools.html
/ftptest/samplereport.html
/ftptest/usage.html

crontab:

View crontab entry for a specific user
# crontab -u john -l

Schedule a cron job every 10 minutes.
*/10 * * * * /home/ramesh/check-disk-space

service:

Service command is used to run the system V init scripts. i.e Instead of calling the scripts located in the /etc/init.d/ directory with their full path, you can use the service command.

Check the status of a service:
# service ssh status

Check the status of all the services.
service --status-all

Restart a service.
# service ssh restart

ps: ps command is used to display information about the processes that are running in the system.
While there are lot of arguments that could be passed to a ps command, following are some of the common ones.

To view current running processes.
$ ps -ef | more

To view current running processes in a tree structure. H option stands for process hierarchy.
$ ps -efH | more

free: This command is used to display the free, used, swap memory available in the system.

Typical free command output. The output is displayed in bytes.
$ free
             total       used       free     shared    buffers     cached
Mem:       3566408    1580220    1986188          0     203988     902960
-/+ buffers/cache:     473272    3093136
Swap:      4000176          0    4000176

If you want to quickly check how many GB of RAM your system has use the -g option. -b option displays in bytes, -k in kilo bytes, -m in mega bytes.
$ free -g
             total       used       free     shared    buffers     cached
Mem:             3          1          1          0          0          0
-/+ buffers/cache:          0          2
Swap:            3          0          3

If you want to see a total memory ( including the swap), use the -t switch, which will display a total line as shown below.

sundar@sundar-laptop:~$ free -t
             total       used       free     shared    buffers     cached
Mem:       3566408    1592148    1974260          0     204260     912556
-/+ buffers/cache:     475332    3091076
Swap:      4000176          0    4000176
Total:     7566584    1592148    5974436

top: top command displays the top processes in the system ( by default sorted by cpu usage ). To sort top output by any column, Press O (upper-case O) , which will display all the possible columns that you can sort by as shown below.

Current Sort Field:  P  for window 1:Def
Select sort field via field letter, type any other key to return

  a: PID        = Process Id              v: nDRT       = Dirty Pages count
  d: UID        = User Id                 y: WCHAN      = Sleeping in Function
  e: USER       = User Name               z: Flags      = Task Flags
  ........

To displays only the processes that belong to a particular user use -u option. The following will show only the top processes that belongs to oracle user.
$ top -u oracle

df: Displays the file system disk space usage. By default df -k displays output in bytes.

$ df -k
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/sda1             29530400   3233104  24797232  12% /
/dev/sda2            120367992  50171596  64082060  44% /home

df -h displays output in human readable form. i.e size will be displayed in GB’s.
sundar@sundar-laptop:~$ df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda1              29G  3.1G   24G  12% /
/dev/sda2             115G   48G   62G  44% /home

Use -T option to display what type of file system.
sundar@sundar-laptop:~$ df -T
Filesystem    Type   1K-blocks      Used Available Use% Mounted on
/dev/sda1     ext4    29530400   3233120  24797216  12% /
/dev/sda2     ext4   120367992  50171596  64082060  44% /home

kill: Use kill command to terminate a process. First get the process id using ps -ef command, then use kill -9 to kill the running Linux process as shown below. You can also use killall, pkill, xkill to terminate a unix process.

$ ps -ef | grep vim
sundar    7243  7222  9 22:43 pts/2    00:00:00 vim
$ kill -9 7243

rm: 

Get confirmation before removing the file.
$ rm -i filename.txt

It is very useful while giving shell metacharacters in the file name argument.

Print the filename and get confirmation before removing the file.
$ rm -i file*

Following example recursively removes all files and directories under the example directory. This also removes the example directory itself.
$ rm -r example

cp:

Copy file1 to file2 preserving the mode, ownership and timestamp.
$ cp -p file1 file2

Copy file1 to file2. if file2 exists prompt for confirmation before overwritting it.
$ cp -i file1 file2

mv:

Rename file1 to file2. if file2 exists prompt for confirmation before overwritting it.

$ mv -i file1 file2
Note: mv -f is just the opposite, which will overwrite file2 without prompting.

mv -v will print what is happening during file rename, which is useful while specifying shell metacharacters in the file name argument.
$ mv -v file1 file2

cat:

You can view multiple files at the same time. Following example prints the content of file1 followed by file2 to stdout.
$ cat file1 file2

While displaying the file, following cat -n command will prepend the line number to each line of the output.
$ cat -n /etc/logrotate.conf
    1 /var/log/btmp {
    2     missingok
    3     monthly
    4     create 0660 root utmp
    5     rotate 1
    6 }

mount:

To mount a file system, you should first create a directory and mount it as shown below.
# mkdir /u01
# mount /dev/sdb1 /u01

You can also add this to the fstab for automatic mounting. i.e Anytime system is restarted, the filesystem will be mounted.
/dev/sdb1 /u01 ext2 defaults 0 2

chmod: chmod command is used to change the permissions for a file or directory.

Give full access to user and group (i.e read, write and execute ) on a specific file.
$ chmod ug+rwx file.txt

Revoke all access for the group (i.e read, write and execute ) on a specific file.
$ chmod g-rwx file.txt

Apply the file permissions recursively to all the files in the sub-directories.
$ chmod -R ug+rwx file.txt

chown: chown command is used to change the owner and group of a file. \

To change owner to oracle and group to db on a file. i.e Change both owner and group at the same time.
$ chown oracle:dba dbora.sh

Use -R to change the ownership recursively.
$ chown -R oracle:dba /home/oracle

passwd:

Change your password from command line using passwd. This will prompt for the old password followed by the new password.
$ passwd

Super user can use passwd command to reset others password. This will not prompt for current password of the user.
# passwd USERNAME

Remove password for a specific user. Root user can disable password for a specific user. Once the password is disabled, the user can login without entering the password.
# passwd -d USERNAME

mkdir:

Following example creates a directory called temp under your home directory.
$ mkdir ~/temp

Create nested directories using one mkdir command. If any of these directories exist already, it will not display any error. If any of these directories doesn’t exist, it will create them.
$ mkdir -p dir1/dir2/dir3/dir4/

ifconfig: Use ifconfig command to view or configure a network interface on the Linux system.

View all the interfaces along with status.
$ ifconfig -a

Start or stop a specific interface using up and down command as shown below.
$ ifconfig eth0 up
$ ifconfig eth0 down

uname: Uname command displays important information about the system such as — Kernel name, Host name, Kernel release number,
Processor type, etc.,

Sample uname output from a Ubuntu laptop is shown below.
$ uname -a
Linux john-laptop 2.6.32-24-generic #41-Ubuntu SMP Thu Aug 19 01:12:52 UTC 2010 i686 GNU/Linux

whereis: 

When you want to find out where a specific Unix command exists (for example, where does ls command exists?), you can execute the following command.
$ whereis ls
ls: /bin/ls /usr/share/man/man1/ls.1.gz /usr/share/man/man1p/ls.1p.gz

When you want to search an executable from a path other than the whereis default path, you can use -B option and give path as argument to it. This searches for the executable lsmk in the /tmp directory, and displays it, if it is available.
$ whereis -u -B /tmp -f lsmk
lsmk: /tmp/lsmk

whatis: Whatis command displays a single line description about a command.

$ whatis ls
ls  (1)  - list directory contents

$ whatis ifconfig
ifconfig (8)         - configure a network interface

locate: Using locate command you can quickly search for the location of a specific file (or group of files). Locate command uses the database created by updatedb.

The example below shows all files in the system that contains the word crontab in it.
$ locate crontab
/etc/anacrontab
/etc/crontab
/usr/bin/crontab
/usr/share/doc/cron/examples/crontab2english.pl.gz
/usr/share/man/man1/crontab.1.gz
/usr/share/man/man5/anacrontab.5.gz
/usr/share/man/man5/crontab.5.gz
/usr/share/vim/vim72/syntax/crontab.vim

man: 

Display the man page of a specific command.
$ man crontab

When a man page for a command is located under more than one section, you can view the man page for that command from a specific section as shown below.
$ man SECTION-NUMBER commandname
Following 8 sections are available in the man page.

General commands
System calls
C library functions
Special files (usually devices, those found in /dev) and drivers
File formats and conventions
Games and screensavers
Miscellaneous
System administration commands and daemons
For example, when you do whatis crontab, you’ll notice that crontab has two man pages (section 1 and section 5). To view section 5 of crontab man page, do the following.

$ whatis crontab
crontab (1)          - maintain crontab files for individual users (V3)
crontab (5)          - tables for driving cron

$ man 5 crontab

tail:

Print the last 10 lines of a file by default.
$ tail filename.txt

Print N number of lines from the file named filename.txt
$ tail -n N filename.txt

View the content of the file in real time using tail -f. This is useful to view the log files, that keeps growing. The command can be terminated using CTRL-C.
$ tail -f log-file

less:

less is very efficient while viewing huge log files, as it doesn’t need to load the full file while opening.
$ less huge-log-file.log

Once you open a file using less command, following two keys are very helpful.
CTRL+F – forward one window
CTRL+B – backward one window

su:

Switch to a different user account using su command. Super user can switch to any other user without entering their password.
$ su - USERNAME

Execute a single command from a different account name. In the following example, john can execute the ls command as raj username. Once the command is executed, it will come back to john’s account.

[john@dev-server]$ su - raj -c 'ls'

[john@dev-server]$

Login to a specified user account, and execute the specified shell instead of the default shell.
$ su -s 'SHELLNAME' USERNAME

mysql:

mysql is probably the most widely used open source database on Linux. Even if you don’t run a mysql database on your server, you might end-up using the mysql command ( client ) to connect to a mysql database running on the remote server.

To connect to a remote mysql database. This will prompt for a password.
$ mysql -u root -p -h 192.168.1.2

To connect to a local mysql database.
$ mysql -u root -p
Note: If you want to specify the mysql root password in the command line itself, enter it immediately after -p (without any space).

yum: 

To install apache using yum.
$ yum install httpd

To upgrade apache using yum.
$ yum update httpd

To uninstall/remove apache using yum.
$ yum remove httpd

rpm:

To install apache using rpm.
# rpm -ivh httpd-2.2.3-22.0.1.el5.i386.rpm

To upgrade apache using rpm.
# rpm -uvh httpd-2.2.3-22.0.1.el5.i386.rpm

To uninstall/remove apache using rpm.
# rpm -ev httpd

ping:

Ping a remote host by sending only 5 packets.
$ ping -c 5 gmail.com

date:

Set the system date:
# date -s "01/31/2010 23:59:53"

Once you’ve changed the system date, you should syncronize the hardware clock with the system date as shown below.
# hwclock –systohc
# hwclock --systohc –utc

wget:

The quick and effective method to download software, music, video from internet is using wget command.
$ wget http://prdownloads.sourceforge.net/sourceforge/nagios/nagios-3.2.1.tar.gz

Download and store it with a different name.
$ wget -O taglist.zip http://www.vim.org/scripts/download_script.php?src_id=7701

grep:

Search for a given pattern(not case sensitive) in a file 
$ grep -i "pattern you want to search" file1

Print the 3 lines after the matched string
$ grep -A 3 -i "pattern you want to search" file1

Search for a given string in all files recursively
$ grep -r "ramesh" *


find:

Find files using file name(not case sensitive) 
$ find -iname "file1.cpp"

Execute commands on files found by the find command
$ find -iname "file1.cpp" -exec md5sum {} \;

Find all empty files in home directory
$ find ~ -empty

rsync:

Sync a single file on a local machine from one location to another location.
$ rsync -zvh backup.tar /tmp/backups/

Sync all the files of from one directory to a different directory in the same machine
$ rsync -avzh /root/rpmpkgs /tmp/backups/

Sync a directory from a local machine to a remote machine
$ rsync -avz rpmpkgs/ root@192.168.0.101:/home/

Sync a remote directory to a local directory
$ rsync -avzh root@192.168.0.100:/home/tarunika/rpmpkgs /tmp/myrpms

wc:

Count number of newlines in a file.
$ wc -l filename.txt

Count number of words in a file.
$ wc -w filename.txt

Count number of bytes in a file.
$ wc -c filename.txt (or) $ wc -m filename.txt

errno.h - C Error Codes in Linux

All the Linux/C error codes are listed below.
I occasionally google C error codes, but always end up grepping through /usr/include to find the answer. To save myself, and a few others, some time in the future...
/usr/include/asm-generic/errno-base.h

#ifndef _ASM_GENERIC_ERRNO_BASE_H
#define _ASM_GENERIC_ERRNO_BASE_H

#define EPERM        1  /* Operation not permitted */
#define ENOENT       2  /* No such file or directory */
#define ESRCH        3  /* No such process */
#define EINTR        4  /* Interrupted system call */
#define EIO       5  /* I/O error */
#define ENXIO        6  /* No such device or address */
#define E2BIG        7  /* Argument list too long */
#define ENOEXEC      8  /* Exec format error */
#define EBADF        9  /* Bad file number */
#define ECHILD      10  /* No child processes */
#define EAGAIN      11  /* Try again */
#define ENOMEM      12  /* Out of memory */
#define EACCES      13  /* Permission denied */
#define EFAULT      14  /* Bad address */
#define ENOTBLK     15  /* Block device required */
#define EBUSY       16  /* Device or resource busy */
#define EEXIST      17  /* File exists */
#define EXDEV       18  /* Cross-device link */
#define ENODEV      19  /* No such device */
#define ENOTDIR     20  /* Not a directory */
#define EISDIR      21  /* Is a directory */
#define EINVAL      22  /* Invalid argument */
#define ENFILE      23  /* File table overflow */
#define EMFILE      24  /* Too many open files */
#define ENOTTY      25  /* Not a typewriter */
#define ETXTBSY     26  /* Text file busy */
#define EFBIG       27  /* File too large */
#define ENOSPC      28  /* No space left on device */
#define ESPIPE      29  /* Illegal seek */
#define EROFS       30  /* Read-only file system */
#define EMLINK      31  /* Too many links */
#define EPIPE       32  /* Broken pipe */
#define EDOM        33  /* Math argument out of domain of func */
#define ERANGE      34  /* Math result not representable */

#endif
/usr/include/asm-generic/errno.h
#ifndef _ASM_GENERIC_ERRNO_H
#define _ASM_GENERIC_ERRNO_H

#include <asm-generic/errno-base.h>

#define EDEADLK     35  /* Resource deadlock would occur */
#define ENAMETOOLONG    36  /* File name too long */
#define ENOLCK      37  /* No record locks available */
#define ENOSYS      38  /* Function not implemented */
#define ENOTEMPTY   39  /* Directory not empty */
#define ELOOP       40  /* Too many symbolic links encountered */
#define EWOULDBLOCK EAGAIN  /* Operation would block */
#define ENOMSG      42  /* No message of desired type */
#define EIDRM       43  /* Identifier removed */
#define ECHRNG      44  /* Channel number out of range */
#define EL2NSYNC    45  /* Level 2 not synchronized */
#define EL3HLT      46  /* Level 3 halted */
#define EL3RST      47  /* Level 3 reset */
#define ELNRNG      48  /* Link number out of range */
#define EUNATCH     49  /* Protocol driver not attached */
#define ENOCSI      50  /* No CSI structure available */
#define EL2HLT      51  /* Level 2 halted */
#define EBADE       52  /* Invalid exchange */
#define EBADR       53  /* Invalid request descriptor */
#define EXFULL      54  /* Exchange full */
#define ENOANO      55  /* No anode */
#define EBADRQC     56  /* Invalid request code */
#define EBADSLT     57  /* Invalid slot */

#define EDEADLOCK   EDEADLK

#define EBFONT      59  /* Bad font file format */
#define ENOSTR      60  /* Device not a stream */
#define ENODATA     61  /* No data available */
#define ETIME       62  /* Timer expired */
#define ENOSR       63  /* Out of streams resources */
#define ENONET      64  /* Machine is not on the network */
#define ENOPKG      65  /* Package not installed */
#define EREMOTE     66  /* Object is remote */
#define ENOLINK     67  /* Link has been severed */
#define EADV        68  /* Advertise error */
#define ESRMNT      69  /* Srmount error */
#define ECOMM       70  /* Communication error on send */
#define EPROTO      71  /* Protocol error */
#define EMULTIHOP   72  /* Multihop attempted */
#define EDOTDOT     73  /* RFS specific error */
#define EBADMSG     74  /* Not a data message */
#define EOVERFLOW   75  /* Value too large for defined data type */
#define ENOTUNIQ    76  /* Name not unique on network */
#define EBADFD      77  /* File descriptor in bad state */
#define EREMCHG     78  /* Remote address changed */
#define ELIBACC     79  /* Can not access a needed shared library */
#define ELIBBAD     80  /* Accessing a corrupted shared library */
#define ELIBSCN     81  /* .lib section in a.out corrupted */
#define ELIBMAX     82  /* Attempting to link in too many shared libraries */
#define ELIBEXEC    83  /* Cannot exec a shared library directly */
#define EILSEQ      84  /* Illegal byte sequence */
#define ERESTART    85  /* Interrupted system call should be restarted */
#define ESTRPIPE    86  /* Streams pipe error */
#define EUSERS      87  /* Too many users */
#define ENOTSOCK    88  /* Socket operation on non-socket */
#define EDESTADDRREQ    89  /* Destination address required */
#define EMSGSIZE    90  /* Message too long */
#define EPROTOTYPE  91  /* Protocol wrong type for socket */
#define ENOPROTOOPT 92  /* Protocol not available */
#define EPROTONOSUPPORT 93  /* Protocol not supported */
#define ESOCKTNOSUPPORT 94  /* Socket type not supported */
#define EOPNOTSUPP  95  /* Operation not supported on transport endpoint */
#define EPFNOSUPPORT    96  /* Protocol family not supported */
#define EAFNOSUPPORT    97  /* Address family not supported by protocol */
#define EADDRINUSE  98  /* Address already in use */
#define EADDRNOTAVAIL   99  /* Cannot assign requested address */
#define ENETDOWN    100 /* Network is down */
#define ENETUNREACH 101 /* Network is unreachable */
#define ENETRESET   102 /* Network dropped connection because of reset */
#define ECONNABORTED    103 /* Software caused connection abort */
#define ECONNRESET  104 /* Connection reset by peer */
#define ENOBUFS     105 /* No buffer space available */
#define EISCONN     106 /* Transport endpoint is already connected */
#define ENOTCONN    107 /* Transport endpoint is not connected */
#define ESHUTDOWN   108 /* Cannot send after transport endpoint shutdown */
#define ETOOMANYREFS    109 /* Too many references: cannot splice */
#define ETIMEDOUT   110 /* Connection timed out */
#define ECONNREFUSED    111 /* Connection refused */
#define EHOSTDOWN   112 /* Host is down */
#define EHOSTUNREACH    113 /* No route to host */
#define EALREADY    114 /* Operation already in progress */
#define EINPROGRESS 115 /* Operation now in progress */
#define ESTALE      116 /* Stale NFS file handle */
#define EUCLEAN     117 /* Structure needs cleaning */
#define ENOTNAM     118 /* Not a XENIX named type file */
#define ENAVAIL     119 /* No XENIX semaphores available */
#define EISNAM      120 /* Is a named type file */
#define EREMOTEIO   121 /* Remote I/O error */
#define EDQUOT      122 /* Quota exceeded */

#define ENOMEDIUM   123 /* No medium found */
#define EMEDIUMTYPE 124 /* Wrong medium type */
#define ECANCELED   125 /* Operation Canceled */
#define ENOKEY      126 /* Required key not available */
#define EKEYEXPIRED 127 /* Key has expired */
#define EKEYREVOKED 128 /* Key has been revoked */
#define EKEYREJECTED    129 /* Key was rejected by service */

/* for robust mutexes */
#define EOWNERDEAD  130 /* Owner died */
#define ENOTRECOVERABLE 131 /* State not recoverable */

#endif

CNAME RECORD - What? Why? How?

What is CNAME ?

CNAME is Canonical Name record, which is used to create alias for another domain. 


Why CNAME ?

If you want to create a local alias for a server, you need to create a CNAME.
Eg: If you want to access www.google.com with different name, for example www.testcname.com, from your computer, you have to create a CNAME.


How to create a CNAME ?

1. Open C:\Windows\System32\drivers\etc\hosts file
2. Add an entry in the below format and save.
   <ip address> <alias>
   For the above shown example it will be:
   <google ip> www.testcname.com
3. If your traffic goes through a proxy, you have to add "www.testcname.com" to proxy settings.
   On chrome, goto
   settings -> Advanced settings -> Change proxy settings -> connections tab -> LAN settings -> Advanced 
   And add "www.testcname.com" to exceptions.
4. Done. If you type www.testcname.com in the browser, it will goto www.google.com