/* Ajith - Syntax Higlighter - End ----------------------------------------------- */

8.26.2009

Implementation of Singly Linked List

This article is part of article series - "Datastructures"

Generally a Linked List means "Singly Linked List". It is a chain of records known as Nodes. Each node has at least two members, one of which points to the next Node in the list and the other holds the data.

Figure 1: Singly Linked List
Basically Single Linked Lists are uni-directional as they can only point to the next Node in the list but not to the previous. We use below structure for a Node in our example.
 struct Node
 {
   int Data;
   struct Node *Next;
 }; 
Variable Data holds the data in the Node (It can be a pointer variable pointing to the dynamically allocated memory) while Next holds the address to the next Node in the list.

Figure 2: Node in a Singly Linked List
Head is a pointer variable of type struct Node which acts as the Head to the list. Initially we set 'Head' as NULL which means list is empty.

8.25.2009

Signals in Linux - Standard Signals

Every signal has a unique signal name, an abbreviation that begins with SIG (SIGINT for interrupt signal, for example). Each signal name is a macro which stands for a positive integer - the signal number for that kind of signal. Your programs should never make assumptions about the numeric code for a particular kind of signal, but rather refer to them always by the names defined. This is because the number for a given kind of signal can vary from system to system, but the meanings of the names are standardized and fairly uniform.

The signal names are defined in signal.h (/usr/include/bits/signum.h), which must be included by any C program that uses signals.

Several signal numbers are architecture-dependent, as indicated in the "Value" column. (Where three values are given, the first one is usually valid for alpha and sparc, the middle one for ix86, ia64, ppc, s390, arm and sh, and the last one for mips. A - denotes that a signal is absent on the corresponding architecture.)



The signals SIGKILL and SIGSTOP cannot be caught, blocked, or ignored.

Next the signals not in the POSIX.1-1990 standard but described in SUSv2 and POSIX.1-2001.


Up to and including Linux 2.2, the default behavior for SIGSYS, SIGXCPU, SIGXFSZ, and (on architectures other than SPARC and MIPS) SIGBUS was to terminate the process (without a core dump). Linux 2.4 conforms to the POSIX.1-2001 requirements for these signals, terminating the process with a core dump.

Next various other signals.


For detailed information about the side-effects and reasons causing these signals checout libc manual.

8.23.2009

Signals in Linux - Basics

What is a Signal ?

A signal is a software interrupt delivered to notify a process or thread of a particular event. The operating system uses signals to report exceptional situations to an executing program. Some signals report errors such as references to invalid memory addresses; others report asynchronous events, such as disconnection of a phone line.

The lifetime of a signal is the interval between its generation and its delivery. A signal that has been generated but not yet delivered is said to be pending. There may be considerable time between signal generation and signal delivery. The process must be running on a processor at the time of signal delivery.
Many computer science researchers compare signals with hardware interrupts, which occur when a hardware subsystem, such as a disk I/O (input/output) interface, generates an interrupt to a processor when the I/O completes. This event in turn causes the processor to enter an interrupt handler, so subsequent processing can be done in the operating system based on the source and cause of the interrupt. When a signal is sent to a process or thread, a signal handler may be entered (depending on the current disposition of the signal), which is similar to the system entering an interrupt handler as the result of receiving an interrupt.

What causes a Signal ?

Let us see some of the events that can cause (or generate, or raise) a signal:
  • A program error such as dividing by zero or issuing an address outside the valid range.
  • A user request to interrupt or terminate the program. Most environments are set up to let a user suspend the program by typing Ctrl-z, or terminate it with Ctrl-c. Whatever key sequence is used, the operating system sends the proper signal to interrupt the process.
  • The termination of a child process.
  • Expiration of a timer or alarm.
  • A call to kill or raise system calls by the same process.
  • A call to kill from another process. Signals are a limited but useful form of interprocess communication.
  • An attempt to perform an I/O operation that cannot be done. Examples are reading from a pipe that has no writer, and reading or writing to a terminal in certain situations.
Each of these kinds of events (excepting explicit calls to kill and raise) generates its own particular kind of signal.

Signals may be generated synchronously or asynchronously. A synchronous signal pertains to a specific action in the program, and is delivered (unless blocked) during that action. Most errors generate signals synchronously, and so do explicit requests by a process to generate a signal for that same process. On some machines, certain kinds of hardware errors (usually floating-point exceptions) are not reported completely synchronously, but may arrive a few instructions later.

Asynchronous signals are generated by events outside the control of the process that receives them. These signals arrive at unpredictable times during execution. External events generate signals asynchronously, and so do explicit requests that apply to some other process. One obvious example would be the sending of a signal to a process from another process or thread via a kill system call. Asynchronous signals are also aptly referred to as interrupts.

A given type of signal is either typically synchronous or typically asynchronous. For example, signals for errors are typically synchronous because errors generate signals synchronously. But any type of signal can be generated synchronously or asynchronously with an explicit request.

How Signals Are Delivered ?

When a signal is generated, it becomes pending. Normally it remains pending for just a short period of time and then is delivered to the process that was signaled. However, if that kind of signal is currently blocked, it may remain pending indefinitely—until signals of that kind are unblocked. Once unblocked, it will be delivered immediately.

When the signal is delivered, whether right away or after a long delay, the specified action for that signal is taken. For certain signals, such as SIGKILL and SIGSTOP, the action is fixed, but for most signals, the program has a choice. Possible default dispositions are
Term   Default action is to terminate the process.
Ign Default action is to ignore the signal.
Core Default action is to terminate the process
and dump core.
Stop Default action is to stop the process.
Cont Default action is to continue the process
if it is currently stopped.
The program can also specify its own way of handling the signals (signal handlers). We sometimes say that a handler catches the signal. While the handler is running, that particular signal is normally blocked.

References:

1. Libc Manual

7.30.2009

Mysterious C program

Never thought in my life that one can write such a mysterious 'C' program which compiles without any errors and provides a strange output.

#include <stdio.h>

main(t,_,a)
char *a;
{return!0<t?t<3?main(-79,-13,a+main(-87,1-_,
main(-86, 0, a+1 )+a)):1,t<_?main(t+1, _, a ):3,main ( -94, -27+t, a
)&&t == 2 ?_<13 ?main ( 2, _+1, "%s %d %d\n" ):9:16:t<0?t<-72?main(_,
t,"@n'+,#'/*{}w+/w#cdnr/+,{}r/*de}+,/*{*+,/w{%+,/w#q#n+,/#{l,+,/n{n+\
,/+#n+,/#;#q#n+,/+k#;*+,/'r :'d*'3,}{w+K w'K:'+}e#';dq#'l q#'+d'K#!/\
+k#;q#'r}eKK#}w'r}eKK{nl]'/#;#q#n'){)#}w'){){nl]'/+#n';d}rw' i;# ){n\
l]!/n{n#'; r{#w'r nc{nl]'/#{l,+'K {rw' iK{;[{nl]'/w#q#\
n'wk nw' iwk{KK{nl]!/w{%'l##w#' i; :{nl]'/*{q#'ld;r'}{nlwb!/*de}'c \
;;{nl'-{}rw]'/+,}##'*}#nc,',#nw]'/+kd'+e}+;\
#'rdq#w! nr'/ ') }+}{rl#'{n' ')# }'+}##(!!/")
:t<-50?_==*a ?putchar(a[31]):main(-65,_,a+1):main((*a == '/')+t,_,a\
+1 ):0<t?main ( 2, 2 , "%s"):*a=='/'||main(0,main(-61,*a, "!ek;dc \
i@bK'(q)-[w]*%n+r3#l,{}:\nuwloca-O;m .vpbks,fxntdCeghiry"),a+1);}
Any Clue of the output ???

Output for the above code is

On the first day of Christmas my true love gave to me
a partridge in a pear tree.

On the second day of Christmas my true love gave to me
two turtle doves
and a partridge in a pear tree.

On the third day of Christmas my true love gave to me
three french hens, two turtle doves
and a partridge in a pear tree.

On the fourth day of Christmas my true love gave to me
four calling birds, three french hens, two turtle doves
and a partridge in a pear tree.

On the fifth day of Christmas my true love gave to me
five gold rings;
four calling birds, three french hens, two turtle doves
and a partridge in a pear tree.

On the sixth day of Christmas my true love gave to me
six geese a-laying, five gold rings;
four calling birds, three french hens, two turtle doves
and a partridge in a pear tree.

On the seventh day of Christmas my true love gave to me
seven swans a-swimming,
six geese a-laying, five gold rings;
four calling birds, three french hens, two turtle doves
and a partridge in a pear tree.

On the eighth day of Christmas my true love gave to me
eight maids a-milking, seven swans a-swimming,
six geese a-laying, five gold rings;
four calling birds, three french hens, two turtle doves
and a partridge in a pear tree.

On the ninth day of Christmas my true love gave to me
nine ladies dancing, eight maids a-milking, seven swans a-swimming,
six geese a-laying, five gold rings;
four calling birds, three french hens, two turtle doves
and a partridge in a pear tree.

On the tenth day of Christmas my true love gave to me
ten lords a-leaping,
nine ladies dancing, eight maids a-milking, seven swans a-swimming,
six geese a-laying, five gold rings;
four calling birds, three french hens, two turtle doves
and a partridge in a pear tree.

On the eleventh day of Christmas my true love gave to me
eleven pipers piping, ten lords a-leaping,
nine ladies dancing, eight maids a-milking, seven swans a-swimming,
six geese a-laying, five gold rings;
four calling birds, three french hens, two turtle doves
and a partridge in a pear tree.

On the twelfth day of Christmas my true love gave to me
twelve drummers drumming, eleven pipers piping, ten lords a-leaping,
nine ladies dancing, eight maids a-milking, seven swans a-swimming,
six geese a-laying, five gold rings;
four calling birds, three french hens, two turtle doves
and a partridge in a pear tree.
Can anyone explain me step by step way the process to get above output ....

4.29.2009

Beware of NVIDIA Graphic cards in Laptops

NVIDIA famously known for its high end graphics cards which makes everyone to own a machine with NVIDIA graphics card. But beware of NVIDIA graphics card as they suffer OVER HEATING issue because of weak die/packaging material. It seems NVIDIA G84 & G86 graphic chipsets had this problem.
According to our sources, the failures are caused by a solder bump that connects the I/O termination of the silicon chip to the pad on the substrate. In Nvidia’s GPUs, this solder bump is created using high-lead. A thermal mismatch between the chip and the substrate has substantially grown in recent chip generations, apparently leading to fatigue cracking. Add into the equation a growing chip size (double the chip dimension, quadruple the stress on the bump) as well as generally hotter chips and you may have the perfect storm to take high lead beyond its limits. Apparently, problems arise at what Nvidia claims to be "extreme temperatures" and what we hear may be temperatures not too much above 70 degrees Celsius.

What supports the theory that a high-lead solder bump in fact is at fault is the fact that Nvidia ordered an immediate switch to use eutectic solders instead of high-lead versions in the last week of July.
source
Major laptop manufacturers like DELL, HP has accepted that there are some issues with laptops shipped with a NVIDIA graphics cards.

According to NVIDIA
, these affected GPUs are experiencing higher than expected failure rates causing video problems and it is because of weak die/packaging material set, which may fail with higher GPU temperature fluctuations.

If your NVIDIA GPU fails, you may see intermittent symptoms during early stages of failure that include:

* Multiple images
* Random characters on the screen
* Lines on the screen
* No video
* Black Screen

DELL agreed that some of its laptop models namely Inspiron 1420, Latitude D630, Latitude D630c, Dell Precision M2300, Vostro Notebook 1310, Vostro Notebook 1400, Vostro Notebook 1510, Vostro Notebook 1710, XPS M1330, XPS M1530 are facing the issue and released a BIOS upgradation to minimize the effect of the GPU failure problem and new systems are being shipped with the updated BIOS revisions.
DELL is offering an additional 12-month limited warranty enhancement specific to this GPU failure issue. For all customers worldwide, DELL plans to add 12 months of coverage for this issue to the existing limited warranty up to 60 months from the date of purchase for the given list of systems.

DELL's BIOS fix is simply turning on the laptop fan much more than usual as a result laptop battery is drained. The 'fix' keeps the fan on much more and destroys battery life.

HP also identified a hardware issue with certain HP Pavilion dv2000/dv6000/dv9000 and Compaq Presario V3000/V6000 series notebook PCs, and has also released a new BIOS for these notebook PCs.
HP says"This service enhancement program is available in North America for 24 months after the start of your original standard limited warranty for issues listed below; otherwise your current standard limited warranty applies. Customers who already have a 24 month or longer warranty period will be covered under their existing standard HP Limited Warranty."
Apple is also facing the same problem with the macbook pro's.
Apple says, NVIDIA assured Apple that Mac computers with these graphics processors were not affected. However, after an Apple-led investigation, Apple has determined that some MacBook Pro computers with the NVIDIA GeForce 8600M GT graphics processor may be affected. If the NVIDIA graphics processor in your MacBook Pro has failed, or fails within two years of the original date of purchase, a repair will be done free of charge, even if your MacBook Pro is out of warranty.
Customers are really angry as the OEM's are not providing a permanent solution to the GPU problem and also not including some of the laptop models that are facing the same issue. Even some of the customers sued NVIDIA to repair their laptops with faulty graphics cards.

We can see still DELL is shipping laptops with defective NVIDIA graphics card.