What is dead-lock....

What is deadlock and mechanism to prevent it…….

A deadlock is a situation wherein two or more competing actions are waiting for the other to finish, and thus neither ever does. It is often seen in a paradox like 'the chicken or the egg'.

In the computing world deadlock refers to a specific condition when two or more processes are each waiting for another to release a resource, or more than two processes are waiting for resources in a circular chain Deadlock is a common problem in multiprocessing where many processes share a specific type of mutually exclusive resource known as a software, or soft, lock. Computers intended for the time-sharing and/or real-time markets are often equipped with a hardware lock (or hard lock) which guarantees exclusive access to processes, forcing serialization. Deadlocks are particularly troubling because there is no general solution to avoid (soft) deadlocks.

This situation may be likened to two people who are drawing diagrams, with only one pencil and one ruler between them. If one person takes the pencil and the other takes the ruler, a deadlock occurs when the person with the pencil needs the ruler and the person with the ruler needs the pencil, before he can give up the ruler. Both requests can't be satisfied, so a deadlock occurs.

The telecommunication’s description of deadlock is a little stronger: deadlock occurs when none of the processes meet the condition to move to another state (as described in the process's finite state machine) and all the communication channels are empty. The second condition is often left out on other systems but is important in the telecommunication context.

Necessary conditions

There are four necessary conditions for a deadlock to occur, known as the Coffman conditions from their first description in a 1971 article by E. G. Coffman.

1. Mutual exclusion condition: a resource is either assigned to one process or it is available
2. Hold and wait condition: processes already holding resources may request new resources
3. No preemption condition: only a process holding a resource may release it
4. Circular wait condition: two or more processes form a circular chain where each process waits for a resource that the next process in the chain holds

Deadlock only occurs in systems where all 4 conditions happen.

Circular wait prevention

Circular wait prevention consists in allowing processes to wait for resources, but ensure that the waiting can't be circular. One approach might be to assign a precedence to each resource and force processes to allocate resources in order of increasing precedence. That is to say that if a process holds some resources and the highest precedence of these resources is m, then this process cannot request any resource with precedence smaller than m. This forces resource allocation to follow a particular and non-circular ordering, so circular wait cannot occur. Another approach is to allow holding only one resource per process; if a process requests another resource, it must first free the one it's currently holding (or hold-and-wait).

Examples

An example of a deadlock which may occur in database products is the following. Client applications using the database may require exclusive access to a table, and in order to gain exclusive access they ask for a lock. If one client application holds a lock on a table and attempts to obtain the lock on a second table that is already held by a second client application, this may lead to deadlock if the second application then attempts to obtain the lock that is held by the first application. (But this particular type of deadlock is easily prevented, e.g., by using an all-or-none resource allocation algorithm.)

Another example might be a text formatting program that accepts text sent to it to be processed and then returns the results, but does so only after receiving "enough" text to work on (e.g. 1KB). A text editor program is written that sends the formatter with some text and then waits for the results. In this case a deadlock may occur on the last block of text. Since the formatter may not have sufficient text for processing, it will suspend itself while waiting for the additional text, which will never arrive since the text editor has sent it all of the text it has. Meanwhile, the text editor is itself suspended waiting for the last output from the formatter. This type of deadlock is sometimes referred to as a deadly embrace (properly used only when only two applications are involved) or starvation. However, this situation, too, is easily prevented by having the text editor send a forcing message (eg. EOF) with its last (partial) block of text, which will force the formatter to return the last (partial) block after formatting, and not wait for additional text.

Nevertheless, since there is no general solution for deadlock prevention, each type of deadlock must be anticipated and specially prevented. But general algorithms can be implemented within the operating system so that if one or more applications becomes blocked, it will usually be terminated after a time (and, in the meantime, is allowed no other resources and may need to surrender those it already has, rolled back to a state prior to being obtained by the application).

Avoidance

Deadlock can be avoided if certain information about processes is available in advance of resource allocation. For every resource request, the system sees if granting the request will mean that the system will enter an unsafe state, meaning a state that could result in deadlock. The system then only grants request that will lead to safe states. In order for the system to be able to figure out whether the next state will be safe or unsafe, it must know in advance at any time the number and type of all resources in existence, available, and requested. One known algorithm that is used for deadlock avoidance is the Banker's algorithm, which requires resource usage limit to be known in advance. However, for many systems it is impossible to know in advance what every process will request. This means that deadlock avoidance is often impossible.

Two other algorithms are Wait/Die and Wound/Wait, each of which is uses a symmetry-breaking technique. In both these algorithms there exists an older process (O) and a younger process (Y). Process age can be determined by a time stamp at process creation time. Smaller time stamps are older processes, while larger timestamps represent younger processes.

	Wait/Die	Wound/Wait
O is waiting for a resource that is being held by Y	O waits	Y dies
Y is waiting for a resource that is being held by O	Y dies	Y waits

It is important to note that a process may be in unsafe state but would not result in a deadlock. The notion of safe/unsafe state only refers to the ability of the system to enter a deadlock state or not. For example, if a process requests A which would result in an unsafe state, but releases B which would prevent circular wait, then the state is unsafe but the system is not in deadlock.

Prevention

Deadlocks can be prevented by ensuring that at least one of the following four conditions occur:

• Removing the mutual exclusion condition means that no process may have exclusive access to a resource. This proves impossible for resources that cannot be spooled, and even with spooled resources deadlock could still occur. Algorithms that avoid mutual exclusion are called non-blocking synchronization algorithms.
• The "hold and wait" conditions may be removed by requiring processes to request all the resources they will need before starting up (or before embarking upon a particular set of operations); this advance knowledge is frequently difficult to satisfy and, in any case, is an inefficient use of resources. Another way is to require processes to release all their resources before requesting all the resources they will need. This too is often impractical. (Such algorithms, such as serializing tokens, are known as the all-or-none algorithms.)
• A "no preemption" (lockout) condition may also be difficult or impossible to avoid as a process has to be able to have a resource for a certain amount of time, or the processing outcome may be inconsistent or thrashing may occur. However, inability to enforce preemption may interfere with a priority algorithm. (Note: Preemption of a "locked out" resource generally implies a rollback, and is to be avoided, since it is very costly in overhead.) Algorithms that allow preemption include lock-free and wait-free algorithms and optimistic concurrency control.
• The circular wait condition: Algorithms that avoid circular waits include "disable interrupts during critical sections" , and "use a hierarchy to determine a partial ordering of resources" (where no obvious hierarchy exists, even the memory address of resources has been used to determine ordering) and Dijkstra's solution.

Detection

Often neither deadlock avoidance nor deadlock prevention may be used. Instead deadlock detection and process restart are used by employing an algorithm that tracks resource allocation and process states, and rolls back and restarts one or more of the processes in order to remove the deadlock. Detecting a deadlock that has already occurred is easily possible since the resources that each process has locked and/or currently requested are known to the resource scheduler or OS.

Detecting the possibility of a deadlock before it occurs is much more difficult and is, in fact, generally undecidable, because the halting problem can be rephrased as a deadlock scenario. However, in specific environments, using specific means of locking resources, deadlock detection may be decidable. In the general case, it is not possible to distinguish between algorithms that are merely waiting for a very unlikely set of circumstances to occur and algorithms that will never finish because of deadlock.

Distributed deadlock

Distributed deadlocks can occur in distributed systems when distributed transactions or concurrency control is being used. Distributed deadlocks can be detected either by constructing a global wait-for graph from local wait-for graphs at a deadlock detector or by a distributed algorithm like edge chasing.

Phantom deadlocks are deadlocks that are detected in a distributed system but don't actually exist - they have either been already resolved or no longer exist due to transactions aborting.

Livelock

A livelock is similar to a deadlock, except that the states of the processes involved in the livelock constantly change with regard to one another, none progressing. Livelock is a special case of resource starvation; the general definition only states that a specific process is not progressing.

As a real-world example, livelock occurs when two people meet in a narrow corridor, and each tries to be polite by moving aside to let the other pass, but they end up swaying from side to side without making any progress because they always both move the same way at the same time.

Livelock is a risk with some algorithms that detect and recover from deadlock. If more than one process takes action, the deadlock detection algorithm can repeatedly trigger. This can be avoided by ensuring that only one process (chosen randomly or by priority) takes action.

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What is Dangling pointers in the world of C, C++ and VC++ languages?

Dangling pointer

Dangling pointers and wild pointers in computer programming are pointers that do not point to a valid object of the appropriate type, or to a distinguished null pointer value in languages which support this. Dangling pointers arise when an object is deleted or de-allocated, without modifying the value of the pointer, so that the pointer still points to the memory location of the de-allocated memory. As the system may reallocate the previously freed memory to another process, if the original program then dereferences the (now) dangling pointer, unpredictable behavior may result, as the memory may now contain completely different data. This is especially the case if the program writes data to memory pointed by a dangling pointer, as silent corruption of unrelated data may result, leading to subtle bugs that can be extremely difficult to find, or general protection faults (Windows). If the overwritten data is bookkeeping data used by the system's memory allocator, the corruption can cause system instabilities. Wild pointers arise when a pointer is used prior to initialization to some known state, which is possible in some programming languages. They show the same erratic behavior as dangling pointers, though they are less likely to stay undetected.

Cause of dangling pointers

In many languages (particularly the C programming language, which assumes the programmer will take care of all design issues, and hence do not include many of the checks that are present in higher-level languages), deleting an object from memory explicitly or by destroying the stack frame on return does not alter any associated pointers. The pointer still points to the location in memory where the object or data was, even though the object or data has since been deleted and the memory may now be used for other purposes, creating a dangling pointer.

A straightforward example is shown below:

       {

           char *cp = NULL;

           /* ... */

           {

               char c;

               cp = &c;

           } /* The memory location, which c was occupying, is released here */          

           /* cp here is now a dangling pointer */

       }

In the above, one solution to avoid the dangling pointer is to make cp a null pointer after the inner block is exited, or to otherwise guarantee that cp won't be used again without further initialization in the code which follows.

Another frequent source of creating dangling pointers is a jumbled combination of malloc () and free () library calls. In such a case, a pointer becomes dangling when the block of memory it points to is freed. As with the previous example, one way to avoid this is to make sure to set the pointer back to null after freeing the memory, as demonstrated below:

       #include 

       {

           char *cp = malloc ( A_CONST );

           /* ... */

           free ( cp );      /* cp now becomes a dangling pointer */

           cp = NULL;        /* cp is no longer dangling */

           /* ... */

       }

Lastly, a common programming misstep to create a dangling pointer is returning the address of a local variable. Since local variables are de-allocated when the function returns, any pointers that point to local variables will become dangling pointers once the stack frame is de-allocated.

       char * func ( void )

       {

           char ca[] = "Pointers and Arrays - II";

           /* ... */

           return ca;

       }

If it is required to return the address of ca, it should be declared with the static storage specifier.

Cause of wild pointers

Wild pointers are created by omitting necessary initialization prior first use. Thus, strictly speaking, every pointer in programming languages which do not enforce initialization begins as a wild pointer.

This most often occurs due to jumping over the initialization, not by omitting it. Most compilers are able to warn about this.

Security holes involving dangling pointers

Like buffer overflow bugs, dangling/wild pointer bugs are frequently security holes. For example, if the pointer is used to make a virtual function call, a different address (possibly pointing at exploit code) may be called due to the vtable pointer being overwritten. Alternatively, if the pointer is used for writing to memory, some other data structure may be corrupted. Even if the memory is only read once the pointer becomes dangling, it can lead to information leaks (if interesting data is put in the next structure allocated there) or privilege escalation (if the now-invalid memory is used in security checks).

Avoiding dangling pointer errors

A popular technique to avoid dangling pointers is to use smart pointers. A smart pointer typically uses reference counting to reclaim objects. Some other techniques include the tombstones method and the locks-and-keys method.

One alternative is to use the DieHard memory allocator^[1], which virtually eliminates dangling pointer errors, as well as a variety of other memory errors (like invalid and double frees).

In languages like Java, dangling pointers cannot occur because there is no mechanism to explicitly de-allocate memory. Rather, the garbage collector may de-allocate memory, but only when the object is no longer reachable from any references.

Dangling pointer detection

To expose dangling pointer errors, one common programming technique is to set pointers to the null pointer or to an invalid address once the storage they point to has been released. When the null pointer is dereferences (in most languages) the program will immediately terminate — there is no potential for data corruption or unpredictable behavior. This makes the underlying programming mistake easier to find and resolve. This technique does not help when there are multiple copies of the pointer.

Some debuggers will automatically overwrite and destroy data that has been freed, usually with a specific pattern, such as 0xdeadbeef (Microsoft's Visual C/C++ debugger, for example, uses 0xCC, 0xCD or 0xDD depending on what has been freed). This usually prevents the data from being reused by making it useless and also very prominent (the pattern serves to show the programmer that the memory has already been freed).

How to resolve memory leakages in C++ language?

Solution is Smart Pointer

A Smart pointer is an abstract data type that simulates a pointer while providing additional features, such as automatic garbage collection or bounds checking. These additional features are intended to reduce bugs caused by the use of pointers while retaining efficiency. Smart pointers typically keep track of the objects they point to for the purpose of memory management.

The use of pointers is a major source of bugs: the constant allocation, de-allocation and referencing that must be performed by a program written using pointers makes it very likely that some memory leaks will occur. Smart pointers try to prevent memory leaks by making the resource de-allocation automatic: when the pointer to an object (or the last in a series of pointers) is destroyed, for example because it goes out of scope, the pointed object is destroyed too.

Several types of smart pointers exist. Some work with reference counting, others assigning ownership of the object to a single pointer. If the language supports automatic garbage collection (for instance, Java), then this use of a smart pointer is unnecessary.

In C++ language, smart pointers may be implemented as a template class that mimics, by means of operator overloading, the behavior of traditional (raw) pointers, (e.g.: dereferencing, assignment) while providing additional memory management algorithms.

Smart pointers can facilitate intentional programming by expressing the use of a pointer in the type itself. For example, if a C++ function returns a pointer, there is no way to know whether the caller should delete the memory pointed to when the caller is finished with the information.

some_type* ambiguous_function(); // What should be done with the result?

Traditionally, this has been solved with comments, but this can be error-prone. By returning a C++ auto_ptr,

auto_ptr obvious_function1();

the function makes explicit that the caller will take ownership of the result, and further more, that if the caller does nothing, no memory will be leaked. Similarly, if the intention is to return a pointer to an object managed elsewhere, the function could return by reference:

some_type& obvious_function2();

Example

Let SmartPointer be a template smart pointer for class X.

void test_smartpointers()

 {

 //first, we create two objects and we keep raw pointers to them

 //since these pointers are not smart, they will not affect the object lifecycle

 Object* obj_1 = new Object();               

 Object* obj_2 = new Object();       

 //then we declare two smart pointers and we assign them with the objects

 //both obj_1 and obj_2 will have counter==1

 SmartPointer