A variable is deallocated when the system reclaims the memory from the variable, so it no longer has an area to store its value. For a variable, the period of time from its allocation until its deallocation is called its lifetime. The most common memory related error is using a deallocated variable.
For local variables , modern languages automatically protect against this error. In other words, most of the time, the local variables appear automatically when we need them, and they disappear automatically when we are done with them. With pointers , however, programmers must make sure that allocation is handled correctly. The most common variables we use are local variables within functions such as the variables number and result in the following function.
All of the local variables and parameters taken together are called its local storage or just its locals :. The variables are called local to represent the idea that their lifetime is tied to the function where they are declared. Whenever the function runs, its local variables are allocated. When the function exits, its locals are deallocated.
For the above example, that means that when the Square function is called, local storage is allocated for number and result. When the function finally exits, its local storage is deallocated. Local parameters are basically local copies of the information from the caller.
This is also known as pass by value. The caller is not sharing the parameter value with the callee. In other words, the callee is getting its own copy. This has the advantage that the callee can change its local copy without affecting the caller. This independence is good since it keeps the operation of the caller and callee functions separate which follows the rules of good software engineering keep separate components as independent as possible. However, since locals are copies of the caller parameters, they do not provide a means of communication from the callee back to the caller.
This is the downside of the independence advantage. Also, sometimes making copies of a value is very expensive. So the caller is left with a pointer to a deallocated variable.
We are essentially running into the lifetime constraint of local variables. We want the int to exist, but it gets deallocated automatically. When we actually run the small code, it appears to be OK. But the bug is still lurking there. We may see immediate effect when the code gets more complicated. In other words, in case when the system reclaim the memory area of the pointer:. When the called exits, its local memory is deallocated and so the pointer no longer has a pointee.
The pointer remains valid for the callee to use because the caller locals continue to exist while the called is running. The object that the pointer is pointing to will remain valid due to the simple constraint that the caller can only exit sometime after its called function exits.
The reverse case, from the callee to the caller, is when the bug occurs as shown in the example above. Before we go into manual memory management, it might be better look at automatic memory management. Automatic memory management is closely related to local variables. A local variable occupies memory that the system allocates when it sees the variable's definition during execution.
The system also deallocates that memory automatically at the end of the block that contains the definition. Programmers sometimes make a mistake of returning invalid pointer as we see in the example below. A pointer becomes invalid once the corresponding variable has been deallocated.
The function badPointer returns the address of the local variable i. However, when the function returns, actually ends the execution of the block and deallocates i. Actually, the content of the variable i is correct at the moment the function returns.
The problem is that the memory for i is allocated in the stack frame for badPointer. When badPointer returns, all the memory in its stack frame is deallocated and made available for use by other functions. Still, the function tries to return it anyway. What's going to happen? Only the compiler knows. This says that i is static and thus we allocate it once and we do not want to deallocate it as long as the code is running. In general, computers have three locations for storing data - physical memory, cache, and registers.
Memory is usually large compared with the other two types of storage. Each memory cell is accessed using an address, and the memory does not have to be consecutive. Cache is a smaller version of memory, stored either directly in the CPU level 1 cache , or on the motherboard level 2 cache. It stores a copy of recently used parts of the main memory, in a location that can be accessed much faster.
Usually, because the cache is hidden from our our programs by the hardware, we do not need only worry about the cache unless we're dealing with kernel. Registers are storage units inside the CPU with very fast access. They can be accessed much faster than memory, and are often used to store data that is needed for a short calculation, such as contents of local variables in a function, or intermediate results of arithmetic calculations.
Since modern compilers are well optimized, it might be better to let the compiler decide which variables should be kept in registers. When we talk about memory management, it's about deallocation since proper deallocation is crucial to the memory management. The determination of when an object ought to be created is trivial and is not problematic. The critical issue, however, is the determination of when an object is no longer needed and arranging for its underlying storage to be returned to the free store heap so that it may be re-used to satisfy future memory requests.
For more info on memory, please visit Taste of Assembly - heap memory and Taste of Assembly - stack memory. Memory Leaks Memory leaks occur when data that are allocated at runtime but not deallocated once they are no longer needed.
A program which forgets to deallocate a block is said to have a memory leak which may or may not be a serious problem. The result will be that the heap gradually fill up as there continue to be allocation requests, but no deallocation requests to return blocks for reuse. For a program which runs, computes something, and exits immediately, memory leaks are not usually a concern. Such a one shot program could omit all of its deallocation requests and still mostly work.
Memory leaks are more of a problem for a program which runs for an indeterminate amount of time. In that case, the memory leaks can gradually fill the heap until allocation requests cannot be satisfied, and the program stops working or crashes.
Many commercial programs have memory leaks, so that when run for long enough, or with large data-sets, it will consume memory resource, and eventually it will slow down our machine because of page swapping. Then, we get failure with an out-of-memory error.
Finding those leaks with normal debugger is very tough because there is no clear faulty line of code. Often the error detection and avoidance code for the heap-full error condition is not well tested, precisely because the case is rarely encountered with short runs of the program - that's why filling the heap often results in a real crash instead of a polite error message.
Most compilers have a heap debugging utility which adds debugging code to a program to track every allocation and deallocation. When an allocation has no matching deallocation, that's a leak, and the heap debugger can help us find them.
Buffer Overruns Buffer overruns occur when memory outside of the allocated boundaries is overwritten. We call it data corruption. This is nasty because it may not become visible at the place where the memory is overwritten. Sharing your research helps everyone. Also see How to Ask — gnat. I'am learning java Add a comment. Active Oldest Votes. Improve this answer. SJuan76 SJuan76 2, 1 1 gold badge 15 15 silver badges 15 15 bronze badges.
But a small doubt.. Suppose if i create a 10 objects of type Student then only 1 set of fresh memory is allocated to the methods present in class Student whereas 10 sets of fresh memory is allocated to store instance variables for 10 objects.. Am i right?
Think that it is not only the properties that take memory, there is a small overhead related to the instance itself an instance of a class with no properties will use more than 0 bytes of memory. One more thing I asked question keeping java in mind Does the same thing happens in java The Java Language Specification doesn't say anything about how much memory is allocated when and for what purpose.
That is left to the implementer, and every implementer may choose differently. Instance fields including property backing fields get N-copies for N-objects. Static fields get a single copy per class. What you are saying makes sense, but is that actually guaranteed by the JLS? Normally, the JLS gives implementors a lot of leeway in questions like this.
You may be right. The point I tried to make is "new T " doesn't allocate a new instance of a method. As to the specifics of the JVM, classloaders do indeed store bytecode in the heap, and I guess there are possible scenarios where classes themselves are instantiated and even garbage collected.
But it is implementation detail of the runtime, and conceptually, the heap I'm talking about is the "user" heap. But since we can also control a classloader from userland, I guess I don't know.
The JLS doesn't even talk about a heap at all, doesn't it? It's perfectly legal to implement Java with a dynamic stack and no heap instead of a finite fixed-size stack and a dynamic heap. This all happens using some predefined routines in the compiler. A programmer does not have to worry about memory allocation and de-allocation of stack variables.
This kind of memory allocation also known as Temporary memory allocation because as soon as the method finishes its execution all the data belongs to that method flushes out from the stack automatically.
It allocates or de-allocates the memory automatically as soon as the corresponding method completes its execution. We receive the corresponding error Java. Stack memory allocation is considered safer as compared to heap memory allocation because the data stored can only be access by owner thread. Memory allocation and de-allocation is faster as compared to Heap-memory allocation.
Stack-memory has less storage space as compared to Heap-memory. Note that the name heap has nothing to do with the heap data structure. It is called heap because it is a pile of memory space available to programmers to allocated and de-allocate. Every time when we made an object it always creates in Heap-space and the referencing information to these objects are always stored in Stack-memory. If a programmer does not handle this memory well, a memory leak can happen in the program.
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