Anything on the stack has been declared before compiling the program. Sometimes it becomes necessary to generate new variables for data while a program is running. In order to accomplish this goal we can use pointers and dynamic memory. Dynamically allocated variables live in a piece of memory known as the heap, these are requested by the running program using the keyword "new". For this example, it is assumed hat there is a 10K heap.
First, an area of 3K is requested, thus:. This results in a failure — NULL is returned into p1 — because, even though 6K of memory is available, there is not a 4K contiguous block available. This is memory fragmentation. It would seem that an obvious solution would be to de-fragment the memory, merging the two 3K blocks to make a single one of 6K.
However, this is not possible because it would entail moving the 4K block to which p2 points. Moving it would change its address, so any code that has taken a copy of the pointer would then be broken. This is only possible because these languages do not support direct pointers, so moving the data has no adverse effect upon application code.
This defragmentation may occur when a memory allocation fails or there may be a periodic garbage collection process that is run. In either case, this would severely compromise real time performance and determinism. A real time operating system may provide a service which is effectively a reentrant form of malloc.
However, it is unlikely that this facility would be deterministic. Memory management facilities that are compatible with real time requirements — i. This creates a partition pool with the descriptor MyPool, containing bytes of memory, filled with partitions of size 40 bytes i. The pool is located at address 0xB The pool is configured such that, if a task attempts to allocate a block, when there are none available, and it requests to be suspended on the allocation API call, suspended tasks will be woken up in a first-in, first-out order.
The other option would have been task priority order. Another API call is available to request allocation of a partition. Here is an example using Nucleus OS:. This requests the allocation of a partition from MyPool. When successful, a pointer to the allocated block is returned in ptr. If a task of higher priority was suspended pending availability of a partition, it would now be run. There is no possibility for fragmentation, as only fixed size blocks are available.
The only failure mode is true resource exhaustion, which may be controlled and contained using task suspend, as shown. Additional API calls are available which can provide the application code with information about the status of the partition pool — for example, how many free partitions are currently available.
Care is required in allocating and de-allocating partitions, as the possibility for the introduction of memory leaks remains. The potential for programmer error resulting in a memory leak when using partition pools is recognized by vendors of real time operating systems. Typically, a profiler tool is available which assists with the location and rectification of such bugs. Having identified a number of problems with dynamic memory behavior in real time systems, some possible solutions and better approaches can be proposed.
It is possible to use partition memory allocation to implement malloc in a robust and deterministic fashion. The idea is to define a series of partition pools with block sizes in a geometric progression; e. A malloc function may be written to deterministically select the correct pool to provide enough space for a given allocation request.
This approach takes advantage of the deterministic behavior of the partition allocation API call, the robust error handling e. Dynamic memory includes stack and heap. Dynamic behavior in embedded real time systems is generally a source of concern, as it tends to be non-deterministic and failure is hard to contain. Using the facilities provided by most real time operating systems, a dynamic memory facility may be implemented which is deterministic, immune from fragmentation and with good error handling.
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Design And Reuse. Dynamic Memory in C In C, dynamic memory is allocated from the heap using some standard library functions. There are two other variants of the malloc function: calloc and realloc. The computer of course obliges.
The problem with this is that the computer needs to know how much memory to set aside before your program starts running. When you run your program, the computer gives it the memory it needs to hold all of the variables you've declared; in other words, you've statically allocated memory.
But this method fails in the above case with the professor. What we'd like to be able to do is to create an array whose size is specified at run time. This time, the computer doesn't oblige; it fact, neither does the compiler. The reason is that at compile time the compiler has absolutely no idea how big an array arr will need to be.
The user could enter any value he wanted for steve , which means the arr could be any size at all. Since the compiler needs to know how much space to tell the computer to set aside, this code won't work. So, how do we get around this? The answer is dynamic memory allocation, and for that, we need pointers.
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