These routines are defined in the header file called stdlib. It is a function which is used to allocate a block of memory dynamically. It reserves memory space of specified size and returns the null pointer pointing to the memory location. The pointer returned is usually of type void. It means that we can assign malloc function to any pointer.
The full form of malloc is memory allocation. Calloc function is used to allocate multiple blocks of memory. It is a dynamic memory allocation function which is used to allocate the memory to complex data structures such as arrays and structures. If this function fails to allocate enough space as specified, it returns null pointer.
The full form of calloc function is contiguous allocation. When this statement is successfully executed, a memory space of 50 bytes is reserved. A good rule of thumb is to avoid call frames larger than a few kilobytes.
So using a VLA makes sense only if you are sure that your len is small enough at most a few dozens of thousands. Otherwise you have a stack overflow and that is a case of undefined behavior , a very scary situation. However, using manual C dynamic memory allocation e. In your case you might call both calloc and free in the same routine. If you know that most of the time your result a very poor name, you never should return the address of an automatic variable VLA; so I'm using buf instead of result below is small you could special case it, e.
However, the above code is less readable and is probably premature optimization. It is however more robust than a pure VLA solution. Some systems e. This is a feature I dislike and usually disable on my Linux machines. So, if this function is one that is called frequently which you will, of course, determine by profiling , the VLA is a good optimisation option. This is a very common C solution I use for the problem which might be of help.
Unlike VLAs, it doesn't face any practical risk of stack overflow in pathological cases. What this does in the above case is use the stack if the string fits into bytes or less. Otherwise it uses a heap allocation. However, let's say there's some crazy exotic case you might occasionally need to handle where the string is 32 kilobytes where the user fell asleep on his keyboard or something like that.
This allows both situations to be handled without problems. It does have the caveat that it's dangerous to copy around and you shouldn't return pointers allocated through it they could end up being invalidated as the FastMem instance is destroyed. It's not a general-purpose allocator and should not be used as such. I actually created it ages ago in response to a situation in a legacy codebase using C89 that a former team thought would never happen where a user managed to name an item with a name that was over characters long maybe he fell asleep on his keyboard.
My colleagues actually tried to increase the size of the arrays allocated in various places to 16, in response at which point I thought it was getting ridiculous and just exchanging a greater risk of stack overflow in exchange for lesser risk of buffer overflow. This provided a solution that was very easy to plug in to fix those cases by just adding a couple of lines of code.
This allowed the common case to be handled very efficiently and still utilize the stack without those crazy rare cases that demanded the heap crashing the software.
However, I've found it useful since then even after C99 since VLAs still can't protect us against stack overflows. This one can but still pools from the stack for small allocation requests. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more.
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Is the assumption that the code is targeted for a non-embedded environement? Add a comment. Active Oldest Votes.
One of them includes changing the size of an array. An array is a collection of items stored at contiguous memory locations. Take a step-up from those "Hello World" programs. Learn to implement data structures like Heap, Stacks, Linked List and many more!
Check out our Data Structures in C course to start learning today. As it can be seen that the length size of the array above made is 9. But what if there is a requirement to change this length size. For Example, If there is a situation where only 5 elements are needed to be entered in this array. In this case, the remaining 4 indices are just wasting memory in this array. So there is a requirement to lessen the length size of the array from 9 to 5. Take another situation.
In this, there is an array of 9 elements with all 9 indices filled. But there is a need to enter 3 more elements in this array. In this case, 3 indices more are required. So the length size of the array needs to be changed from 9 to This procedure is referred to as Dynamic Memory Allocation in C. Therefore, C Dynamic Memory Allocation can be defined as a procedure in which the size of a data structure like Array is changed during the runtime. C provides some functions to achieve these tasks.
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