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. Array or Malloc? Ask Question. Asked 9 years, 7 months ago.
Active 11 months ago. Viewed 16k times. Improve this question. Dynamic 5, 9 9 gold badges 42 42 silver badges 73 73 bronze badges. Dev Bag Dev Bag 1 1 gold badge 1 1 silver badge 3 3 bronze badges.
Is the assumption that the code is targeted for a non-embedded environement? Add a comment. Active Oldest Votes. If you declare char result[len]; and len exceeds the amount of available stack space, your program's behavior is undefined.
Improve this answer. Keith Thompson Keith Thompson 6, 2 2 gold badges 27 27 silver badges 34 34 bronze badges. Variable-length automatic arrays were introduced to C in C Unless you have concerns about backwards comparability to older standards, it's fine. Philip Philip 6, 25 25 silver badges 43 43 bronze badges. The memory allocated using functions malloc and calloc is not de-allocated on their own. Hence the free method is used, whenever the dynamic memory allocation takes place.
It helps to reduce wastage of memory by freeing it. Malloc Memory successfully freed. Memory successfully allocated using calloc. Calloc Memory successfully freed. In other words, if the memory previously allocated with the help of malloc or calloc is insufficient, realloc can be used to dynamically re-allocate memory. The elements of the array are: 1, 2, 3, 4, 5, Enter the new size of the array: 10 Memory successfully re-allocated using realloc.
Skip to content. Change Language. Related Articles. Table of Contents. Save Article. Allocation would then work like this:. Clearly some allocations would be very efficient: 16 bytes from the byte pool. Some would be very good; 31 bytes from the byte pool. Others would be OK; 9 bytes from the byte pool. Yet others would be inefficient; 65 bytes from the byte pool. Overall, these inefficiencies are a small price to pay for the benefits of speed, determinism and elimination of fragmentation.
Interesting post, however your pool scheme does not solve the case where allocation sizes tend to grow regularly up to a given size, as smaller chunks are not coalesced once freed. A better alternative, especially if sticking to power of two block siszes would be to use a Buddy allocator, as it tends to merge small blocks when they are freed. By the way, the buddy algorithm, combined, for small sizes, with a pool algorithm proposing non-power of two sizes always multiple of the cache line size is what is used very effectively by the Linux kernel to handle unpredictible allocations in a fast and reliable way.
You must Sign in or Register to post a comment. This site uses Akismet to reduce spam. Learn how your comment data is processed. You must verify your email address before signing in. Check your email for your verification email, or enter your email address in the form below to resend the email. Please confirm the information below before signing in. Already have an account? Sign In. Please check your email and click on the link to verify your email address.
We've sent an email with instructions to create a new password. Your existing password has not been changed. Sorry, we could not verify that email address. Connect and share knowledge within a single location that is structured and easy to search. As said, it is dynamic allocation which means you allocate the memory at run time.
For example when you don't know the amount of memory during compile time. One example should clear this. Say you know there will be maximum 20 students. So you can create an array with static 20 elements.
Your array will be able to hold maximum 20 students. But what if you don't know the number of students? Say the first input is the number of students. It could be 10, 20, 50 or whatever else. This is just one example. There are many situations like this where dynamic allocation is needed. Have a look at the man page malloc 3. You use malloc when you need to allocate objects that must exist beyond the lifetime of execution of the current block where a copy-on-return would be expensive as well , or if you need to allocate memory greater than the size of that stack ie: a 3mb local stack array is a bad idea.
C a is language that implicitly passes by value, rather than by reference. In this example, if we passed 'p' to a function to do some work on it, we would be creating a copy of the entire structure. This uses additional memory the total of how much space that particular structure would require , is slower, and potentially does not scale well more on this in a minute.
We only are passing an address in memory that refers to this structure. The amount of data passed is smaller size of a pointer , therefore the operation is faster.
Now, knowing this, imagine a program like a student information system which will have to create and manage a set of records in the thousands, or even tens of thousands. If you pass the whole structure by value, it will take longer to operate on a set of data, than it would just passing a pointer to each record. The reason being malloc allocates the space on heap while the other allocates it on the stack.
The C programming language manages memory statically, automatically, or dynamically. Static-duration variables are allocated in main memory, usually along with the executable code of the program, and persist for the lifetime of the program; automatic-duration variables are allocated on the stack and come and go as functions are called and return.
For static-duration and automatic-duration variables, the size of the allocation must be compile-time constant except for the case of variable-length automatic arrays[5].
0コメント