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# How to allocate memory which is 2^k aligned

 P: n/a Hi, can somebody help me figure out how can I make write a function which inturn uses malloc routine to allocate memory which is 2^k aligned? The condition is such that there should not be any memory leak and it should not also waste memory. Thanks, Divick Sep 5 '06 #1
11 Replies

 P: n/a Divick wrote: can somebody help me figure out how can I make write a function which inturn uses malloc routine to allocate memory which is 2^k aligned? The condition is such that there should not be any memory leak and it should not also waste memory. I think you're looking to calculate the next power of 2 from a number. Given that you need to allocate N bytes, the N is not necessarily any power of 2, so you need to find the next power of 2 that is not smaller than N. You can do it in a loop. V -- Please remove capital 'A's when replying by e-mail I do not respond to top-posted replies, please don't ask Sep 5 '06 #2

 P: n/a Divick posted: Can somebody help me figure out how can I make write a function which inturn uses malloc routine to allocate memory which is 2^k aligned? "malloc" does this already -- it allocates memory whose alignment is strict enough to satisfy the alignment of any kind of object. You might like "aligned_storage" aswell (Google for it), although it allocates on the stack rather than on the heap. -- Frederick Gotham Sep 5 '06 #3

 P: n/a Divick wrote: > can somebody help me figure out how can I make write a function which inturn uses malloc routine to allocate memory which is 2^k aligned? The condition is such that there should not be any memory leak and it should not also waste memory. For more or less arbitrary values of k, the only way to do it is to allocate a block that's sure to be large enough (i.e. 2^(k+1)) and pick off an address within that block that meets your alignment requirements. For values of k such that 2^k is the alignment of some type (typically 2, 4, 8, or 16 bytes), malloc returns a pointer that's suitably aligned for any type, so you don't need to do anything special. -- -- Pete Author of "The Standard C++ Library Extensions: a Tutorial and Reference." For more information about this book, see www.petebecker.com/tr1book. Sep 5 '06 #4

 P: n/a Frederick Gotham wrote: Divick posted: Can somebody help me figure out how can I make write a function which inturn uses malloc routine to allocate memory which is 2^k aligned? "malloc" does this already -- it allocates memory whose alignment is strict enough to satisfy the alignment of any kind of object. But malloc only satisfies the alignment requirements of the implementation - e.g. hardware alignment requirements. The application may have stricter requirements depending on what you want to do. Regards, Bart. Sep 5 '06 #5

 P: n/a For more or less arbitrary values of k, the only way to do it is to allocate a block that's sure to be large enough (i.e. 2^(k+1)) and pick off an address within that block that meets your alignment requirements. For values of k such that 2^k is the alignment of some type (typically 2, 4, 8, or 16 bytes), malloc returns a pointer that's suitably aligned for any type, so you don't need to do anything special. Since I have constraint that I have to use malloc in my application and I also want that once that memory is passed to me I should be able to free the memory. Thus simply returning the offset will not solve my purpose as once the application passes the same location to me , I will pass the location to free and free wouldn't know that it was actually allocated. See the code below The problem with the code below is that free does not know how to free the memory because I pass it the address which malloc didn't return. The other problem is that lot of memory gets wasted. Is there a way to overcome such problems? #define ALIGNMENT_K 3 void * allocate_memory(size_t size) { //allocate memory so that at least there is an address which is 2^k aligned char * mem = (char *)malloc(size + 2^ALIGNMENT_K); //calculate offset and if it is not 2^k aligned then //increment the pointer till it is. while( (*mem & ~(0xffffffff << ALIGNMENT_K)) != 0 ) { mem++; } return (void *)mem; } //If the mem pointer is not the same that malloc returned then it //is a problem and free won't be able to free the memory void free_memory(void * mem) { free(mem); } I hope I am clear in my question and intent. Thanks, Divick Sep 6 '06 #6

 P: n/a But malloc only satisfies the alignment requirements of the implementation - e.g. hardware alignment requirements. The application may have stricter requirements depending on what you want to do. Yup exactly that is what my problem is. Thanks for asserting in a clear way. Divick Sep 6 '06 #7

 P: n/a Divick wrote: Hi, can somebody help me figure out how can I make write a function which inturn uses malloc routine to allocate memory which is 2^k aligned? The condition is such that there should not be any memory leak and it should not also waste memory. There is no portable way. But why do you want a specific alignment in the first place? The reason for this must be for some nonportable reason and the platform your program is to run on likely will support such a call - or at least just waste virtual (not real) memory. /Peter Sep 6 '06 #8

 P: n/a Divick Since I have constraint that I have to use malloc in my application andI also want that once that memory is passed to me I should be able tofree the memory. Thus simply returning the offset will not solve mypurpose as once the application passes the same location to me , I willpass the location to free and free wouldn't know that it was actuallyallocated. See the code belowThe problem with the code below is that free does not know how to freethe memory because I pass it the address which malloc didn't return.The other problem is that lot of memory gets wasted. Is there a way toovercome such problems? It seems you have to write your own version of free() that contains a map between the original values returned by malloc() and the aligned values passed to the application. This should be reasonably easy using a std::map. Steve Sep 6 '06 #9

 P: n/a Divick wrote: Hi, can somebody help me figure out how can I make write a function which inturn uses malloc routine to allocate memory which is 2^k aligned? The condition is such that there should not be any memory leak and it should not also waste memory. you could try something like template struct aligned_str { boost::aligned_storage(malloc(sizeof(my_aligned) ); // and use p->aligned_.address(); Sep 7 '06 #10

 P: n/a dasjotre wrote: sorry, I was beeing lazy ... template class aligned_str { boost::aligned_storageaddress(); Sep 7 '06 #11

 P: n/a dasjotre wrote: Divick wrote: >>Hi, can somebody help me figure out how can I make write a functionwhich inturn uses malloc routine to allocate memory which is 2^kaligned?The condition is such that there should not be any memory leak and itshould not also waste memory. you could try something like template struct aligned_str { boost::aligned_storage(malloc(sizeof(my_aligned) ); // and use p->aligned_.address(); I don't think that will work. aligned_storage is basically a union of a char array of the appropriate size and some other type with suitable alignment. For TR1 we made it clear that, essentially, the value of Align must be a value that is the alignment of some type. It doesn't handle arbitrary alignment values. -- -- Pete Author of "The Standard C++ Library Extensions: a Tutorial and Reference." For more information about this book, see www.petebecker.com/tr1book. Sep 7 '06 #12

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