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Text of the page (random words):
he sbrk system call to request memory from the operating system 6 the 6th edition unix documentation gives alloc and free as the low level memory allocation functions 7 the malloc and free routines in their modern form are completely described in the 7th edition unix manual 8 9 some platforms provide library or intrinsic function calls which allow run time dynamic allocation from the c stack rather than the heap e g alloca 10 this memory is automatically freed when the calling function ends overview of functions edit the c dynamic memory allocation functions are defined in stdlib h header cstdlib header in c 1 function description malloc allocates the specified number of bytes aligned_alloc allocates the specified number of bytes at the specified alignment realloc increases or decreases the size of the specified block of memory moving it if necessary calloc allocates the specified number of bytes and initializes them to zero free releases the specified block of memory back to the system differences between malloc and calloc edit malloc takes a single argument the amount of memory to allocate in bytes while calloc takes two arguments the number of elements and the size of each element malloc only allocates memory while calloc allocates and sets the bytes in the allocated region to zero 11 usage example edit creating an array of ten integers with automatic scope is straightforward in c int a 10 however the size of the array is fixed at compile time if one wishes to allocate a similar array dynamically without using a variable length array which is not guaranteed to be supported in all c11 implementations an array can be allocated using malloc which returns a void pointer indicating that it is a pointer to a region of unknown data type which can be cast for safety int a int malloc 10 sizeof int this computes the number of bytes that ten integers occupy in memory then requests that many bytes from malloc and assigns the result to a pointer named a due to c syntax pointers and arrays can be used interchangeably in some situations because malloc might not be able to service the request it might return a null pointer and it is good programming practice to check for this int a int malloc 10 sizeof int if a fprintf stderr malloc failed n return 1 when the program no longer needs the dynamic array it must eventually call free to return the memory it occupies to the free store free a the memory set aside by malloc is not initialized and may contain cruft the remnants of previously used and discarded data after allocation with malloc elements of the array are uninitialized variables the command calloc will return an allocation that has already been cleared int a int calloc 10 sizeof int with realloc we can resize the amount of memory a pointer points to for example if we have a pointer acting as an array of size n displaystyle n and we want to change it to an array of size m displaystyle m we can use realloc int a int malloc 2 sizeof int a 0 1 a 1 2 a int realloc a 3 sizeof int a 2 3 note that realloc must be assumed to have changed the base address of the block i e if it has failed to extend the size of the original block and has therefore allocated a new larger block elsewhere and copied the old contents into it therefore any pointers to addresses within the original block are also no longer valid type safety edit as stated earlier malloc returns a void pointer void which indicates that it is a pointer to a region of unknown data type the use of casting is required in c due to the strong type system whereas this is not the case in c one may cast see type conversion this pointer to a specific type without a cast int ptr1 malloc 10 sizeof ptr with a cast int ptr2 int malloc 10 sizeof ptr there are advantages and disadvantages to performing such a cast including the cast improves interoperability between c and c allowing c code to be compiled or used as c furthermore the cast allows for pre 1989 versions of malloc that originally returned a char 12 additionally casting can help the developer identify inconsistencies in type sizing should the destination pointer type change particularly if the pointer is declared far from the malloc call although modern compilers and static analysers can warn on such behaviour without requiring the cast 13 however under the c standard the cast is not necessary and adding the cast may mask failure to include the header stdlib h in which the function prototype for malloc is found 12 14 in the absence of a prototype for malloc the c90 standard requires that the c compiler assume malloc returns an int if there is no cast c90 requires a diagnostic when this integer is assigned to the pointer however with the cast this diagnostic would not be produced hiding a bug on certain architectures and data models such as lp64 on 64 bit systems where long and pointers are 64 bit and int is 32 bit this error can actually result in undefined behaviour as the implicitly declared malloc returns a 32 bit value whereas the actually defined function returns a 64 bit value depending on calling conventions and memory layout this may result in stack smashing this issue is less likely to go unnoticed in modern compilers as c99 does not permit implicit declarations so the compiler must produce a diagnostic even if it does assume int return additionally if the type of the pointer is changed at its declaration the lines where malloc is called and the cast will have to be updated as well in c if std malloc must be used it is preferable to static_cast it rather than use a raw cast int p static_cast int malloc 10 sizeof ptr common errors edit the improper use of dynamic memory allocation can frequently be a source of bugs these can include security bugs or program crashes most often due to segmentation faults most common errors are as follows 15 not checking for allocation failures memory allocation is not guaranteed to succeed and may instead return a null pointer using the returned value without checking if the allocation is successful invokes undefined behavior this usually leads to crash due to the resulting segmentation fault on the null pointer dereference but there is no guarantee that a crash will happen so relying on that can also lead to problems memory leaks failure to deallocate memory using free leads to the buildup of non reusable memory which is no longer used by the program this wastes memory resources and can lead to allocation failures when these resources are exhausted logical errors all allocations must follow the same pattern allocation using malloc usage to store data deallocation using free failures to adhere to this pattern such as memory usage after a call to free dangling pointer or before a call to malloc wild pointer calling free twice double free etc usually causes a segmentation fault and results in a crash of the program these errors can be transient and hard to debug for example freed memory is usually not immediately reclaimed by the os and thus dangling pointers may persist for a while and appear to work in addition as an interface that precedes ansi c standardization malloc and its associated functions have behaviors that were intentionally left to the implementation to define for themselves one of them is the zero length allocation which is more of a problem with realloc since it is more common to resize to zero 16 although both posix and the single unix specification require proper handling of 0 size allocations by either returning null or something else that can be safely freed 17 not all platforms are required to abide by these rules among the many double free errors that it has led to the 2019 whatsapp rce was especially prominent 18 a way to wrap these functions to make them safer is by simply checking for 0 size allocations and turning them into those of size 1 returning null has its own problems it otherwise indicates an out of memory failure in the case of realloc it would have signaled that the original memory was not moved and freed which again is not the case for size 0 leading to the double free 19 implementations edit the implementation of memory management depends greatly upon operating system and architecture some operating systems supply an allocator for malloc while others supply functions to control certain regions of data the same dynamic memory allocator is often used to implement both malloc and the operator new in c 20 heap based edit see also sbrk implementation of legacy allocators was commonly done using the heap segment the allocator would usually expand and contract the heap to fulfill allocation requests the heap method suffers from a few inherent flaws a linear allocator can only shrink if the last allocation is released even if largely unused the heap can get stuck at a very large size because of a small but long lived allocation at its tip which could waste any amount of address space although some allocators on some systems may be able to release entirely empty intermediate pages to the os a linear allocator is sensitive to fragmentation a good allocator will attempt to track and reuse free slots through the entire heap but as allocation sizes and lifetimes get mixed it can be difficult and expensive to find or coalesce free segments large enough to hold new allocation requests a linear allocator has extremely poor concurrency characteristics as the heap segment is per process every thread has to synchronise on allocation and concurrent allocations from threads which may have very different work loads amplifies the previous two issues dlmalloc and ptmalloc edit doug lea has developed the public domain dlmalloc doug lea s malloc as a general purpose allocator starting in 1987 the gnu c library glibc is derived from wolfram gloger s ptmalloc pthreads malloc a fork of dlmalloc with threading related improvements 21 22 23 as of november 2023 the latest version of dlmalloc is version 2 8 6 from august 2012 in 2023 it was relicensed from cc0 to mit 0 24 dlmalloc is a boundary tag allocator memory on the heap is allocated as chunks an 8 byte aligned data structure which contains a header and usable memory allocated memory contains an 8 or 16 byte overhead for the size of the chunk and usage flags similar to a dope vector unallocated chunks also store pointers to other free chunks in the usable space area making the minimum chunk size 16 bytes on 32 bit systems and 24 32 depends on alignment bytes on 64 bit systems 22 24 2 8 6 minimum allocated size unallocated memory is grouped into bins of similar sizes implemented by using a double linked list of chunks with pointers stored in the unallocated space inside the chunk bins are sorted by size into three classes 22 24 overlaid data structures for requests below 256 bytes a smallbin request a simple two power best fit allocator is used if there are no free blocks in that bin a block from the next highest bin is split in two for requests of 256 bytes or above but below the mmap threshold dlmalloc since v2 8 0 use an in place bitwise trie algorithm treebin if there is no free space left to satisfy the request dlmalloc tries to increase the size of the heap usually via the brk system call this feature was introduced way after ptmalloc was created from v2 7 x and as a result is not a part of glibc which inherits the old best fit allocator for requests above the mmap threshold a largebin request the memory is always allocated using the mmap system call the threshold is usually 128 kb 25 the mmap method averts problems with huge buffers trapping a small allocation at the end after their expiration but always allocates an entire page of memory which on many architectures is 4096 bytes in size 26 game developer adrian stone argues that dlmalloc as a boundary tag allocator is unfriendly for console systems that have virtual memory but do not have demand paging this is because its pool shrinking and growing callbacks sysmalloc systrim cannot be used to allocate and commit individual pages of virtual memory in the absence of demand paging fragmentation becomes a greater concern 27 freebsd s and netbsd s jemalloc edit since freebsd 7 0 and netbsd 5 0 the old malloc implementation phkmalloc by poul henning kamp was replaced by jemalloc written by jason evans the main reason for this was a lack of scalability of phkmalloc in terms of multithreading in order to avoid lock contention jemalloc uses separate arenas for each cpu experiments measuring number of allocations per second in multithreading application have shown that this makes it scale linearly with the number of threads while for both phkmalloc and dlmalloc performance was inversely proportional to the number of threads 28 openbsd s malloc edit openbsd s implementation of the malloc function makes use of mmap for requests greater in size than one page the entire allocation is retrieved using mmap smaller sizes are assigned from memory pools maintained by malloc within a number of bucket pages also allocated with mmap 29 better source needed on a call to free memory is released and unmapped from the process address space using munmap this system is designed to improve security by taking advantage of the address space layout randomization and gap page features implemented as part of openbsd s mmap system call and to detect use after free bugs as a large memory allocation is completely unmapped after it is freed further use causes a segmentation fault and termination of the program the grapheneos project initially started out by porting openbsd s memory allocator to android s bionic c library 30 hoard malloc edit main article hoard memory allocator hoard is an allocator whose goal is scalable memory allocation performance like openbsd s allocator hoard uses mmap exclusively but manages memory in chunks of 64 kilobytes called superblocks hoard s heap is logically divided into a single global heap and a number of per processor heaps in addition there is a thread local cache that can hold a limited number of superblocks by allocating only from superblocks on the local per thread or per processor heap and moving mostly empty superblocks to the global heap so they can be reused by other processors hoard keeps fragmentation low while achieving near linear scalability with the number of threads 31 mimalloc edit main article mimalloc an open source compact general purpose memory allocator from microsoft research with focus on performance 32 the library is about 11 000 lines of code thread caching malloc tcmalloc edit every thread has a thread local storage for small allocations for large allocations mmap or sbrk can be used tcmalloc a malloc developed by google 33 has garbage collection for local storage of dead threads the tcmalloc is considered to be more than twice as fast as glibc s ptmalloc for multithreaded programs 34 35 in kernel edit operating system kernels need to allocate memory just as application programs do the implementation of malloc within a kernel often differs sign...