I am writing C for an MPC 555 board and need to figure out how to allocate dynamic memory without using malloc.
Typically malloc() is implemented on Unix using sbrk() or mmap(). (If you use the latter, you want to use the MAP_ANON flag.)
If you're targetting Windows, VirtualAlloc may help. (More or less functionally equivalent to anonymous mmap().)
Update: Didn't realize you weren't running under a full OS, I somehow got the impression instead that this might be a homework assignment running on top of a Unix system or something...
If you are doing embedded work and you don't have a malloc(), I think you should find some memory range that it's OK for you to write on, and write your own malloc(). Or take someone else's.
Pretty much the standard one that everybody borrows from was written by Doug Lea at SUNY Oswego. For example glibc's malloc is based on this. See: malloc.c, malloc.h.
You might want to check out Ralph Hempel's Embedded Memory Manager.
If your runtime doesn't support malloc, you can find an open source malloc and tweak it to manage a chunk of memory yourself.
malloc() is an abstraction that is use to allow C programs to allocate memory without having to understand details about how memory is actually allocated from the operating system. If you can't use malloc, then you have no choice other than to use whatever facilities for memory allocation that are provided by your operating system.
If you have no operating system, then you must have full control over the layout of memory. At that point for simple systems the easiest solution is to just make everything static and/or global, for more complex systems, you will want to reserve some portion of memory for a heap allocator and then write (or borrow) some code that use that memory to implement malloc.
An answer really depends on why you might need to dynamically allocate memory. What is the system doing that it needs to allocate memory yet cannot use a static buffer? The answer to that question will guide your requirements in managing memory. From there, you can determine which data structure you want to use to manage your memory.
For example, a friend of mine wrote a thing like a video game, which rendered video in scan-lines to the screen. That team determined that memory would be allocated for each scan-line, but there was a specific limit to how many bytes that could be for any given scene. After rendering each scan-line, all the temporary objects allocated during that rendering were freed.
To avoid the possibility of memory leaks and for performance reasons (this was in the 90's and computers were slower then), they took the following approach: They pre-allocated a buffer which was large enough to satisfy all the allocations for a scan-line, according to the scene parameters which determined the maximum size needed. At the beginning of each scan-line, a global pointer was set to the beginning of the scan line. As each object was allocated from this buffer, the global pointer value was returned, and the pointer was advanced to the next machine-word-aligned position following the allocated amount of bytes. (This alignment padding was including in the original calculation of buffer size, and in the 90's was four bytes but should now be 16 bytes on some machinery.) At the end of each scan-line, the global pointer was reset to the beginning of the buffer.
In "debug" builds, there were two scan buffers, which were protected using virtual memory protection during alternating scan lines. This method detects stale pointers being used from one scan-line to the next.
The buffer of scan-line memory may be called a "pool" or "arena" depending on whome you ask. The relevant detail is that this is a very simple data structure which manages memory for a certain task. It is not a general memory manager (or, properly, "free store implementation") such as malloc, which might be what you are asking for.
Your application may require a different data structure to keep track of your free storage. What is your application?
You should explain why you can't use malloc(), as there might be different solutions for different reasons, and there are several reasons why it might be forbidden or unavailable on small/embedded systems:
concern over memory fragmentation. In this case a set of routines that allocate fixed size memory blocks for one or more pools of memory might be the solution.
the runtime doesn't provide a malloc() - I think most modern toolsets for embedded systems do provide some way to link in a malloc() implementation, but maybe you're using one that doesn't for whatever reason. In that case, using Doug Lea's public domain malloc might be a good choice, but it might be too large for your system (I'm not familiar with the MPC 555 off the top of my head). If that's the case, a very simple, custom malloc() facility might be in order. It's not too hard to write, but make sure you unit test the hell out of uit because it's also easy to get details wrong. For example, I have a set of very small routines that use a brain dead memory allocation strategy using blocks on a free list (the allocator can be compile-time configured for first, best or last fit). I give it an array of char at initialization, and subsequent allocation calls will split free blocks as necessary. It's nowhere near as sophisticated as Lea's malloc(), but it's pretty dang small so for simple uses it'll do the trick.
many embedded projects forbid the use of dynamic memory allocation - in this case, you have to live with statically allocated structures
Write your own. Since your allocator will probably be specialized to a few types of objects, I recommend the Quick Fit scheme developed by Bill Wulf and Charles Weinstock. (I have not been able to find a free copy of this paper, but many people have access to the ACM digital library.) The paper is short, easy to read, and well suited to your problem.
If you turn out to need a more general allocator, the best guide I have found on the topic of programming on machines with fixed memory is Donald Knuth's book The Art of Computer Programming, Volume 1. If you want examples, you can find good ones in Don's epic book-length treatment of the source code of TeX, TeX: The Program.
Finally, the undergraduate textbook by Bryant and O'Hallaron is rather expensive, but it goes through the implementation of malloc in excruciating detail.
Write your own. Preallocate a big chunk of static RAM, then write some functions to grab and release chunks of it. That's the spirit of what malloc() does, except that it asks the OS to allocate and deallocate memory pages dynamically.
There are a multitude of ways of keeping track of what is allocated and what is not (bitmaps, used/free linked lists, binary trees, etc.). You should be able to find many references with a few choice Google searches.
malloc() and its related functions are the only game in town. You can, of course, roll your own memory management system in whatever way you choose.
If there are issues allocating dynamic memory from the heap, you can try allocating memory from the stack using alloca(). The usual caveats apply:
The memory is gone when you return.
The amount of memory you can allocate is dependent on the maximum size of your stack.
You might be interested in: liballoc
It's a simple, easy-to-implement malloc/free/calloc/realloc replacement which works.
If you know beforehand or can figure out the available memory regions on your device, you can also use their libbmmm to manage these large memory blocks and provide a backing-store for liballoc. They are BSD licensed and free.
FreeRTOS contains 3 examples implementations of memory allocation (including malloc()) to achieve different optimizations and use cases appropriate for small embedded systems (AVR, ARM, etc). See the FreeRTOS manual for more information.
I don't see a port for the MPC555, but it shouldn't be difficult to adapt the code to your needs.
If the library supplied with your compiler does not provide malloc, then it probably has no concept of a heap.
A heap (at least in an OS-less system) is simply an area of memory reserved for dynamic memory allocation. You can reserve such an area simply by creating a suitably sized statically allocated array and then providing an interface to provide contiguous chunks of this array on demand and to manage chunks in use and returned to the heap.
A somewhat neater method is to have the linker allocate the heap from whatever memory remains after stack and static memory allocation. That way the heap is always automatically as large as it possibly can be, allowing you to use all available memory simply. This will require modification of the application's linker script. Linker scripts are specific to the particular toolchain, and invariable somewhat arcane.
K&R included a simple implementation of malloc for example.
Related
Currently we use malloc/free Linux commands for memory allocation/de-allocation in our C based embedded application. I heard that this would cause memory fragmentation as the heap size increases/decreases because of memory allocation/de-allocation which would result in performance degradation. Other programming languages with efficient Garbage Collection solves this issue by freeing the memory when not in use.
Are there any alternate approaches which would solve this issue in C based embedded programs ?
You may take a look at a solution called memory pool allocation.
See: Memory pools implementation in C
Yes, there's an easy solution: don't use dynamic memory allocation outside of initialization.
It is common (in my experience) in embedded systems to only allow calls to malloc when a program starts (this is usually done by convention, there's nothing in C to enforce this. Although you can create your own wrapper for malloc to do this). This requires more work to analyze what memory your program could possibly use since you have to allocate it all at once. The benefit you get, however, is a complete understanding of what memory your program uses.
In some cases this is fairly straightforward, in particular if your system has enough memory to allocate everything it could possibly need all at once. In severely memory-limited systems, however, you're left with the managing the memory yourself. I've seen this done by writing "custom allocators" which you allocate and free memory from. I'll provide an example.
Let's say you're implementing some mathematical program that needs lots of big matrices (not terribly huge, but for example 1000x1000 floats). Your system may not have the memory to allocate many of these matrices, but if you can allocate at least one of them, you could create a pool of memory used for matrix objects, and every time you need a matrix you grab memory from that pool, and when you're done with it you return it to the pool. This is easy if you can return them in the same order you got them in, meaning the memory pool works just like a stack. If this isn't the case, perhaps you could just clear the entire pool at the end of each "iteration" (assuming this math system is periodic).
With more detail about what exactly you're trying to implement I could provide more relevant/specific examples.
Edit: See sg7's answer as well: that user provides a link to well-established frameworks which implement what I describe here.
C (and C++) include a family of dynamic memory allocation functions, most of which are intuitively named and easy to explain to a programmer with a basic understanding of memory. malloc() simply allocates memory, while calloc() allocates some memory and clears it eagerly. There are also realloc() and free(), which are pretty self-explanatory.
The manpage for malloc() also mentions valloc(), which allocates (size) bytes aligned to the page border.
Unfortunately, my background isn't thorough enough in low-level intricacies; what are the implications of allocating and using page border-aligned memory, and when is this appropriate as opposed to regular malloc() or calloc()?
The manpage for valloc contains an important note:
The function valloc() appeared in 3.0BSD. It is documented as being obsolete in 4.3BSD, and as legacy in SUSv2. It does not appear in POSIX.1-2001.
valloc is obsolete and nonstandard - to answer your question, it would never be appropriate to use in new code.
While there are some reasons to want to allocate aligned memory - this question lists a few good ones - it is usually better to let the memory allocator figure out which bit of memory to give you. If you are certain that you need your freshly-allocated memory aligned to something, use aligned_alloc (C11) or posix_memalign (POSIX) instead.
Allocations with page alignment usually are not done for speed - they're because you want to take advantage of some feature of your processor's MMU, which typically works with page granularity.
One example is if you want to use mprotect(2) to change the access rights on that memory. Suppose, for instance, that you want to store some data in a chunk of memory, and then make it read only, so that any buggy part of your program that tries to write there will trigger a segfault. Since mprotect(2) can only change permissions page by page (since this is what the underlying CPU hardware can enforce), the block where you store your data had better be page aligned, and its size had better be a multiple of the page size. Otherwise the area you set read-only might include other, unrelated data that still needs to be written.
Or, perhaps you are going to generate some executable code in memory and then want to execute it later. Memory you allocate by default probably isn't set to allow code execution, so you'll have to use mprotect to give it execute permission. Again, this has to be done with page granularity.
Another example is if you want to allocate memory now, but might want to mmap something on top of it later.
So in general, a need for page-aligned memory would relate to some fairly low-level application, often involving something system-specific. If you needed it, you'd know. (And as mentioned, you should allocate it not with valloc, but using posix_memalign, or perhaps an anonymous mmap.)
First of all valloc is obsolete, and memalignshould be used instead.
Second thing it's not part of the C (C++) standard at all.
It's a special allocation which is aligned to _SC_PAGESIZE boundary.
When is it useful to use it? I guess never, unless you have some specific low level requirement. If you would need it, you would know to need it, since it's rarely useful (maybe just when trying some micro-optimizations or creating shared memory between processes).
The self-evident answer is that it is appropriate to use valloc when malloc is unsuitable (less efficient) for the application (virtual) memory usage pattern and valloc is better suited (more efficient). This will depend on the OS and libraries and architecture and application...
malloc traditionally allocated real memory from freed memory if available and by increasing the brk point if not, in which case it is cleared by the OS for security reasons.
calloc in a dumb implementation does a malloc and then (re)clears the memory, while a smart implementation would avoid reclearing newly allocated memory that is automatically cleared by the operating system.
valloc relates to virtual memory. In a virtual memory system using the file system, you can allocate a large amount of memory or filespace/swapspace, even more than physical memory, and it will be swapped in by pages so alignment is a factor. In Unix creation of file of a specified file and adding/deleting pages is done using inodes to define the file but doesn't deal with actual disk blocks till needed, in which case it creates them cleared. So I would expect a valloc system to increase the size of the data segment swap without actually allocating physical or swap pages, or running a for loop to clear it all - as the file and paging system does that as needed. Thus valloc should be a heck of a lot faster than malloc. But as with calloc, how particular idiotsyncratic *x/C flavours do it is up to them, and the valloc man page is totally unhelpful about these expectations.
Traditionally this was implemented with brk/sbrk. Of course in a virtual memory system, whether a paged or a segmented system, there is no real need for any of this brk/sbrk stuff and it is enough to simply write the last location in a file or address space to extend up to that point.
Re the allocation to page boundaries, that is not usually something the user wants or needs, but rather is usually something the system wants or needs.
A (probably more expensive) way to simulate valloc is to determine the page boundary and then call aligned_alloc or posix_memalign with this alignment spec.
The fact that valloc is deprecated or has been removed or is not required in some OS' doesn't mean that it isn't still useful and required for best efficiency in others. If it has been deprecated or removed, one would hope that there are replacements that are as efficient (but I wouldn't bet on it, and might, indeed have, written my own malloc replacement).
Over the last 40 years the tradeoffs of real and (once invented) virtual memory have changed periodically, and mainstream OS has tended to go for frills rather than efficiency, with programmers who don't have (time or space) efficiency as a major imperative. In the embedded systems, efficiency is more critical, but even there efficiency is often not well supported by the standard OS and/or tools. But when in doubt, you can roll your own malloc replacement for your application that does what you need, rather than depend on what someone else woke up and decided to do/implement, or to undo/deprecate.
So the real answer is you don't necessarily want to use valloc or malloc or calloc or any of the replacements your current subversion of an OS provides.
I have to do a project in C where I have to constantly allocate memory for big data structures and then free it. Does there exista a library with a function that helps to keep track of the memory usage so I can be sure if I am doing things correctly? (I'm new to C)
For example, a function that returns:
A) The total of memory used by the program at the moment, OR
B) The total of memory left,
would do the job. I already googled for that and searched in other answers.
Thanks!
Try tcmalloc: you are looking for a heap profiler, although valgrind might be more useful initially.
If you're worried about memory leaks, valgrind is probably what you need. On the other hand, if you're more concerned just with whether you're data structures are using excessive memory, you might just use the common mallinfo function included as an extension to malloc in many unix standard libraries including glibc on Linux.
Although some people excoriate it, the book "Writing Solid Code" by Steve Maguire has a lot of reasonable ideas about how to track your memory usage without modifying the system memory allocation functions. Basically, instead of calling the raw malloc() etc functions directly, you call your own memory allocation API built on top of the standard one. Your API can track allocations and frees, detect double frees, frees of non-allocated memory, unreleased (leaked) memory, complete dumps of what is allocated, etc. You either need to crib the code from the book or write your own equivalent code. One interesting problem is providing a stack trace for each allocation; there isn't a standard way to determine the call stack. (The book is a bit dated now; it was written just a few years after the C89 standard was published and does not exploit const qualifiers.)
Some will argue that these services can be provided by the system malloc(); indeed, they can, and these days often are. You should look carefully at the manual provided for your version of malloc(), and decide whether it provides enough for you. If not, then the wrapper API mechanism is reasonable. Note that using your own API means you track what you explicitly allocate, while leaving library functions not written to use your API using the system services - as, indeed, does your code, under the covers.
You should also look into valgrind. It does a superb job tracking memory abuses, and in particular will report leaked memory (memory that was allocated but not freed). It also spots when you read or write outside the bounds of an allocated space, spotting buffer overflows.
Nevertheless, ultimately, you need to be disciplined in the way you write your code, ensuring that every time you allocate memory, you know when it will be released.
Every time you allocate/free memory, you could log how big your data structure is.
Today, I appeared for an interview and the interviewer asked me this,
Tell me the steps how will you design your own free( ) function for
deallocate the allocated memory.
How can it be more efficient than C's default free() function ? What can you conclude ?
I was confused, couldn't think of the way to design.
What do you think guys ?
EDIT : Since we need to know about how malloc() works, can you tell me the steps to write our own malloc() function
That's actually a pretty vague question, and that's probably why you got confused. Does he mean, given an existing malloc implementation, how would you go about trying to develop a more efficient way to free the underlying memory? Or was he expecting you to start discussing different kinds of malloc implementations and their benefits and problems? Did he expect you to know how virtual memory functions on the x86 architecture?
Also, by more efficient, does he mean more space efficient or more time efficient? Does free() have to be deterministic? Does it have to return as much memory to the OS as possible because it's in a low-memory, multi-tasking environment? What's our criteria here?
It's hard to say where to start with a vague question like that, other than to start asking your own questions to get clarification. After all, in order to design your own free function, you first have to know how malloc is implemented. So chances are, the question was really about whether or not you knew anything about how malloc can be implemented.
If you're not familiar with the internals of memory management, the easiest way to get started with understanding how malloc is implemented is to first write your own.
Check out this IBM DeveloperWorks article called "Inside Memory Management" for starters.
But before you can write your own malloc/free, you first need memory to allocate/free. Unfortunately, in a protected mode OS, you can't directly address the memory on the machine. So how do you get it?
You ask the OS for it. With the virtual memory features of the x86, any piece of RAM or swap memory can be mapped to a memory address by the OS. What your program sees as memory could be physically fragmented throughout the entire system, but thanks to the kernel's virtual memory manager, it all looks the same.
The kernel usually provides system calls that allow you to map in additional memory for your process. On older UNIX OS's this was usually brk/sbrk to grow heap memory onto the edge of your process or shrink it off, but a lot of systems also provide mmap/munmap to simply map a large block of heap memory in. It's only once you have access to a large, contiguous looking block of memory that you need malloc/free to manage it.
Once your process has some heap memory available to it, it's all about splitting it into chunks, with each chunk containing its own meta information about its size and position and whether or not it's allocated, and then managing those chunks. A simple list of structs, each containing some fields for meta information and a large array of bytes, could work, in which case malloc has to run through the list until if finds a large enough unallocated chunk (or chunks it can combine), and then map in more memory if it can't find a big enough chunk. Once you find a chunk, you just return a pointer to the data. free() can then use that pointer to reverse back a few bytes to the member fields that exist in the structure, which it can then modify (i.e. marking chunk.allocated = false;). If there's enough unallocated chunks at the end of your list, you can even remove them from the list and unmap or shrink that memory off your process's heap.
That's a real simple method of implementing malloc though. As you can imagine, there's a lot of possible ways of splitting your memory into chunks and then managing those chunks. There's as many ways as there are data structures and algorithms. They're all designed for different purposes too, like limiting fragmentation due to small, allocated chunks mixed with small, unallocated chunks, or ensuring that malloc and free run fast (or sometimes even more slowly, but predictably slowly). There's dlmalloc, ptmalloc, jemalloc, Hoard's malloc, and many more out there, and many of them are quite small and succinct, so don't be afraid to read them. If I remember correctly, "The C Programming Language" by Kernighan and Ritchie even uses a simple malloc implementation as one of their examples.
You can't blindly design free() without knowing how malloc() works under the hood because your implementation of free() would need to know how to manipulate the bookkeeping data and that's impossible without knowing how malloc() is implemented.
So an unswerable question could be how you would design malloc() and free() instead which is not a trivial question but you could answer it partially for example by proposing some very simple implementation of a memory pool that would not be equivalent to malloc() of course but would indicate your presence of knowledge.
One common approach when you only have access to user space (generally known as memory pool) is to get a large chunk of memory from the OS on application start-up. Your malloc needs to check which areas of the right size of that pool are still free (through some data structure) and hand out pointers to that memory. Your free needs to mark the memory as free again in the data structure and possibly needs to check for fragmentation of the pool.
The benefits are that you can do allocation in nearly constant time, the drawback is that your application consumes more memory than actually is needed.
Tell me the steps how will you design your own free( ) function for deallocate the allocated memory.
#include <stdlib.h>
#undef free
#define free(X) my_free(X)
inline void my_free(void *ptr) { }
How can it be more efficient than C's default free() function ?
It is extremely fast, requiring zero machine cycles. It also makes use-after-free bugs go away. It's a very useful free function for use in programs which are instantiated as short-lived batch processes; it can usefully be deployed in some production situations.
What can you conclude ?
I really want this job, but in another company.
Memory usage patterns could be a factor. A default implementation of free can't assume anything about how often you allocate/deallocate and what sizes you allocate when you do.
For example, if you frequently allocate and deallocate objects that are of similar size, you could gain speed, memory efficiency, and reduced fragmentation by using a memory pool.
EDIT: as sharptooth noted, only makes sense to design free and malloc together. So the first thing would be to figure out how malloc is implemented.
malloc and free only have a meaning if your app is to work on top of an OS. If you would like to write your own memory management functions you would have to know how to request the memory from that specific OS or you could reserve the heap memory right away using existing malloc and then use your own functions to distribute/redistribute the allocated memory through out your app
There is an architecture that malloc and free are supposed to adhere to -- essentially a class architecture permitting different strategies to coexist. Then the version of free that is executed corresponds to the version of malloc used.
However, I'm not sure how often this architecture is observed.
The knowledge of working of malloc() is necessary to implement free(). You can find a implementation of malloc() and free() using the sbrk() system call in K&R The C Programming Language Chapter 8, Section 8.7 "Example--A Storage Allocator" pp.185-189.
I read that some games rewrite their own malloc to be more efficient. I don't understand how this is possible in a virtual memory world. If I recall correctly, malloc actually calls an OS specific function, which maps the virtual address to a real address with the MMU. So then how can someone make their own memory allocator and allocate real memory, without calling the actual runtime's malloc?
Thanks
It's certainly possible to write an allocator more efficient than a general purpose one.
If you know the properties of your allocations, you can blow general purpose allocators out of the water.
Case in point: many years ago, we had to design and code up a communication subsystem (HDLC, X.25 and proprietary layers) for embedded systems. The fact that we knew the maximum allocation would always be less than 128 bytes (or something like that) meant that we didn't have to mess around with variable sized blocks at all. Every allocation was for 128 bytes no matter how much you asked for.
Of course, if you asked for more, it returned NULL.
By using fixed-length blocks, we were able to speed up allocations and de-allocations greatly, using bitmaps and associated structures to hold accounting information rather than relying on slower linked lists. In addition, the need to coalesce freed blocks was not needed.
Granted, this was a special case but you'll find that's so for games as well. In fact, we've even used this in a general purpose system where allocations below a certain threshold got a fixed amount of memory from a self-managed pre-allocated pool done the same way. Any other allocations (larger than the threshold or if the pool was fully allocated) were sent through to the "real" malloc.
Just because malloc() is a standard C function doesn't mean that it's the lowest level access you have to the memory system. In fact, malloc() is probably implemented in terms of lower-level operating system functionality. That means you could call those lower level interfaces too. They might be OS-specific, but they might allow you better performance than you would get from the malloc() interface. If that were the case, you could implement your own memory allocation system any way you want, and maybe be even more efficient about it - optimizing the algorithm for the characteristics of the size and frequency of allocations you're going to make, for example.
In general, malloc will call an OS-specific function to obtain a bunch of memory (at least one VM page), and will then divide that memory up into smaller chunks as needed to return to the caller of malloc.
The malloc library will also have a list (or lists) of free blocks, so it can often meet a request without asking the OS for more memory. Determining how many different block sizes to handle, deciding whether to attempt to combine adjacent free blocks, and so forth, are the choices the malloc library implementor has to make.
It's possible for you to bypass the malloc library and directly invoke the OS-level "give me some memory" function and do your own allocation/freeing within the memory you get from the OS. Such implementations are likely to be OS-specific. Another alternative is to use malloc for initial allocations, but maintain your own cache of freed objects.
One thing you can do is have your allocator allocate a pool of memory, then service requests from than (and allocate a bigger pool if it runs out). I'm not sure if that's what they're doing though.
If I recall correctly, malloc actually
calls an OS specific function
Not quite. Most hardware has a 4KB page size. Operating systems generally don't expose a memory allocation interface offering anything smaller than page-sized (and page-aligned) chunks.
malloc spends most of its time managing the virtual memory space that has already been allocated, and only occasionally requests more memory from the OS (obviously this depends on the size of the items you allocate and how often you free).
There is a common misconception that when you free something it is immediately returned to the operating system. While this sometimes occurs (particularly for larger memory blocks) it is generally the case that freed memory remains allocated to the process and can then be re-used by later mallocs.
So most of the work is in bookkeeping of already-allocated virtual space. Allocation strategies can have many aims, such as fast operation, low memory wastage, good locality, space for dynamic growth (e.g. realloc) and so on.
If you know more about your pattern of memory allocation and release, you can optimise malloc and free for your usage patterns or provide a more extensive interface.
For instance, you may be allocating lots of equal-sized objects, which may change the optimal allocation parameters. Or you may always free large amounts of objects at once, in which case you don't want free to be doing fancy things.
Have a look at memory pools and obstacks.
See How do games like GTA IV not fragment the heap?.