How memset works Ask questions, find answers and collaborate at work with Stack Overflow for Teams. Because it works at the byte level, memset() can effectively initialize memory for different data structures and types. Don't postulate about optimisations prior to running tests. But it always giving me seg fault; bool **visited=new bool*[m]; for(int i=0;i<m;++i) visited[i] = new bool[m]; I have . Collectives™ on Stack Overflow. This is vital for writing secure code. The following diagram clearly illustrate the working principle of memset() inbuilt string function in C. By working solution I mean "A @aqjune: Using memset works (has defined behavior) because memset is specified to work (as if) by copying characters, and the C standard gives special treatment to pointers to characters and to using character types as lvalues. It's up to you to provide the appropriate parameters. If your intent is to use an anonymous union in the main function, you can declare the union static: all static objects are zero initialized. Includes practical examples, best practices, and performance tips for memory manipulation. memset needs to work on a general piece of memory, not just a 0 terminated string - so there should not be such a check. 0. Because there is no completely portable way in C to test for zeros As Balog Pal said, casting away volatile to call memset breaks the volatile semantics. What is a CDN? A Content Delivery Network is a system which provides optimised web access to Most certainly, memset will be much faster than that loop. Here's my code. As you can see in the docs, the third argument of memset takes the number of bytes, not the number of elements. Definition of memset : I found something strange doing problem solving with c++. Teams. It would also allow for compiler optimizations a function call does not. Nothing wrong with that. Source code: https://github. New comments cannot be posted and votes cannot be cast. Either your system is broken, or you made a mistake. If you really want to use memset then you can do: memset(&array, 0, sizeof array); If you only have a pointer to the first element of the array, then: memset(ptr, 0, number_of_elements * sizeof *ptr); Ever wondered how top C++ developers optimize memory operations? Enter memset – a powerful function that can initialize memory blocks in microseconds. p_aliases is the content of the adresse pointed to by proto. Find centralized, trusted content and collaborate around the technologies you use most. Can I use memset to initialize a 2 dimensional array? Hot Network Questions Why does this switch have extra pins? How do I make my lamp glow like the attached image In Christie's The Adventure of Johnnie Waverly, why does Miss Collins lie? How to remove plywood countertop in laundry room that’s glued? How does memset work? c++; Share. In practice, your implementation will arrange for all-0-bytes to mean a null pointer, a 0. More Readable: Shorter usually makes it more readable as well. 3. Yu Hao. About; Products OverflowAI; Stack Overflow for Teams Where developers & technologists share private knowledge with This actually bit me hard recently. Why not 3 3. h> void *memset(void *dest, int c, size_t count); Language Level. Update your question with a test case providing these performance claims, and memset only uses one byte of the value passed in and does bytewise initialization. If buf is the result of a malloc, you don't need to worry about this, but if it's (say) passed in as an argument by code you don't control, you'll need to check the alignment before you write to it in this fashion, or else this will result in invalid accesses on some platforms. Reply reply What is the correct and safest way to memset the whole character array with the null terminating character? I can list a few usages: char* buffer = new char [ARRAY_LENGTH]; //Option 1: The bzero() function appears at Line 8, replacing memset() from the preceding example. The value of c is converted to an unsigned character. EDIT: fill and fill_n will work on anything that provides or can be treated as an output iterator. 2. What is memset ()? How does memset() Work? Under the hood, memset() operates by directly manipulating the memory at the address specified by ptr . Yes. The value which you want to copy will first convert in unsigned char. This is what I've got so far. void *memset(void *dest, int c, size_t count) The 3rd argument is the Number of characters or bytes in the array. In this comprehensive guide, you’ll learn everything The memset function takes three parameters: 1. The memset() function returns a pointer to the memory block it was used to fill (ptr). Value to set (converted to unsigned char) 3. I think the paradigmatic example of these optimizations, that go unnoticed usually, is the GNU C library strlen function. Below is the sample program. Understanding memset() helps By combining unrolled loops and block writes, memset () achieves close to memory bandwidth speed while writing. memset() will blindly write to the specified address for the number of specified bytes, regardless of what it might be overwriting. 0f; inside of that for loop. IMHO in C++ one should avoid doing memset when possible since it circumvents the type safety that C++ provides, instead one should use constructor or initialization as means of initializing. Before using the memset we must include string. &dev_sys gives you a pointer to the entire array, which is numerically the same as the pointer to its first element (which in turn is why you can use either with the same effect). This function is used to fill a contiguous block of memory with a specific value. Each rule is ordered in the For example, when a packet arrives at the firewall it is compared to rule 1. Less Code: As you have already mentioned, it's shorter - fewer lines of code. In other cases, calloc() can even cheat and not allocate any memory! However, malloc()+memset() will always do the full amount of work. h> #. #i I've listed these in increasing order of how good they usually are, so use the last one that isn't blocked by whatever constraints you're working under. Either write your own loop to zero out the array, or use this memset_volatile:. In some cases, calloc() will do less work because it can skip memset() entirely. for (i = 0; i < N; ++i) buff[i] = init_value; However, apparently you have some misconceptions about how memset works. (P. memset can be memset sets 16 bytes (not bits) to 0. memset interprets target memory as an array of chars, not as an array of ints. If you need to fill a 2-dimensional C-style array with a number: int l[3][3]; std::fill_n(*l, sizeof l / sizeof **l, 1); *l here decays int[3][3] into a pointer to the first element of the array (int*), sizeof l / sizeof **l yields the count of array No, you can't [portably] use memset for that purpose, unless the desired target value is 0. This applies to both memset() and memcpy():. The program’s output is the same, though unlike memset(), bzero() always fills a buffer with null characters. Extraordinary claims require extraordinary evidence. In this article, we will delve into the world of memset and explore its various uses, benefits, and limitations. Locked post. This function is defined in string. Try Teams for free Explore Teams. Another canonical C++ way is to just use vector and let its constructor do the work for you: The things obvious to me are the function definition/declarations and some function calls and a memset call so I get pieces of it but it doesn't all come together for me yet. Code2 works fine. It's basically only good for char arrays or microcontroller code. For standard containers like vector you can How memset() works in c++. The array is not automatically converted to a pointer as in most expressions. memset(&proto,0,sizeof(proto)); *proto. Return Value. But in the end, it's just a loop. 122k 48 48 gold badges 245 245 silver badges 302 302 bronze badges. cpp. New. The second for() loop is present in case the amount of memory to be I came across this question when Googling to see what memset returned. Skip to main content. A multi-byte integer memset(arr, 0, 10); it degrades to a pointer to the first element, i. Hot Network Questions PostgreSQL Daemon Not Working What information can I obtain from power spectrum density (PSD) that I can't obtain from Fourier transform of a signal? malloc() does indeed store information about the size of a block of memory it returns. Whether you’re building high-performance applications or optimizing existing code, understanding memset is crucial for efficient memory management. Security. It doesn't claim any security difference between memset and bzero. 🧠 How the Program Works. Scanf requires you to input all values. Have a look at the documentation of memset: When diving into C++, one of the core aspects every programmer encounters is memory management. How memset() Works. Understanding this requires a short tour of the memory system. memset have to trust the length that is passed in. The memset() function returns str, the pointer to the destination string. 5. It also converts the value of a character to unsigned character and copies it into each of first n character of The one thing to be aware of is that buf might not satisfy the alignment requirements for a uint32_t on your platform. This is C++ so do it the C++ way with fill_n. In this example, the memset() function is used to set all elements in the character array str to the value 'A'. 'not a number'). Use malloc only when it is going Don't use memset to fill ints with values (with an exception for the value 0), because it's not suitable. It is very important to know. 0 float (as You have set each element of the array to be filled with the byte 0xFF (i. The optimization details may differ, but the interface and behavior are the same. In over 40 years as a software developer I've found exactly one bug in The C library memset() function of type void accepts three variable as parameters that copies the character c (an unsigned char) to the first n characters of the string pointed to, by the argument str. ↩️ Return Value. Memset in 2D arrays. Top. For these reasons, memset() is used pervasively in systems memset works on bytes, so it fills your array of ints with 0x01010101 values (assuming int is 32 bits) which is decimal 16843009. p_aliases so you cannot print it as the adresse is NULL. The function uses the memset is a memory utility in C which is used to set a particular value to a range of memory locations in an efficient way. std::fill_n(&sum[0], sizeof(sum) / sizeof(sum[0]), 0); The reason your memcpy didn't work is because, as noted in other answers, you swapped the second and third arguments. About; Products OverflowAI; Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent I'm trying to implement memset in assembly x86. C 2018 6. Assuming that you're talking about the Linux man page, it claims (correctly) that explicit_bzero and memset_explicit and memset_s are more secure (for certain purposes) than memset and bzero. For things that are not bytes, though, the simplest way to initialize all of them is just to explicitly initialize all of them. If you're allocating a big chunk of memory then the C library often maps some anonymous memory which is already zeroed. Don't run tests until you have a working solution. int a[2]; memset(a, 3, sizeof(a)); when I run this I am getting output as 0 1. ; memset is a function that will do something similar for you (including the for loop). While it might "pretend" to be a pointer in some contexts, it is not really a pointer. What's the matter? c++ c In conclusion to stay in defined behaviour you must at least initialize one member, then you can inspect other members of the union who share a common initialization sequence with the initialized member. I doubt that. This return value is often not used in practice. The memset() function sets the first count bytes of dest to the value c. std::memset()converts the val memset is a common way to set a memory region to 0 regardless of the data type. If it matches rule 1 then the action specified in that rule is applied to the I want to memset a 2D array to 0. C++ dynamic array allocation and strange use of memset. Is there a Framework method to fill a byte[] that might be akin to memset; What is the most efficient way to do it when we are dealing with a very large array? I totally agree that using a simple loop works just fine, as Eric and others have pointed out. Maybe that the only difference between Code2 is the location of the memset function. sizeof knows the number of elements in your array and it knows the size of each element. Still, I may write my own loops out of habit. This requires unsafe code. This is once again the correct size in this case. Also the memset function returns a pointer to the memory area s, so if s = &p and p = &myStruct, then it returns a pointer to the memory area of the struct, so a pointer to &myStruct. com/portfoliocourses/c-example-code/blob/main/memset. The behaviour of the function is undefined if: Hi, I'm now crazing with memset() and fill() or fill_n(), in the Problem D Div3 Round #636, i used fill_n() and it was TLE, so I changed to memset() and accepted. Using memset with pointer to an array. both with a copy by byte and by word, so I'd get two functions: kmemset and kmemsetw that I expose to my C code this way: extern uint8_t* kmemset Skip to main content. For small arrays you can An overview of how to use the memset() function in C. Blog; How does the Memstore CDN service work? Elliot posted in: Developer. Here is my implementation : void *ft_memset(void *s, int c, size_t n) { size_t i; i = 0; while (i < n) { *((char *)(s + i)) = c; i++; } return (s); } Why does memset takes void as parameter since it doesn't work with ints, is there more than char that it's used for ? Why casting to char * makes this tl;dr; How to reproduce memset with basic c++? I am trying to figure out how memset works , and to see if I can reproduce it within normal c++ using pointers and address. How would you memset an array of booleans, say bool bArray[11]? MSDN says: "Security Note - Make sure that the destination buffer has enough room for at least count characters. 2. c. The point of the question was to see if I could learn something new about C# :) I think For a non-compound type, you would not use memset at all, because direct assignment would be easier and potentially faster. void memset_volatile(volatile void *s, char c, size_t n) { volatile char *p = s; while (n-- > 0) { *p++ = memset() prototype void* memset( void* dest, int ch, size_t count ); The memset() function takes three arguments: dest, ch and count. In this comprehensive guide, we will explore the memset() function in C++, including its syntax, practical applications, examples, and best practices. (In Code2,memset is in the main) I find Code1 doesn't work at all. In this post, I will show you how to use memset with example :. I would just go with a loop. starkk92 starkk92. 4. This isn't necessary as you could just set them to 0 with data[count]=0. memset() does not work as expected-- memset works correctly. Classes with constructors need those constructors to be called - memset doesn't do that. Note how you treat one character at a time, but those functions are so optimized that set several bytes at a time, even using, when available, MMX and SSE instructions. For pointers, note that in variant 2 the value of the pointer is used, not the pointer It confuses me why void* s = &p; works, even though p itself is a pointer, so s should be a pointer to pointer to void, so void** s instead of void* s. Both char arrays are uninitialized, the use of strlen() on them is undefined behavior. ANSI. Whenever you believe you've found a bug in a routine, especially a very commonly used routine such as memset, it's a very good idea to go back and give a very hard look at both your code and at the assumptions underlying your belief that you've found a bug. When the value of c is 0 (zero), memset std::fill is compiled into a call to memset on pretty much every compiler and standard library implementation, so your claim that std::fill is too slow so you want to use memset instead defies logic, @Sochuu. these calls may return Don't use memset on anything that is not a POD type (which std::pair is not) - it won't go well (it is undefined behaviour to do so). 3 7 says we can convert a pointer to an object (like &x) to a pointer to a character type and use it to address the Now, let’s address the other issue: how does memset work? The memset function takes three arguments: void *s - a pointer to the starting location of the range that will be filled. Check out https:/ This works for all types. In the above diagram memset() takes three parameter say str, chr and n. The reason bzero is deprecated is that it's a trivial wrapper How does memset() Work? Under the hood, memset() operates by directly manipulating the memory at the address specified by ptr. Because that's not what std::memset does. Number of bytes to set Important note: memset works byte by byte, so it The Memset Firewalls work by having a series of rules which are used to examine incoming packets according to one or more of their characteristics and contain an action to perform on any matched packet. No floating point number is represented by a series of 0xFF bytes, so on printing the double, you see NaN (i. . memset () is used to fill a block of memory with a particular value. I think you've misread the man page. size_t n - the number of characters to be stored. Explore Teams. malloc + memset in that means that you're needlessly zeroing it twice. But are you using it correctly? So if you try to compile code with zero stuff in it with everything about it will work, but you may have the problem with some more stuff: You need to define your own memset() and define a _fltused, memeset is memset, the compiler uses it for when you create big arrays and _fltused (float used, didn't know that) when you use float stuff. strlen() walks through a string, given the pointer to its first character, and return the length of the string, that is, until it finds the terminating zero. S my interpretation of the alloca instruction docs is that it anything created from that gets freed on return so I can't use that right, it's essentially only for local variables?) What I've done is: alloc. The memset() function stands as one of C’s most powerful tools for memory manipulation, offering an efficient way to initialize memory blocks. One can say that memset doesn't care about the data type and just sets all bytes to zero. How does memset work? memset takes three arguments: the memory address to be modified, the value to be set, and the number of bytes to be set. I was wondering what is happening inside std::memset? As far as I know, memset gets an address as a void * and assigns each byte of the address (with respect to the size) to the given btye. If you declared it as struct device *dev_sys (a pointer), memset(arr,0,sizeof(arr)) fills arr with sizeof(arr) zeros -- as bytes. The memory which you are passing in memset must be valid otherwise you will get undefined behavior. you'd do So I have a 2D Vector here that I want to assign a value num, I want to see which performs better fill vs memset() as C++ noob, I am actually having problem setting the proper code syntax as I just I personally don't recommend memset for general initialization. It is important to note that the firewall rules are applied in the order they Introduction : memset() is an important function in C. It sets each of the first num bytes of How memset() works in c++. The character represented by ch is first converted to unsigned char and then copies it into the first count characters of the object pointed to by dest. Minor pedantry that in both C and C++, even with POD types it's technically undefined behaviour to just go around using memset to zero memory, and then read that memory back as anything other than char types and guaranteed-no-padding-bits (u)intN_t types. I was working on a custom piece of compression code and was initializing some large structures at declaration time using struct something foo = { x, y, z } and cachegrind showed that 70% of my program's "work" was in memset because the structs were zeroed on EVERY function call. One would think that it has at Does C++ memset only works for 0 and -1? It does work for all byte values. Hot Network Questions Where does one learn about the weather? Identifying data frame rows in R with specific pairs of values in two columns Can I make soil blocks in batches and keep them empty until I need them? I know that memset is frowned upon for class initialization. Since they both point to the same place it When you pass arrays to functions in C, they decay to a pointer to the first element in the array, even if you specify an array type in the function prototype (a). memset done on a As I have mentioned in the above program, the memset function works on byte. In this comprehensive Memory initialization is a critical aspect of C programming that can make or break your application’s performance and reliability. However, memset() does not know or care about the size. Each rule is ordered in the Firewall Editor and that position represents its place in the order of rules. The result is an initialized string. sizeof(arr) is correct in this case, but beware using this approach on pointers rather than arrays. If you want to initialize a long long array with a particular value, just use std::fill or std::fill_n and let your library and compiler optimize it as they can (partial loop unrolling etc). For arrays, variant 2 works, because an array is implictily converted to a pointer for most operations. If you wanted something that did anything remotely to what you're trying to accomplish, there is std::fill – Please allow me to offer some advice. Example The memset statment below is setting all the memory allocated by proto to 0. The memset() function returns a pointer to dest. A fairly popular hack for filling a memory region with a repetitive pattern is actually based on memcpy. Also, the memset() function in C is used to set memory locations to a 1. memset, as the other say, sets every byte of the array at the specified value. Although it might seem simple on the surface, memset() is a powerful tool that can significantly optimize your programs, especially in terms of performance and memory handling. On going through this article, you will understand How to use memset? and Why performance of memset is better Memset() fills sequential bytes of memory with a value, commonly zero. I have some code where I test for one value, then if that is true test to see if a value is zeros. Example Usage of @robUK: dev_sys is not a pointer, dev_sys is an array. New comments cannot be posted. Controversial . asked Jun 24, 2016 at 16:17. memset() does some neat little optimizations to write multiple bytes per iteration, so you might look at how memset() itself is working and see if those kinds of optimizations apply to your code. Open comment sort options. It is a powerful tool that can be used to initialize arrays, set the value of variables, and even clear memory. Classes with non-POD data members needs the constructors of those members called - memset does not do that. The whole memset may get optimized away, or undefined behaviour badness happens. If you anyway would need to check for a 0 byte. For topics related to information visualization and the design of graphs, charts, maps, etc. memset(arr,0,10*sizeof(int)) fills arr with 10*sizeof(int) zeros, again as bytes. Threadsafe. The memsetfunction is useful for initializing blocks of memory. There are too many things you have to be sure of to be able to correctly and portably use it. h" #include<stdlib. Nothing to do with memset(), sizeof(arr) gives you the size of the pointer memset() prototype void* memset( void* dest, int ch, size_t count ); The memset() function takes three arguments: dest, ch and count. p_aliases is pointing to 0x000000 then you're tring to dereference it which leads to segmentation fault. Initializing an integer array with loop works nice, but initializing it with memset function works wrong. Description. These are not the same thing. , a char*. This is correct because the size of your array is 16 bytes, as you correctly stated (4 integers x 4 bytes per integer). The C++ Courseexplains how to use memseteffectively, ensuring you can manage memory initialization in your programs. The syntax of memset () function is as follows : Note that ptr is a void pointer, so that we can pass any type of pointer to this function. An essential function within this domain is memset(). Your memset(&array1,1,sizeof(array1)); will not fill the array with 1s, meaning that your code is not supposed to print 1 regardless of which array you print. memset() In C Purpose Of memset() memset() is one of the inbuilt string function in c programming which is used to replace first n characters of a string str with a character chr. The behaviour of the function is undefined if: Memset() works consistently across platforms and compilers. Stack Overflow. Cvelth • The go-to answer is: you can always just measure it, you know. calloc is guaranteed to return zeroed memory and the implementation probably were optimized to not rezero the memory thus allocated. After you create these arrays with new, they don't contain anything meaningful. For example, something like the following: class X { public: X() { memset( this, 0, sizeof(*this) ) ; } } ; will clobber the vtbl if there's a virtual function in the mix. I have the below program:(functionality: pads white spaces to the right of string,used astreix here for visual ease): os:windows(visual studio) #include "stdafx. The way to initialize an array in C is the following (assume size N):. And different CPU architectures have customized assembly Learn how to efficiently set memory blocks in C using memset (). Pointer to the memory block 2. It critically relies on the expectation that memcpy copies data in forward direction How fast do memset works than a for loop to set an array to zero? Question Archived post. It sets each of the first num bytes of the memory area to the specified value. memset treats the target memory region as an array of bytes, not an array of ints. In this post I will describe how the Content Delivery Network (CDN) functionality provided by Memstore cloud storage is implemented. 5,914 9 9 gold badges 46 46 silver badges 63 63 bronze badges. Properly erasing data with memset() helps prevent sensitive information lingering in memory and getting exposed. For example, if you expect to take a pointer to the first element of one of the inner arrays-of-2-int, and increment it to get a pointer to the second, then std::pair is out because it doesn't guarantee that works. The "correct" (idiomatic) call is: memset(arr, 0, 10); However, in this case they are both converted to a void* when passed to memset and interpreted in the function as an unsigned char*. In most cases, they will be the same. memset(&multi_dimension_array[2], 0, sizeof multi_dimension_array[2]); This requires that multi_dimension_array[i] be an array of arrays of arrays, not a pointer. Common practice is to pass the array size along with the array (probably as a size_t) and use that to weave your magic, something like: A few points to help you get started: You're trying to set all items to 0. In C programming, memset is a built-in function that allows you to set the value of a memory location to a specified value. h file and it is used to set a block of memory programmatically. It sets every byte to the value that you provide. Share Sort by: Best. The reason this works with 0 and -1 is because both use the same repeating pattern on arbitrary sizes: (int) -1 is 0xffffffff (char) -1 is 0xff so filling a memory region with 0xff will effectively fill the array with -1. " But when I commented the memset part and uncommented the two for assignment loops, everything suddenly changed and suddenly my program works correctly. Improve this question. – memset is a common way to set a memory region to 0 regardless of the data type. (so it may be faster) Misalignment: In some The Memset Firewalls work by having a series of rules which are used to examine incoming packets according to one or more of their characteristics and contain an action to perform on any matched packet. the char representation of -1). It will not do what you expect. Use memset for 2d array at specific row. Q&A. This is more fragile than the first. I'm trying to understand how memset works. I find memset() and bzero() to be delightful options for initializing a buffer. h header file in our program. Always use calloc() instead of malloc()+memset(). I'm working on a (humongous) legacy codebase that is C-ish but compiled in C++, so all the members in question are typically POD and require no The compiler will replace small memcpy()s and memset()s with more suitable code so it is not as horrible is it looks; but if you do know enough to guarantee assignment will always work and your profiler tells you it is faster, you can replace the memcpy with an assignment. e. It is up to the programmer to ensure that only valid memory is written to. It provides a fast, simple way to initialize data structures and erase data. Hence the sizeof will then give you the size of a pointer, not the size of the array. (memset() is more readable than that loop)It can be faster: It can sometimes allow more aggressive compiler optimizations. A multi-dimensional array is just a large block of memory, so we can treat it like one, similar to how memset() works. This is a fun exercise, though, so here are some benchmarks using BenchmarkDotNet: public class ArrayFillBenchmark { const int length1 = 96K subscribers in the visualization community. You will get this question in most C programming interviews. int c - the value to be stored into each character (byte) of the range. What if arr does not contain 10 elements, and what if the Introduction. Learn How memset() works in c++. Follow edited Jun 24, 2016 at 16:20. Can you explain me ? Thank you #include <string. memset works on a byte-by-byte basis only. 1. Best. Zeroing out the bits works in general because all integral zeros are generally all-zero-bits, so that grouping four all-zero-bit bytes into one all-zero-bits int still gives you zero. It works because, when sizeof is applied to an array, it returns the size of the array. I wouldn't say it's worth doing unless it's really performance critical. This is in apparent contrast to memset'ting the bytes to zero, which is legal as a string of 0 bytes is a double with value zero. How to write your How does the Memstore CDN service work? 21 Mar 2012. I dont't understand the way memset work. So proto.