memset | 易学教程

memset in parallel with threads bound to each physical core

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问题 I have been testing the code at In an OpenMP parallel code, would there be any benefit for memset to be run in parallel? and I'm observing something unexpected. My system is a single socket Xeon E5-1620 which is an Ivy Bridge processor with 4 physical cores and eight hyper-threads. I'm using Ubuntu 14.04 LTS, Linux Kernel 3.13, GCC 4.9.0, and EGLIBC 2.19. I compile with gcc -fopenmp -O3 mem.c When I run the code in the link it defaults to eight threads and gives Touch: 11830.448 MB/s Rewrite:

Is there a memset-like function which can set integer value in visual studio?

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问题 1, It is a pity that memset(void* dst, int value, size_t size) fools a lot of people when they first use this function! 2nd parameter "int value" should be "uchar value" to describe the real operation inside. Don't misunderstand me, I am asking a memset-like function! 2, I know there are some c++ candy function like std::fill_n(my_array, array_length, constant_value); even a pure c function in OS X: memset_pattern4(grid, &pattern, sizeof grid); mentioned in a perfect thread Why is memset()

Why calloc wasn't intended to assign arbitrary values?

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问题 As per Why malloc+memset is slower than calloc? malloc + memset is slower than calloc under certain conditions. Why wasn't calloc written in such a way that it can take an extra value argument ( like memset ) to override default assignment by zero? What would have been the effect of that if it were done? 回答1: These calloc or memset initializations operate on a byte level, so even memset with a value different from 0 is not that usefull. At least I don't remember having it used with different

使用memset()要注意

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原型如下： ptr是要写入的内存块的指针，value是要写入的值，num是从ptr指向的首地址开始一共要写入的字节数。要注意num传入的参数错误示范：实际上sizeof(InDegree)是4，等于sizeof(int)。也就是说这里的num参数只是一个指针的大小，没有完成初始化工作，除了第一个数组元素外，剩下的都是随机值。应该改成：来源： https://www.cnblogs.com/mrlonely2018/p/12020171.html

How do I do a memset for a pointer to an array?

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问题 How do I do a memset for a pointer to an array? int (*p)[2]; p=(int(*))malloc(sizeof(*p)*100); memset(p,0,sizeof(*p)*100); Is this allocation an correct? 回答1: you can use calloc . calloc will replace both malloc and memset . p = calloc(100, sizeof (*p)); 回答2: I'll summarize a lot of answers (although I've ignored some of the stylistic variants in favor of my own preferences). In C: How to use malloc: int (*p)[2] = malloc(100 * sizeof(*p)); How to use memset: memset(p, 0, 100 * sizeof(*p));

Glib memory allocation error

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问题 I am using a library libfprint on ubuntu nd I am trying to call a function through my java code. API_EXPORTED struct fp_img *fpi_img_new(size_t length) { struct fp_img *img = g_malloc(sizeof(*img) + length); memset(img, 0, sizeof(*img)); fp_dbg("length=%zd", length); img->length = length; return img; } I am passing integer value 5 from my java code to this function. When I try to execute above function I got following errors: GLib-ERROR **: /build/buildd/glib2.0-2.30.0/./glib/gmem.c:170:

What is the fastest way to zero an existing array?

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问题 I have an existing 1D array, is memset the fastest way to zero it? 回答1: Fastest ... probably yes. Buggy almost sure! It mostly depends on the implementation, platform and ... what type the array contains. In C++ when a variable is defined its constructor is called. When an array is defined, all the array's elements' constructors are called. Wiping out the memory can be considered "good" only for the cases when the array type is know to have an initial state that can be represented by all zero

memset not filling array

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问题 u32 iterations = 5; u32* ecx = (u32*)malloc(sizeof(u32) * iterations); memset(ecx, 0xBAADF00D, sizeof(u32) * iterations); printf("%.8X\n", ecx[0]); ecx[0] = 0xBAADF00D; printf("%.8X\n", ecx[0]); free(ecx); Very simply put, why is my output the following? 0D0D0D0D BAADF00D ps: u32 is a simple typedef of unsigned int edit: Compiling with gcc 4.3.4 string.h is included 回答1: The second parameter to memset is typed as an int but is really an unsigned char. 0xBAADF00D converted to an unsigned char

【网络流24题】最长k可重区间集问题

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这是一道我一开始没想出来的题。题面 https://www.luogu.org/problemnew/show/P3358 题解离散化。图的基础是一条长链，流量为$k$，区间是从$l$指向$r$的边，流量为$1$，费用为长度。 #include<cstdio> #include<iostream> #include<cstring> #include<algorithm> #include<queue> #include<vector> #define ri register int #define N 5000 #define S 0 #define INF 1000000007 using namespace std; int n,k,T; int l[550],r[550],v[550],dc[1450]; int read() { int ret=0,f=0; char ch=getchar(); while (ch>'9' || ch<'0') f|=(ch=='-'),ch=getchar(); while (ch>='0' && ch<='9') ret*=10,ret+=(ch-'0'),ch=getchar(); return f?-ret:ret; } struct graph { vector<int> to,w,c; vector<int> ed[N]

Is it guaranteed that memset will zero out the padding bits in a structure?

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问题 In general ,as per C standard is it guaranteed that memset() with 0 will zero out the padding bits in a C structure? What about gcc? For example , something like: struct MyStruct { unsigned char member1; unsigned int member2; char member3; unsigned char member4; float member5; }; struct MyStruct ms; memset(&ms, 0, sizeof( struct MyStruct)); 回答1: Perhaps worth noting that memset doesn't know anything about your struct (or array, or primitive, or any chunk of memory whatsoever that you happen