[Original] A knowledge point caused by memset

[Original] A knowledge point caused by memset

 

When debugging a base Sorting Problem, I want to assign an initial value to the int * that was initially allocated, similar to zeromemory on windows, but I found that things are not that simple. Example: # include <iostream> # include <cstring> using namespace STD; int main (){ Int * C = new int [10]; Memset (C, 0x0, 10 ); For (INT I = 0; I <10; I ++) cout <C [I] <"->"; cout <"\ n "; return 0 ;} Output: 0-> 0-> 1193541632-> 1397052160-> 1313818963-> 1162690894-> 1852785469-> 1701605235-> 1937330944-> 1148020084-> 1. Memset (C, 0x0, 10 ); => Memset (C, 0x0, 10 * sizeof (INT) ); Output: 0-> 0-> 0-> 0-> 0-> 0-> 0-> 0-> 0-> 0-> 2. memset (C, 0x0 , 10); => memset (C, 0x1 , 10); output: 16843009-> 16843009 -> 1193541889-> 1397052160-> 1313818963-> 1162690894-> 1852785469-> 1701605235-> 1937330944-> 1148020084-> 3. memset (C, 0x0, 10); => memset (C, 0x1, 10 * sizeof (INT) ); Output: 16843009-> 16843009-> 16843009-> 16843009-> 16843009-> 16843009-> 16843009-> 16843009-> 16843009-> reason (http://blog.ednchina.com/fpga2006/16745/message.aspx ): memset prototype: _ cribd void * _ cdecl _ mingw_nothrow memset (void *, Int, size_t); memset (A, 1, 20 ); it is to assign a value to the 20 bytes of memory that a points to. Each character is filled with the ASCII value 1. After being converted to binary, 1 is 00000001, which occupies one byte. An int element is 4 bytes, starting with 1000000010000000100000001, which is equal to 16843009, The assignment of an int element is completed. Another article Article The general usage of http://www.cppblog.com/SHFACM/archive/2009/02/04/73004.aspx memset can only be used to initialize an array of the char type, that is, it only accepts the value of 0x00-0xff, however, in most cases, it is incorrect to assign a relatively large or small value to an array of double or Int values: memset (ARR, 2147483647, sizeof (ARR )); however, you can use some tips to get a similar maximum value, such as memset (ARR, 0x7f, sizeof (ARR). It assigns all values in arr to 2139062143 (note: here, the value of ARR [0] of Int Is 0x7f7f7f7f = 2139062143. You may try it.) This is the maximum value that can be achieved by assigning values to int using memset: memset (ARR, 0x80, sizeof (ARR); // set int to-2139062144 memset (ARR, 0x7f, sizeof (ARR )); // set double to 1.38242e + 306 memset (ARR, 0xfe, sizeof (ARR); // set double to-5.31401e + 303