Detailed explanation of memset() function

1. Header file

 <memory.h> or <string.h>    in C

     <cstring> in C++

2. Prototype and function

void *memset(void *s,int c,size_t n)

    where s is a pointer or array, c is the value assigned to s, and n is the length of s to be modified, that is, the first n bytes of s.

    Function: Set the value of the first n bytes of the allocated memory space s to the value c.

3. Commonly used

    It is often used to clear a variable or array of a structure type, such as clearing a structure:

struct sample_struct
{
char csName[16];
int iSeq;
int iType;
};

// for variables:
struct sample_strcut stTest;

//In general, the method of clearing stTest:
stTest.csName[0]='/0';
stTest.iSeq=0;
stTest.iType = 0;

// with memset:
memset(&stTest,0,sizeof(struct sample_struct));

//if it is an array:
struct sample_struct TEST[10];

memset(TEST,0,sizeof(struct sample_struct)*10);

   It is also commonly used to initialize memory after malloc applies for a piece of memory:

double* wid = (double*)malloc((num+1) *sizeof(double));
if (wid == NULL)  
{  
    cout << "Fail to allocate memory to wid" << endl;  
    exit(1);  
}  
memset(wid, 0, (num + 1) *sizeof(double));//initialize to 0

 4. Pay attention

    The function of memset is to copy the numbers into the specified memory in a single byte-by-byte copy .

    (1) If ptr points to a char address, value can be any character value;

    (2) If ptr points to a non-char type, such as an int type address, if the assignment is to be correct, the value of value can only be -1 or 0, because each bit of -1 and 0 is the same after being converted into binary, set int The type occupies 4 bytes, then -1=0XFFFFFFFF, 0=0X00000000.

Reference: Explanation of the assignment process

   Detailed example