Bscis Knowledge(Function)

extern unsigned int strlen(char *s)

计算字符串s的（unsigned int）长度，不包括(‘\0’)

int strlen(const char *str)
{
    assert(str!=nullptr);
    int len=0;
    while((*str++) != '\0') len++;
    return len;
}

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void *memest(void *s, int ch, size_t count)

将s中前n个字节 （typedef unsigned int size_t）用 ch 替换并返回 s .作用是在一段内存块中填充某个给定的值，它是对较大的结构体或数组进行清零操作的一种最快方法

void *memest(void *src, int ch, size_t count)
{
    assert(str!=nullptr);
    char *tmpsrc = (char*)src;
    while(count--) *tmpsrc++ = (char)ch;
    return src;
}

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void *memcpy(void *dst, const void *src, size_t n)

从源src所指的内存地址其实位置开始拷贝n个字节到目标dest所指的内存地址的起始位置中

void *memcpy(void *dst, const void *src, size_t num)
{
    assert((dst!=nullptr) && (scr!=nullptr));
    assert(des >= scr+num || scr > dst+num);//内存重叠
    byte *psrc = (byte*)src;//byte即为unsigned char类型
    byte *pdst = (byte*)dst;
    while(num-- > 0) *pdst++ = *psrc++;//先取值，再指向下一位
    return dst;
}

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void memmove(void dest, const void* src, size_t count)

memmove用于从src拷贝count个字符到dest，如果目标区域和源区域有重叠的话，memmove能够保证源串在被覆盖之前将重叠区域的字节拷贝到目标区域中。但复制后src内容会被更改。但是当目标区域与源区域没有重叠则和memcpy函数功能相同

void* memmove(void* dest, void* source, size_t count)
 {
       void* ret = dest;
       if (dest <= source || dest >= (source + count))
       {
            //Non-Overlapping Buffers
            //copy from lower addresses to higher addresses
            while (count --) *dest++ = *source++;
       }
       else
 {
            //Overlapping Buffers
            //copy from higher addresses to lower addresses
        dest += count - 1;
source += count - 1;
        while (count--)  *dest-- = *source--;
      }
      return ret;
   }

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char *strstr(char *str1, const char *str2)

str1: 被查找目标　string expression to search.
str2: 要查找对象　The string expression to find.
返回值：若str2是str1的子串，则返回str2在str1的首次出现的地址；如果str2不是str1的子串，则返回NULL。

char str[]="1234xyz";
char *str1=strstr(str,"34");
cout << str1 << endl; //34xyz