strcmp () strcat() strcpy()源码分析学习

这三个库函数都熟悉用法，但是源码还没有读过，网上找了源码学习一下

来源：https://blog.csdn.net/wangningyu/article/details/4662891

/************************************************************************

*char *strcpy(dst, src) - copy one string over another

*

*Purpose:

* Copies the string src into the spot specified by

* dest; assumes enough room.

*

*Entry:

* char * dst - string over which "src" is to be copied

* const char * src - string to be copied over "dst"

*

*Exit:

* The address of "dst"

*

*Exceptions:
************************************************************************/
char* strcpy(char * dst, const char * src)
{
    char * cp = dst;
    while( *cp++ = *src++ )
        ;                            /* Copy src over dst */
    return( dst );
}


/************************************************************************

*char *strcat(dst, src) - concatenate (append) one string to another

*

*Purpose:

* Concatenates src onto the end of dest. Assumes enough

* space in dest.

*

*Entry:

* char *dst - string to which "src" is to be appended

* const char *src - string to be appended to the end of "dst"

*

*Exit:

* The address of "dst"

*

*Exceptions:

*
************************************************************************/
char* strcat ( char * dst , const char * src )
{
    char * cp = dst;
    while( *cp )
        cp++;                        /* find end of dst */
    while( *cp++ = *src++ ) ;        /* Copy src to end of dst */
    return( dst );                    /* return dst */
}


/************************************************************************

*strcmp - compare two strings, returning less than, equal to, or greater than

*

*Purpose:

* STRCMP compares two strings and returns an integer

* to indicate whether the first is less than the second, the two are

* equal, or whether the first is greater than the second.

*

* Comparison is done byte by byte on an UNSIGNED basis, which is to

* say that Null (0) is less than any other character (1-255).

*

*Entry:

* const char * src - string for left-hand side of comparison

* const char * dst - string for right-hand side of comparison

*

*Exit:

* returns -1 if src < dst

* returns 0 if src == dst

* returns +1 if src > dst

*

*Exceptions:

*
************************************************************************/
int strcmp ( const char* src, const char* dst )
{
    int ret = 0 ;
    while( !(ret = *(unsigned char *)src - *(unsigned char *)dst) && *dst)
        ++src, ++dst;
    if ( ret < 0 )
        ret = -1 ;
    else if ( ret > 0 )
        ret = 1 ;
    return( ret );
}

1. strcmp

int strcmp ( const char* src, const char* dst )
{
    int ret = 0 ;
    while( !(ret = *(unsigned char *)src - *(unsigned char *)dst) && *dst)
        ++src, ++dst;
    if ( ret < 0 )
        ret = -1 ;
    else if ( ret > 0 )
        ret = 1 ;
    return( ret );
}

我们要看的是while循环这个语句， ! (ret = *(unsigned char *)src - *(unsigned char *)dst)意思是拿指针变量src所指向的字符值（即*src）减去指针变量dst所指向的字符值（即*dst），差值赋给ret，再取非运算，最后与*dst进行与运算；

! ret表示ret=0才继续比较。

为什么要把src转成unsigned char *类型？

  使用*（unsigned char *）str1而不是用*str1。这是因为传入的参数为有符号数，有符号字符值的范围是-128-127，无符号字符值的范围是0-255，而字符串的ASCII没有负值，若不转化为无符号数，在减法实现时出现错误。

例如：str的值为1，str2的值为255。

作为无符号数计算时ret=-254，结果为负值，正确。

作为有符号数计算时ret=-2，结果为正值，错误。

while循环中（ret=*（unsigned char *）str1 - *（unsigned char *）str2）&& *str1，最后的str1也可以换成str2，因为前面已经做了相减，无论哪个先为''都会退出。

这个函数没有判断参数为NULL时的情况，所以当传入NULL时程序会崩溃。

2. strcat

char* strcat ( char * dst , const char * src )
{
    char * cp = dst;
    while( *cp )
        cp++;                        /* find end of dst */
    while( *cp++ = *src++ ) ;        /* Copy src to end of dst */
    return( dst );                    /* return dst */
}

 while( *cp )
        cp++;
不能写成while(*cp++), 因为这样的话为*cp为0时还会+1，导致cp指向了的后一个。

为什么直接拷贝到strDset中，还要返回char*型呢？为了实现链表表达式
如int length = strlen( strcpy( strDest, “hello world”) );


3.strcpy

char* strcpy(char * dst, const char * src)
{
    char * cp = dst;
    while( *cp++ = *src++ )
        ;                            /* Copy src over dst */
    return( dst );
}

还有另一个strncpy，加上了长度控制count

char * __cdecl strncpy ( char * dest, constchar * source, size_t count )

{

    char *start = dest;

    while (count && (*dest++ = *source++)) /* copy string */   

        count--;

    if (count) /* pad out with zeroes */

    while (--count)

        *dest++ ='';

    return(start);
}