memchr函数实现——string.h库函数

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函数实现：

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信息来自RHEL，man page:
MEMCHR(3)                                      Linux Programmer's Manual                                     MEMCHR(3)

NAME
       memchr, memrchr, rawmemchr - scan memory for a character

SYNOPSIS
       #include <string.h>

       void *memchr(const void *s, int c, size_t n);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       memrchr(), rawmemchr(): _GNU_SOURCE

DESCRIPTION
       The memchr() function scans the initial n bytes of the memory area pointed to by s for the first instance of c.
       Both c and the bytes of the memory area pointed to by s are interpreted as unsigned char.

       The memrchr() function is like the memchr() function, except that it searches backward from the end  of  the  n bytes pointed to by s instead of forward from the beginning.

 The  rawmemchr()  function  is similar to memchr(): it assumes (i.e., the programmer knows for certain) that an
 instance of c lies somewhere in the memory area starting at the location pointed to by s, and  so  performs  an
 optimized  search for c (i.e., no use of a count argument to limit the range of the search).  If an instance of c is not found, the results are unpredictable.  The following call is a fast means of locating a string's  ter‐minating null byte:

           char *p = rawmemchr(s, '\0');

RETURN VALUE
       The  memchr()  and  memrchr() functions return a pointer to the matching byte or NULL if the character does not occur in the given memory area.

       The rawmemchr() function returns a pointer to the matching byte, if one is  found.   If  no  matching  byte  is found, the result is unspecified.

memchr()函数实现：

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各功能类比strchr函数：传送门：http://blog.youkuaiyun.com/riyadh_linux/article/details/50222053

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0.函数原型：

#include <string.h>

void *memchr(const void *s, int c, size_t n);

1.参数：

1.s：指定查询内存。
2.c：欲查询字符’c’。
3.n：指定内存前n个字节。

2.返回值：

如果指针s指向的内存前n个字节中，按字节分割:

->可以和'c'字符匹配，那么返指向回字符第一次匹配成功位置的指针。
->如果没有匹配，返回NULL。

3.功能描述：

memchr函数在s指定内存空间前n个字节中查找是否有可以和字符’c’匹配的字节，并返回该字节第一次出现位置的指针。没找到则返回NULL。

4.实现：

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void *my_memchr(const void *s, int c, size_t n)
{
    char *s_func = (char *)s;

    //参数判断
    if(NULL == s_func || 0 >= n){
        return NULL;
    }

    //具体实现
    while(n-- && *s_func && *s_func != c){
        ++s_func;
    }

    return (s_func ? s_func : NULL);
}

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