快速排序：

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#define LENGTH(x) sizeof(x)/sizeof(x[0])

/**輸出陣列元素
*\param arr:指向陣列的指標
*\param len:陣列元素的個數
*/
void print(char (*arr)[10],int len)
{
    int i;
    for (i=0;i<len;i++)
    {
        printf("%s ",arr[i]);
    }
    printf("\n");
}

int main()
{
    char arr[][10]={"bac","bca","abc","acb","cba","cab"}; /* 定義二維字元陣列*/
    char *key="bca";/* 要查詢的字串*/
    char *ptr=NULL; /* 字元指標*/
	// 輸出未排序時字元陣列的內容
    printf("before qsort :");
    print(arr,LENGTH(arr));
    /* 使用qsort對字元陣列排序*/
    qsort((void *)arr,LENGTH(arr),sizeof(arr[0]),(int (*)(const void *,const void *))strcmp);
    /* 輸出排序後字元陣列的內容*/
    printf("after qsort :");
    print(arr,LENGTH(arr));
    /* 採用二分查詢查詢指定字元*/
    ptr=(char *)bsearch(key,arr,LENGTH(arr),sizeof(arr[0]),(int (*)(const void *,const void *))strcmp);
    if(ptr)
    {
        /* 找到*/
        printf("%s is in the array\n",key);
    }
    else/* 沒找到*/
    {
        printf("%s isn't in the array\n",key);
    }
    return 0;
}
 

#include<stdlib.h>
#include<stdio.h>
#define ArrayLen(arr) (sizeof(arr) / sizeof(arr[0]))

int numarray[] = {123, 145, 512, 627, 800, 933};

int numeric (const int *p1, const int *p2)
{
    return(*p1 - *p2);
}
int* lookup(int key)
{//返回值是指向key的地址 
    int *itemptr;
    // The cast of (int(*)(const void *,const void*)) is needed to avoid a type mismatch error at
	// compile time
    itemptr = (int *)bsearch(&key, numarray, ArrayLen(numarray),	\
                             sizeof(int), (int(*)(const void *,const void *))numeric);
    return (itemptr);
}
int main(void)
{
	int *p = lookup(512);
	if(NULL != p)
		printf("找到的key是%d，", *p);
	
	printf("key的下標是%d\n", (p - numarray));
		
    return 0;
}