#include <stdio.h>
#include <limits.h>

unsigned character_offset( unsigned bit_number )
{

    int c1=bit_number / CHAR_BIT;
    return c1;                       //计算bit_number第几个字节内
}

unsigned bit_offset( unsigned bit_number )
{
    int j=bit_number % CHAR_BIT;
    return j;                       //计算bit_number 在最高字节里是第几位
}

void set_bit( char bit_array[], unsigned bit_number )    //把指定的位设置为 1
{
    printf("char %c\n",bit_array[character_offset(bit_number)]);
    printf("int  %d\n",bit_array[character_offset(bit_number)]);

    bit_array[ character_offset(bit_number) ] |=
    ( 1 << bit_offset(bit_number) );

    printf("char %c\n",bit_array[character_offset(bit_number)]);
    printf("int  %d\n",bit_array[character_offset(bit_number)]);
}

int main()
{
    char a[20] = "10001100001";
    printf("int       a[0]=%d\nchar      a[0]=%c\n",a[0],a[0]);
    printf("CHAR_BIT: %d\n",CHAR_BIT);
    set_bit(a, 10);
    printf( "%s\n",a );

    return 0;
}

#include <stdio.h>
#define BIT 1

void set_bit(char *p,int i)
{
    int j = 0;
    for(j;j < 10;j++)
    {
        if(i == j)
            *(p+i) |= BIT;
        printf("%c",*(p+j));
    }
    printf("\n");
}

int main()
{
    char a[20] = "1000100001";
    //char *p;
    int i;
    //p = a;
    scanf("%d",&i);//输入要想改变的那位
    set_bit(a,i);
   
    return 0;
}

看不出是做什么的。
数组直接元素下标操作就可以了吧，a[i]='1'

改动一下上面的代码  上面的代码是c和指针作者就是没有主函数，主函数是自己写的  

是不是这意思：
    char a[20] = "1000100001";
    char *p=a;
    int i=1;    //要想改变的那位
    p[i] = '1';
    printf("%.10s\n", p); //显示 1100100001