char *S(char *b)
{
    char *s;
    return s;
}

這個s是傳不回去的，因為在函式體裡面定義，是區域性變數。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char *addBinary(char * a, char * b) {
    int lena,lenb,len=0,num_a,num_b,i,j,jin=0;
    int s[100];
    char ss[100];
    lena=strlen(a);
    lenb=strlen(b);
    i=lena-1;
    j=lenb-1;
//printf("a%d b%d\n",lena,lenb);
    while(i!=-1&&j!=-1)
    {
        num_a=a[i]-'0';
        num_b=b[j]-'0';
        s[len]=(num_a+num_b+jin)%2;
        jin=(num_a+num_b+jin)/2;
        len++;
        i--;
        j--;
    }

    while(i!=-1)
    {
        num_a=a[i]-'0';
        s[len]=(num_a+jin)%2;
        jin=(num_a+jin)/2;
        len++;
        i--;
    }
    while(j!=-1)
    {
        num_b=b[j]-'0';
        s[len]=(num_b+jin)%2;
        jin=(num_b+jin)/2;
        len++;
        j--;
    }
    if(jin==0)
    {
        for(i=len-1;i>=0;i--)
        ss[len-i-1]=s[i]+'0';
        ss[len]='\0';
    }
    else
    {
        for(i=len-1;i>=0;i--)
        ss[len-i]=s[i]+'0';
        ss[0]='1';
        ss[++len]='\0';
    }
    a=ss;
    puts(a);
    return a;
}
int main()
{
    char a[]= "11", b[] = "1";
   // printf("a%d  b%d\n",a,b);
    //char *ss;
    printf("%s\n",a);
    return 0;
}