malloc分配的空间在之后的使用中未完全使用完，如何解决？最后四行代码是什么意思？

char * addBinary(char * a, char * b){
    int  i,k,j,len;
    int  c;
    c=0;
    j=strlen(a);
    k=strlen(b);
    len=j>k?j:k;
    char *res=(char *)malloc(sizeof(char)*(len+2));
    res[len+1]='\0';
    i=len-1;
    while(i>=0)
    {
        if(j-1>=0&&k-1>=0){
        if(a[j-1]-'0'+b[k-1]-'0'+c==2)
        {
            res[i]='0';
            c=1;
        }else
        {
            res[i]=a[j-1]-'0'+b[k-1]-'0'+c;
            c=0;
        }}
        if(j-1<0)
        {
            if(b[k-1]-'0'+c==2)
            {
                res[i]='0';
                c=1;
            }else{
                res[i]=b[k-1]-'0'+c;
                c=0;
            }
        }
        if(k-1<0)
        {
            if(a[j-1]-'0'+c==2)
            {
                res[i]='0';
                c=1;
            }
            else{
                res[i]=a[j-1]-'0'+c;
                c=0;
            }
        }
        i--,j--,k--;
    } 
    if(c>0)
        res[0]='1';
    else 
        *res++;
    return res;
}

大佬要是有时间顺便解决一下为什么输出会有"10�"这个问号

[此贴子已经被作者于2019-12-8 17:12编辑过]

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


char* addBinary(char* a, char* b) {
    int  i, k, j, len;
    int  c;
    c = 0;
    j = strlen(a);
    k = strlen(b);
    len = j > k ? j : k;
    char* res = (char*)malloc(sizeof(char) * (len + 2));
    res[len + 1] = '\0';
    i = len - 1;
    while (i >= 0)
    {
        if (j - 1 >= 0 && k - 1 >= 0) {
            if (a[j - 1] - '0' + b[k - 1] - '0' + c == 2)
            {
                res[i] = '0';
                c = 1;
            }
            else
            {
                res[i] = a[j - 1] - '0' + b[k - 1] - '0' + c;
                c = 0;
            }
        }
        if (j - 1 < 0)
        {
            if (b[k - 1] - '0' + c == 2)
            {
                res[i] = '0';
                c = 1;
            }
            else {
                res[i] = b[k - 1] - '0' + c;
                c = 0;
            }
        }
        if (k - 1 < 0)
        {
            if (a[j - 1] - '0' + c == 2)
            {
                res[i] = '0';
                c = 1;
            }
            else {
                res[i] = a[j - 1] - '0' + c;
                c = 0;
            }
        }
        i--, j--, k--;
    }
    if (c > 0)
        res[0] = '1';
    else
        (*res)++;
    return res;
}

int main()
{
    char a[10] = "110";
    char b[10] = "1001";
    char* res;
    int i = 0;
    printf("%s",addBinary(a, b));
}

完整版代码

carry += *--pa - '0';
大佬能解释一下是怎么实现的吗？
 if (c > 0)
        res[0] = '1';
    else
        (*res)++;
还有这是什么意思？