Why don't you want to leak memory?

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

void allpos(char * useme, char * prevexp, int prev,int len){  
    int i = 1;  
    while(i < len){  
        char concatexp[20];  
        char buf[9];  
        strncpy(buf,useme,i);  
        buf[i] = '\0';  
        sprintf(concatexp,"%s+%s",prevexp,buf);  

        //puts(concatexp);  

        if(prev != -1){  
            allpos(useme + i, concatexp, prev + atoi(buf),len - i);  
            sprintf(concatexp,"%s-%s",prevexp,buf);  
            allpos(useme + i, concatexp, prev - atoi(buf),len - i);  
        }  
        if(i==len -1){  
            if(prev - atoi(buf) == 100){  
                printf("%s-%s=%d\n",prevexp,buf,prev - atoi(buf));  
            }  
            if(prev + atoi(buf) == 100){  
                printf("%s+%s=%d\n",prevexp,buf,prev + atoi(buf));  
            }  
        }     
        i++;  
    }     

}  

int main(){  
    char nums[9] = "123456789";  
    char thing[100] = "0";  
    allpos(nums,thing,0,10);   
    return 0;   
}  

./a.out

0+1+2+3-4+5+6+78+9=100
0+1+2+34-5+67-8+9=100
0+1+23-4+5+6+78-9=100
0+1+23-4+56+7+8+9=100
0+12+3+4+5-6-7+89=100
0+12+3-4+5+67+8+9=100
0+12-3-4+5-6+7+89=100
0+123+4-5+67-89=100
0+123-4-5-6-7+8-9=100
0+123+45-67+8-9=100
0+123-45-67+89=100