Program debugging using Valgrind - c

Question

I am trying to solve a problem of "Multiplying a number by 11".Number can be any long.I am getting SIGABRT error.
I tried to debug using valgrind, but i don't know to solve it(saw many questions of valgrind on stackoverflow but no success).I am stuck on this problem since last week.
Code is :
#include <stdio.h>
#include <stdlib.h>
#define CHUNK 10
void get_input(char *val,int *i){
char ch,*tmp=NULL;
int size;
size=CHUNK;
while(ch=getc(stdin),ch!=EOF && ch!='\n'){
val[(*i)++]=ch;
if(*i>=size){
size+=CHUNK;
tmp=realloc(val,size);
if(!tmp){
free(val);
val=NULL;
break;
}
val=tmp;
}
}
val[*i]='\0';
}
void mul_11(char *val,int *i){
int *digit,iter,j,carry=0,num,temp;
iter=*i;
digit=(int*)malloc((iter+2)*sizeof(int));
for(j=iter-1;j>=0;j--){
temp=val[j]-'0';
num=temp*11+carry;
digit[j+2]=num%10;
carry=num/10;
}
digit[1]=carry%10;
digit[0]=carry/10;
if(digit[0]==0)
temp=1;
else
temp=0;
for(j=temp;j<=iter+1;j++)
printf("%d",digit[j]);
free(digit);
digit=NULL;
*i=0;
}
int main() {
int t,i=0;
char *val=NULL;
scanf("%d ",&t);
while(t--){
val=(char*)malloc(CHUNK*sizeof(char));
get_input(val,&i);
mul_11(val,&i);
printf("\n");
free(val);
val=NULL;
}
return 0;
}
Whole code with input can be found out Geeksforgeeks code here
Valgrind error :
5555555555555555555555555555555555555
==2349== Invalid read of size 1
==2349== at 0x4008CE: mul_11 (mul11.c:30)
==2349== by 0x400A8A: main (mul11.c:55)
==2349== Address 0x52044a4 is 20 bytes after a block of size 16 in arena "client"
==2349==
6111111105-3-6-6-6-6-6-6-6-6-6-6-6-6-6-6-6-6-6-6-6-6-6-6-6-6-60-8
==2349== Invalid free() / delete / delete[] / realloc()
==2349== at 0x4C2EDEB: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==2349== by 0x400AA0: main (mul11.c:57)
==2349== Address 0x5204480 is 0 bytes inside a block of size 10 free'd
==2349== at 0x4C2FD5F: realloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==2349== by 0x400809: get_input (mul11.c:13)
==2349== by 0x400A77: main (mul11.c:54)
==2349== Block was alloc'd at
==2349== at 0x4C2DB8F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==2349== by 0x400A60: main (mul11.c:53)
==2349==

Answer 1

If get_input reallocs your val and it gets a new address, the caller (main) will not be told that its value for val has become invalid (realloc has freed it). mul_11 will be using freed memory, and main will then free it for the second time.
One solution is to send in a pointer to the address, if the function can realloc it, so the caller gets the new values.
void get_input(char **valp,int *i){
char *val = *valp;
...
val = realloc ...
*valp = val;
...