Why's _itoa causing my program to crash? - c

Question

The following code just keeps on crashing when it reaches the part with _itoa, I've tried to implement that function instead and then it got even weirder, it just kept on crashing when I ran it without the debugger but worked fine while working with the debugger.
# include "HNum.h"
# include <stdio.h>
# include <stdlib.h>
# include <string.h>
# include <assert.h>
# define START_value 30
typedef enum {
HNUM_OUT_OF_MEMORY = -1,
HNUM_SUCCESS = 0,
} HNumRetVal;
typedef struct _HNum{
size_t Size_Memory;
char* String;
}HNum;
HNum *HNum_alloc(){
HNum* first = (HNum*)malloc(sizeof(HNum));
if(first==NULL){
return NULL;
}
first->String =(char*)malloc(sizeof(START_value));
if(first->String==NULL){
return NULL;
}
first->Size_Memory = START_value; // slash zero && and starting from zero index;
return first;
}
HNumRetVal HNum_setFromInt(HNum *hnum, int nn){
itoa(nn,hnum->String,10);
}
void main(){
HNum * nadav ;
int h = 13428637;
nadav = HNum_alloc();
nadav->String="1237823423423434";
HNum_setFromInt(nadav,h);
printf("nadav string : %s \n ",nadav->String);
//printf("w string %s\n",w->String);
//printf("nadav string %s\n",nadav->String);
HNum_free(nadav);
}
I've been trying to figure this out for hours and couldn't come up with anything...
The IDE I'm using is Visual Studio 2012 express, the crash shows the following:
"PROJECT C.exe has stopped working
windows can check online for a solution to the program."

Answer 1

first->String =(char*)malloc(sizeof(START_value));
should be
first->String = malloc(START_value);
The current version allocates space for sizeof(int)-1 characters (-1 to leave space for the nul terminator). This is too small to hold your target value so _itoa writes beyond memory allocated for first->String. This results in undefined behaviour; it is quite possible for different runs to fail in different places or debug/release builds to behave differently.
You also need to remove the line
nadav->String="1237823423423434";
which leaks the memory allocated for String in HNum_alloc, replacing it with a pointer to a string literal. This new pointer should be considered to be read-only; you cannot write it it inside _itoa

Answer 2

Since I'm not allowed to comment:
simonc's answer is correct. If you find the following answer useful, you should mark his answer as the right one:P
I tried that code myself and the only thing missing is lets say:
strcpy(nadav->String, "1237823423423434"); INSTEAD OF nadav->String="1237823423423434";
and
first->String = malloc(START_value); INSTEAD OF first->String =(char*)malloc(sizeof(START_value));
Also, maybe you'd have to use _itoa instead of itoa, that's one of the things I had to change in my case anyhow.
If that doesn't work, you should probably consider using a different version of VS.