Vector in C crashing - c

Question

I tried to write a vector in c using memory operations.Compiler shows no errors but if I try to print an element from the vector it simply crashes. And whenever I try to print destination variable (printf((int) destination)) the program crashes again.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
typedef struct{
void* elemList;
int elemSize;
int maxSize;
int curSize;
}myvector;
void initVec(myvector * vec, int typeSize){
vec->curSize = 0;
vec->maxSize = 10;
vec->elemSize =typeSize;
vec->elemList = malloc(10*sizeof(typeSize));
}
void add(myvector * vec, void* elem){
if(vec->curSize >= vec->maxSize){
vec->elemList = realloc(vec->elemList, vec->maxSize*2);
}
memcpy(&vec->elemList[vec->curSize],elem,vec->elemSize);
}
void get(myvector * vec, int index, void* destination){
if(index > vec->curSize || index < 0){
printf("Invalid Index");
return;
}
destination = malloc(vec->elemSize);
memcpy(destination,&vec->elemList[index], vec->elemSize);
}
int main()
{
myvector newVec;
initVec(&newVec,sizeof(int));
int a = 5;
add(&newVec,&a);
int* b;
get(&newVec,0,b);
printf(*b);//this is where the program crashes
return 0;
}

Answer 1

Basically the pointer in the get is not handled correctly. It's being passed by value so a copy of the pointer is made, the copy is modified (memory allocation is done for this copy), but the original pointer once you quit the get method is not pointing to a valid memory. You have to pass the address of the pointer. Following is a modified code (note the double ** in the destination in the get method). Basically I pass the address of the "destination" pointer instead of the pointer itself. Additionally I fixed the line sizeof(typeSize) .. it should be typeSize only since you are already calling the initVec method with sizeof operator.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
typedef struct{
void* elemList;
int elemSize;
int maxSize;
int curSize;
}myvector;
void initVec(myvector * vec, int typeSize){
vec->curSize = 0;
vec->maxSize = 10;
vec->elemSize = typeSize;
vec->elemList = malloc(vec->maxSize*typeSize);
}
void add(myvector * vec, void* elem){
if(vec->curSize >= vec->maxSize)
{
vec->elemList = realloc(vec->elemList, vec->maxSize*2);
}
memcpy(&vec->elemList[vec->curSize], elem, vec->elemSize);
vec->curSize++;
}
void get(myvector * vec, int index, void** destination){
if(index > vec->curSize || index < 0)
{
printf("Invalid Index");
return;
}
*destination = malloc(vec->elemSize);
memcpy(*destination, &vec->elemList[index], vec->elemSize);
}
int main()
{
myvector newVec;
initVec(&newVec,sizeof(int));
int a = 5;
add(&newVec,&a);
int* b;
get(&newVec, 0, &b);
printf("value of b is %d\n", *b); // This works correctly now
return 0;
}

Answer 2

A couple of issues with the code :
vec->elemList = malloc(10*sizeof(typeSize)); should be vec->elemList = malloc(10*typeSize);
If you would like get to create a pointer to int I would recommend either defining it like int* get(myvector * vec, int index) and return a newly allocated pointer to int or in the main function use :
int b;
get(&newVec, 0, &b);
the latter will also avoid memory leaks.
printf(*b); is wrong as you are passing an int and it expects a char* use either printf("%d", b); if b is an int or printf("%d", b);if b is aint`
you are using malloc a lot but no free. In this particular program you don't get memory leaks as the OS will reclaim all memory when main returns. But think early about a function to clear your vector and.

Answer 3

*b shouldn't be a valid pointer to string, so it will cause crash.
Try printing it by printf("%d",*b);
To make it better, you should free the buffer that are allocated by malloc.
UPDATE
The get function is wrong since it throws away the buffer allocated to destination
get function and main function should be like this:
void get(myvector * vec, int index, void** destination){
if(index > vec->curSize || index < 0){
printf("Invalid Index");
return;
}
*destination = malloc(vec->elemSize);
memcpy(*destination,&vec->elemList[index], vec->elemSize);
}
int main()
{
myvector newVec;
initVec(&newVec,sizeof(int));
int a = 5;
add(&newVec,&a);
int* b;
get(&newVec,0,&b);
printf("%d",*b);//this is where the program crashes
return 0;
}
But this still gives me Segmentation Fault. I'm working on.
UPDATE 2
You should think about the size of each elements.
You also forget the size information in add function.
This code should work if we don't care about memory leak.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
typedef struct{
void* elemList;
int elemSize;
int maxSize;
int curSize;
}myvector;
void initVec(myvector * vec, int typeSize){
vec->curSize = 0;
vec->maxSize = 10;
vec->elemSize =typeSize;
vec->elemList = malloc(vec->maxSize*vec->elemSize);
}
void add(myvector * vec, void* elem){
if(vec->curSize >= vec->maxSize){
vec->elemList = realloc(vec->elemList, vec->elemSize * vec->maxSize*2);
vec->maxSize *= 2;
}
memcpy(vec->elemList + vec->curSize * vec->elemSize,elem,vec->elemSize);
vec->curSize++;
}
void get(myvector * vec, int index, void** destination){
if(index >= vec->curSize || index < 0){
printf("Invalid Index");
return;
}
*destination = malloc(vec->elemSize);
memcpy(*destination,vec->elemList + index * vec->elemSize, vec->elemSize);
}
int main()
{
myvector newVec;
initVec(&newVec,sizeof(int));
int a = 5;
add(&newVec,&a);
int* b;
get(&newVec,0,(void**)&b);
printf("%d",*b);
return 0;
}