glibc detected free() invalid pointer

I'm having a bit of trouble with some dynamic memory allocation.

Below is just a test code that I've been playing around with to try and fix the problem (it's the same problem in my current project's code, this is just a simpler way to show it).

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

int main(){

int x = 5;
int *ptr = (int*) malloc(sizeof(int));
assert(ptr != NULL);
ptr = &x;

printf("x = %d\n",x);

*ptr = 3;
printf("x = %d\n",x);

free(ptr);

return 0;
}


The program compiles fine and when run I get the correct outputs printed "x = 5 x = 3"
But then I get the error:

glibc detected ./dnam: free(): invalid pointer: 0xbfccf698

dnam is the name of the test program.
From what I've read about the error, it's supposedly caused by freeing memory that you haven't malloc/calloc/realloc'd.


This error message is followed by a backtrace and a memory map. AT the end of the memory map I'm told the program has aborted (core dumped).

int *ptr = (int*) malloc(sizeof(int));

ptr = &x;

You are changing `ptr` value! The compiler will be taking unlimited revenge if you attempt to free it.


Here:

free(ptr);

You are `free`-ing an object not allocated through `malloc`.

You are allocating memory and saving its address into ptr. Then, you make ptr point to x's address, thus when you run `free(ptr)` you essentially freeing `&x`, which doesn't work.

tl;dr: there's no need for `malloc` and `free` when you're assigning the pointer a pointer to another var.

if you want use this code.I think you should use a temp pointer save the init ptr.at last free(temp).like this:

int *temp = ptr;


free(temp);