Getting run time error when accessing the freed pointer variable

Question

i have one scenario like below

#include <stdio.h>
int main(void) {
int *p1=NULL;
int a;
p1=&a;
printf("%p\n",p1);
   p1=NULL;
printf("%p\n",p1);
return 0;
}

In this case i have no problem at all.but if i use like this as below:

#include <stdio.h>
int main(void) {
int *p1=NULL;
int a;
p1=&a;
printf("%p\n",p1);
   free(p1);
 printf("%p\n",p1);
return 0;
}

In this case, i got run time error as below:

Runtime error time: 0 memory: 2052 signal:11

I want to know why it is happens like that. As far i know, freeing the pointer will do same thing as assigning it NULL value(mean when we do free(p) it also delete the pointer link to memory instead of deleting the allocated memory space). I would rather if you could possibly suggest me
anything on that.

Thanks in Advance.

`i got run time error` - please consider adding the error stacktrace to your question. — cel, Apr 02 '15 at 07:33
`%p` requires an argument of type `void*`. So you need to cast it. Use `%p` instead of `%x` to print addresses(pointers) — Spikatrix, Apr 02 '15 at 07:36

score 1 · Answer 1 · answered Apr 02 '15 at 07:33

1

You are trying to free() memory you haven't allocated at all, which results in Undefined Behavior.

Therefore, you need a malloc() before the free() call like

p1 = malloc(sizeof(int));

answered Apr 02 '15 at 07:33

cadaniluk

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score 1 · Answer 2 · answered Apr 02 '15 at 07:34

1

free(p1); with a p1 value not returned by malloc() produces undefined behaviour.

answered Apr 02 '15 at 07:34

Sourav Ghosh

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score 1 · Answer 3 · answered Apr 02 '15 at 07:35

1

You have undefined behaviour:

You can only free what you've malloced. p1 points to stack allocated memory so don't attempt to free it.

Be careful too when using *p1: it has the same rules as accessing a. You need to initialise a (either with a = ... or via the pointer with *p1 = ...) before doing anything else with it. Otherwise the program behaviour is undefined.

answered Apr 02 '15 at 07:35

Bathsheba

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i got your point, you mean if we have static storage allocated memory then we don't need to free it, it will be freed automatically when it goes out of scope. But one more thing i want to ask you, if i have another scenario as below:
Refer to above program with few change, Suppose if i pointed p1 pointer to another pointer(about which i don't know whether it has static allocated address or dynamically allocated address), then what will the proper way to free the p1 pointer? – r13kumar Apr 02 '15 at 08:32
I think you should ask that as another question. – Bathsheba Apr 02 '15 at 08:35

Getting run time error when accessing the freed pointer variable

3 Answers3