Thread: free() not working?

i was playing with malloc() and free() and i notice that free() is not deallocating the memory

Code:

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

int main(void)
{
    
    double *pInt = (double *) malloc(7*sizeof(double));
    
    free(pInt);
  
    for (int j = 0; j < 7; j++)
    {
        *(pInt + j) = j;
    }
    
    for (int k = 0; k < 7; k++)
    {
        printf("Element is %e at address %p\n",*(pInt+k),pInt+k);
    }
    getchar();
    return 0;
    
}

i was expecting the program to terminate without displaying the output but it ran surprisingly and it displayed the output.

why is this so? maybe i don't know how free() works.

i ran this under dev-cpp btw.

You observed that free did not overwrite the memory deallocated. You did not observe that free did not deallocate the memory.

Note that what you did actually results in undefined behaviour.

To test if your frees and mallocs work I suggest using Valgrind.

Originally Posted by scout

thanks for the replies, guys

One other thing they didn't touch on...

Calling free() does not reset the value in your pointer...

All it really does is mark the memory as "available" to the system. Your buffer content will remain in place until it's overwritten by something else.

Checking for NULL doesn't buy you much since free(NULL) is a no-op.

Originally Posted by Adak

free() puts the memory's "name" back into the singles list. It doesn't guarantee mr. RAM a date on Saturday night.

Originally Posted by CommonTater

One other thing they didn't touch on...

Calling free() does not reset the value in your pointer...

All it really does is mark the memory as "available" to the system. Your buffer content will remain in place until it's overwritten by something else.

Or to put it another way (Since I liked Adak's analogy):
It doesn't mean your phone number magically disappears from your ex's cellphone either.

Originally Posted by iMalc

Or to put it another way (Since I liked Adak's analogy):
It doesn't mean your phone number magically disappears from your ex's cellphone either.

Lol. I like this analogy better.