I'm trying to learn C and I'm currently trying to write a basic stack data structure, but I can't seem to get basic malloc
/free
right.
Here's the code I've been using (I'm just posting a small part here to illustrate a specific problem, not the total code, but the error message was generated just by running this example code in valgrind
)
#include <stdio.h>
#include <stdlib.h>
typedef struct Entry {
struct Entry *previous;
int value;
} Entry;
void destroyEntry(Entry entry);
int main(int argc, char *argv[])
{
Entry* apple;
apple = malloc(sizeof(Entry));
destroyEntry(*(apple));
return 0;
}
void destroyEntry(Entry entry)
{
Entry *entry_ptr = &entry;
free(entry_ptr);
return;
}
When I run it through valgrind
with --leak-check=full --track-origins=yes
, I get the following error:
==20674== Invalid free() / delete / delete[] / realloc()
==20674== at 0x4028E58: free (vg_replace_malloc.c:427)
==20674== by 0x80485B2: destroyEntry (testing.c:53)
==20674== by 0x8048477: main (testing.c:26)
==20674== Address 0xbecc0070 is on thread 1's stack
I think this error means that the destroyEntry
function is not allowed to modify memory allocated explicitly in main. Is that right? Why can't I just free
the memory I allocated in main
in another function? (and is this behavior somehow specific to main?)
Whenever you pass a parameter to a function, a copy is made, and the function works on that copy. So in your case, you are trying to free
a copy of the original object, which doesn't make any sense.
You should modify your function to take a pointer, and then you can have it call free
directly on that pointer.