Reputation: 9801
I just wrote some C code:
#include <stdlib.h>
#include <time.h>
#include <string.h>
typedef struct {
void **data;
time_t lastModified;
} container;
container *container_init() {
container *c = malloc(sizeof(container));
void *data = NULL;
c->data = &data;
c->lastModified = time(NULL);
return c;
}
void *container_getData(container *c) {
void **containerData = c->data;
return *containerData;
}
// only pass manually allocated data that can be free()'d!
void container_setData(container *c, void *data) {
free(container_getData(c));
c->data = &data;
}
void container_free(container *c) {
free(container_getData(c)); // <--- THIS LINE
free(c);
}
int main(int argc, const char *argv[]) {
for (int i = 0; i < 100000000; i++) {
char *data = strdup("Hi, I don't understand pointers!");
container *c = container_init();
container_setData(c, data);
container_free(c);
}
}
My logic was the following: When I call container_setData()
, the old data is free()
'd and a pointer to the new data is stored. That new data will have to be released at some point. That happens for the last time during the call to container_free()
.
I have marked a line in the container_free()
function. I would have sworn I'd need that line in order to prevent a memory leak. However, I can't use the line ("object beeing freed was not allocated") and there's no memory leak if I delete it. How does the string from my loop ever get released?!
Could someone explain where the error is?
Upvotes: 1
Views: 169
Reputation: 70538
To explain larsmans answer with code make these changes:
typedef struct {
void *data;
time_t lastModified;
} container;
void *container_getData(container *c) {
return c->data;
}
void container_setData(container *c, void *data) {
free(c->data);
c->data = data;
}
void container_free(container *c) {
free(c->data);
free(c);
}
And other changes too -- this just gets you on the right track.
Upvotes: 1
Reputation: 363807
c->data = &data;
stores the address of the pointer data
(the argument to your function), not the actual pointer. I.e., you're storing a pointer to a temporary.
You could have built the container
structure with just a void *data
member.
Upvotes: 3