C - freeing structs
Let's say I have this struct
typedef struct person{
char firstName[100], surName[51]
} PERSON;
and I am allocating space by malloc and filling it with some values
PERSON *testPerson = (PERSON*) malloc(sizeof(PERSON));
strcpy(testPerson->firstName, "Jack");
strcpy(testPerson->surName, "Daniels");
What is the correct and safe way to free all memory taken by that struct? Is "free(testPerson);" enough or do I need to free each struct's attribute one by one?
It leads me to another question - how are structures stored in memory? I noticed a strange behaviour - when I try to print structure address it's equal to it's first attribute's address.
printf("Structure address %d == firstName address %d", testPerson, testPerson->firstName);
Which means that this free(testPerson) should be equal to this free(testPerson->firstName);
and that's not what I want to do.
Thanks
Solution 1:
Simple answer : free(testPerson)
is enough .
Remember you can use free()
only when you have allocated memory using malloc
, calloc
or realloc
.
In your case you have only malloced memory for testPerson
so freeing that is sufficient.
If you have used char * firstname , *last surName
then in that case to store name you must have allocated the memory and that's why you had to free each member individually.
Here is also a point it should be in the reverse order; that means, the memory allocated for elements is done later so free()
it first then free the pointer to object.
Freeing each element you can see the demo shown below:
typedef struct Person
{
char * firstname , *last surName;
}Person;
Person *ptrobj =malloc(sizeof(Person)); // memory allocation for struct
ptrobj->firstname = malloc(n); // memory allocation for firstname
ptrobj->surName = malloc(m); // memory allocation for surName
.
. // do whatever you want
free(ptrobj->surName);
free(ptrobj->firstname);
free(ptrobj);
The reason behind this is, if you free the ptrobj
first, then there will be memory leaked which is the memory allocated by firstname
and suName
pointers.
Solution 2:
First you should know, how much memory is allocated when you define and allocate memory in below case.
typedef struct person{
char firstName[100], surName[51]
} PERSON;
PERSON *testPerson = (PERSON*) malloc(sizeof(PERSON));
1) The sizeof(PERSON) now returns 151 bytes (Doesn't include padding)
2) The memory of 151 bytes is allocated in heap.
3) To free, call free(testPerson).
but If you declare your structure as
typedef struct person{
char *firstName, *surName;
} PERSON;
PERSON *testPerson = (PERSON*) malloc(sizeof(PERSON));
then
1) The sizeof(PERSON) now returns 8 bytes (Doesn't include padding)
2) Need to allocate memory for firstName and surName by calling malloc() or calloc(). like
testPerson->firstName = (char *)malloc(100);
3) To free, first free the members in the struct than free the struct. i.e, free(testPerson->firstName); free(testPerson->surName); free(testPerson);
Solution 3:
Because you defined the struct
as consisting of char
arrays, the two strings are the structure and freeing the struct
is sufficient, nor is there a way to free the struct
but keep the arrays. For that case you would want to do something like struct { char *firstName, *lastName; }
, but then you need to allocate memory for the names separately and handle the question of when to free that memory.
Aside: Is there a reason you want to keep the names after the struct
has been freed?