C - freeing structs

c struct malloc
user10099 picture user10099 · Nov 27, 2012 · Viewed 105.4k times · Source

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

Answer

Omkant picture Omkant · Nov 27, 2012

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.

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