C - freeing structs

Question

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- gt firstName   Jack    strcpy testPerson- gt 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- gt 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

User · Answer

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

User · Answer

You can t free types that aren t dynamically allocated  Although arrays are syntactically similar  int  x   malloc sizeof int    4  can be used in the same way that int x 4  is   calling free firstName  would likely cause an error for the latter   For example  take this code   int x  free  amp x     free   is a function which takes in a pointer   amp x is a pointer  This code may compile  even though it simply won t work   If we pretend that all memory is allocated in the same way  x is  allocated  at the definition   freed  at the second line  and then  freed  again after the end of the scope  You can t free the same resource twice  it ll give you an error   This isn t even mentioning the fact that for certain reasons  you may be unable to free the memory at x without closing the program   tl dr  Just free the struct and you ll be fine  Don t call free on arrays  only call it on dynamically allocated memory

User · Answer

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- gt 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

User · Answer

This way you only need to free the structure because the fields are arrays with static sizes which will be allocated as part of the structure   This is also the reason that the addresses you see match  the array is the first thing in that structure   If you declared the fields as char   you would have to manually malloc and free them as well

User · Answer

free is not enough  free just marks the memory as unused  the struct data will be there until overwriting  For safety  set the pointer to NULL after free   Ex   if  testPerson        free testPerson       testPerson   NULL      struct is similar like an array  it is a block of memory  You can access to struct member via its offset  The first struct s member is placed at offset 0 so the address of first struct s member is same as the address of struct

User · Answer

Mallocs and frees need to be paired up   malloc grabbed a chunk of memory big enough for Person   When you free you tell malloc the piece of memory starting  here  is no longer needed  it knows how much it allocated and frees it   Whether you call    free testPerson     or   free testPerson- gt firstName    all that free   actually receives is an address  the same address  it can t tell which you called  Your code is much clearer if you use free testPerson  though - it clearly matches up the with malloc

User · Answer

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- gt firstname   malloc n      memory allocation for firstname ptrobj- gt surName   malloc m      memory allocation for surName         do whatever you want  free ptrobj- gt surName   free ptrobj- gt 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

[c] C - freeing structs

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