In C, do I need to manually free pointers within a struct when freeing the struct?

Question

I'm learning about memory management in C.

I have a pointer to a struct which I malloc size for via

some_struct *mystruct = malloc(sizeof(some_struct));

and I later use free on that struct.

Inside this struct there are three char * pointers; I malloc their memory as well.

Do I have to free their memory as well before I free *mystruct or will the destruction of *mystruct also free the memory inside the struct?

DWright · Accepted Answer · 2012-12-23T18:18:55.053

5

You absolutely have to also free the memory pointed to by the char * pointers inside the struct. Do that first, then free your struct

But don't free the char * pointers inside the struct, if you are pointing at the same memory for those char * pointers elsewhere (i.e., if you have copied your struct to another struct instance, those char * pointers need to point to valid memory).

edited Dec 23 '12 at 18:18

answered Dec 23 '12 at 17:37

DWright

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C is not a garbage collected language. The runtime does not reference count allocated memory, so that it can figure out when to deallocate things automatically when nothing else refers to them any more. – DWright Dec 23 '12 at 17:39
1

@DWrigth There isn't even a runtime. – Dec 23 '12 at 17:40
Well, there is a runtime library. – DWright Dec 23 '12 at 17:45
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that's called the C standard library. Since C is not a dynamic language, calling the standard library a "runtime" is an error. Take a look at the Objective-C **runtime** library, for example. Now that is a runtime library. – Dec 23 '12 at 17:50
let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/21602/discussion-between-dwright-and-h2co3) – DWright Dec 23 '12 at 17:52
hey just a quick question. i create my pointer to my struct like in my post then i pass it to a function with a prototype `void *function(void *param)` from inside the function i call free(param) once i am done making a clone of the structure via `some_struct mynewstruct = *((some_struct *)param);` if i had 3 `char *` variables inside my strut how would i free them from inside the function. `free(param.var);` doesnt seem to work and neitehr does using `->` – randy newfield Dec 23 '12 at 18:00
ill take a look at this right now. i also added a extended version of whats happening to the original post along with my error messages from gcc. – randy newfield Dec 23 '12 at 18:15
Big caveat, now that I re-read your edits!!! If you copy the contents of the struct via `some_struct mynewstruct = *pParamAsStruct;`, you do NOT need to free the char * pointers when you free *param, in fact to do so would be a bug. – DWright Dec 23 '12 at 18:15
Instead, free the char * pointers before you free mynewstruct (Eventually). – DWright Dec 23 '12 at 18:16

score 3 · Answer 2 · answered Dec 23 '12 at 17:35

3

There's no magic and no Santa Claus. You have to do the work yourself. Free all the memory you allocated dynamically before you lose track of it.

answered Dec 23 '12 at 17:35

Kerrek SB

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score 0 · Answer 3 · answered Dec 23 '12 at 17:44

Thats the reason why you later want to change to c++, then you can have a struct which does it by its own (if you put the right code into its destructor). In C you have to do the work by yourself, that is, before deleting your struct, delete the things inside the struct. And, of course, when you malloc a struct, you should also properly initialize the members inside the struct, as minimum initialize pointers to 0, so that there are no glitches when deleting them...

score 0 · Answer 4 · edited Jun 20 '20 at 09:12

As far as good practice goes you should free everything you allocate, think of it like opening an html tag and closing it, and as far as good practices go it's a good idea not to think about structs the way you would about classes in C++ or java.

Freeing structs only free pointers to object, not the object itself,

#include <stdio.h>                                                                                                                                                                   
                                                                                                                                                                               
typedef struct                                                                                                                                                                       
{                                                                                                                                                                                    
    int a,b,c,d;                                                                                                                                                                     
    char some_string[1050];                                                                                                                                                          
                                                                                                                                                                                     
} first_type;                                                                                                                                                                        
typedef struct                                                                                                                                                                       
{                                                                                                                                                                                    
    struct first_type *ap;                                                                                                                                                           
} second_type;                                                                                                                                                                       
                                                                                                                                                                                                                                                                                                                                                                    
int main(void)                                                                                                                                                                       
{                                                                                                                                                                                    
    printf("first type: %d\n", sizeof(first_type));                                                                                                                                  
    printf("second type: %d\n", sizeof(second_type));                                                                                                                                
}

if you run this little example you will see something like this

first type: 1068

second type: 8

although second_type has a pointer to a first_type it's size is considerably less than first_type. So when you malloc and free stuff you are only reserving and releasing however many bytes that data type is in memory.

Hope this helps.

In C, do I need to manually free pointers within a struct when freeing the struct?

4 Answers4