Homebrew Quick question about C memory management

[mashers]

Hey guys,

Objective C has spoiled me, especially with ARC, so I need to double check something on C memory management. Take the following example:

typedef struct myStruct {
    u8 * spriteData;
} myStruct;

myStruct myStructArray[16];

void doStuffWithStruct(int i) {
    myStruct aStruct = myStructArray[i];
    u8 * data = malloc(1024);
    doStuffWithData(data);
    aStruct.spriteData = data;
}

int main() {
    doStuffWithStruct(0);
    doStuffWithStruct(0);
}

I'm assuming that the second call to doStuffWithStruct(0) will cause aStruct.spriteData to leak. I'm wondering about two possible solutions.

Solution 1

typedef struct myStruct {
    u8 * spriteData;
} myStruct;

myStruct myStructArray[16];

void doStuffWithStruct(int i) {
    myStruct aStruct = myStructArray[i];
    u8 * data = malloc(1024);
    doStuffWithData(data);
    aStruct.spriteData = data;

/*
Free data here. This is like retaining properties in Obj-C, 
but I'm concerned aStruct.spriteData will then point to 
NULL or garbage in this instance
*/

    free(data);
}

int main() {
    doStuffWithStruct(0);
    doStuffWithStruct(0);
}

Solution 2

typedef struct myStruct {
    u8 * spriteData;
} myStruct;

myStruct myStructArray[16];

void doStuffWithStruct(int i) {
    myStruct aStruct = myStructArray[i];

/*
Free data here if it already exists. Is the if statement needed 
to prevent attempting to free NULL if aStruct.spriteData has 
not yet been allocated, or is freeing a NULL pointer ok?
*/
    if (aStruct.spriteData) {
        free(aStruct.spriteData);
    }

    u8 * data = malloc(1024);
    doStuffWithData(data);
    aStruct.spriteData = data;
}

int main() {
    doStuffWithStruct(0);
    doStuffWithStruct(0);
}

So, which, if either, of these will prevent a memory leak? Also, do I need to go through and free the memory for all of the myStruct objects in myStructArray before terminating the application, or will that be taken care of for me when the array itself is freed?

 

[TheCruel]

You should be able to free a null pointer:

The free() function deallocates the memory allocation pointed to by ptr. If ptr is a NULL pointer, no operation is performed.

But never free the pointer if you plan on using the data sometime later.

 

[Gocario]

The first solution will always free the memory within the function (= no leak IF the pointer data wasn't malloced BEFORE).
The second will need a free function further.

// If ptr is a null pointer, no action occurs.
void free(void*ptr);

Care:

• Once freed, spriteData will still be pointing to the address returned by the previous malloc. You should set it manually to NULL after the free.
• If spriteData isn't initialized to NULL, problems might appear within `if (.spriteData)`.

 

[mashers]

Great, thanks. So solution 2 is the right one since I don't need the old content of spriteData. Is that right?

 

[TheCruel]

Great, thanks. So solution 2 is the right one since I don't need the old content of spriteData. Is that right?

Yeah, most likely. Definitely not the first one.

 

[mashers]

The first solution will always free the memory within the function (= no leak IF the pointer data wasn't malloced BEFORE).
The second will need a free function further.

// If ptr is a null pointer, no action occurs.
void free(void*ptr);

Care:

• Once freed, spriteData will still be pointing to the address returned by the previous malloc. You should set it manually to NULL after the free.
• If spriteData isn't initialized to NULL, problems might appear within `if (.spriteData)`.

Oh right. So I should do

free(aStruct.spriteData);
aStruct.spriteData = NULL;

 

[Gocario]

The free() function deallocates the memory allocation pointed to by ptr. If ptr is a NULL pointer, no operation is performed.

void free(void*ptr); // If ptr is a null pointer, no action occurs.

I am pointing this again, with this link.

PS: What is the default value when declaring a pointer, NULL or not set?

 

[doctorgoat]

God, I hate not having garbage collection. ._.

 

[Gocario]

God, I hate not having garbage collection. ._.

Garbage collectors are bad in this case... :c
I don't like them in this case!

 

[mashers]

I am pointing this again, with this link.

PS: What is the default value when declaring a pointer, NULL or not set?

I found that I got lock ups when freeing the pointer if I didn't check it was NULL first. Perhaps free() works differently in ctrulib.

 

[sarkwalvein]

I found that I got lock ups when freeing the pointer if I didn't check it was NULL first. Perhaps free() works differently in ctrulib.

Does your example work?
There are many privilege.
First when you write:
"myStruct aStruct = myStructArray;"
You are creating a copy, not a reference.
That copy and all modifications to it will die within the function.

Oh... I will look for a computer to finish this reply, the phone sucks.

 

[Gocario]

Does your example work?
There are many privilege.
First when you write:
"myStruct aStruct = myStructArray;"
You are creating a copy, not a reference.
That copy and all modifications to it will die within the function.

Oh... I will look for a computer to finish this reply, the phone sucks.

He is using pointer to reference infact: https://github.com/mashers/3ds_hb_menu/blob/master/source/themegfx.c#L65

 

[sarkwalvein]

He is using pointer to reference infact: https://github.com/mashers/3ds_hb_menu/blob/master/source/themegfx.c#L65

Oh, okay. I was replying based only in the posted example.
Another thing to mention is that free doesn't make the pointer NULL.
So if somebody wants to "check for NULL" in order to assign memory, it is compulsory to set the pointer to null after free manually:
free(pointer);
pointer=NULL;

So:
if(aStruct->spriteData){
free(aStruct->spriteData);
aStruct->spriteData = 0;
}

** But I see this is already being done in the github also.

 

[Gocario]

Oh, okay. I was replying based only in the posted example.
Another thing to mention is that free doesn't make the pointer NULL.
So if somebody wants to "check for NULL" in order to assign memory, it is compulsory to set the pointer to null after free manually:
free(pointer);
pointer=NULL;

So:
if(aStruct->spriteData){
free(aStruct->spriteData);
aStruct->spriteData = 0;
}

As he said:

Oh right. So I should do

free(aStruct.spriteData);
aStruct.spriteData = NULL;

 

[mashers]

He is using pointer to reference infact: https://github.com/mashers/3ds_hb_menu/blob/master/source/themegfx.c#L65

Heh, you realised which function I was talking about I thought the real function was too complex for this questions so I simplified with an example, but got the referencing wrong in the example!

And yes, I'm setting it to NULL manually now. The gridlauncher is probably so littered with uninitialised variables that it's a miracle it works at all. I've got so used to not having to do this in Obj-C, since everything is initialised to nil, 0 or false automatically when the variable is created.

Is there a static analyser which works with ctrulib? It would be good to run the code through something like clang to check for problems like this.

--------------------- MERGED ---------------------------

Oh and one other question, do I need to go through the array and release any existing objects or will that be taken care of for me?

 

[Gocario]

Is there a static analyser which works with ctrulib? It would be good to run the code through something like clang to check for problems like this.
--------------------- MERGED ---------------------------
Oh and one other question, do I need to go through the array and release any existing objects or will that be taken care of for me?

Analysers? What are they?
> Everytime you use a malloc, a free shall be used.

 

[mashers]

Analysers? What are they?
> Everytime you use a malloc, a free shall be used.

Great, thank you. Time to search the source folder for instances of the words "malloc" and "calloc"...

 

[sarkwalvein]

Oh and one other question, do I need to go through the array and release any existing objects or will that be taken care of for me?

You take care of everything memory related.
If you created an array that contains pointers, and then assigned memory to those pointers, you have to use free for each one of those pointers later on.

 

[mashers]

You take care of everything memory related.
If you created an array that contains pointers, and then assigned memory to those pointers, you have to use free for each one of those pointers later on.

Thank you for clarifying. This is different to what I'm used to in Obj-C, even using manual reference counting. Take for example:

NSString *s1 = [[NSString alloc] initWithString:@"Hello];
NSString *s2 = [[NSString alloc] initWithString:@"World];
NSArray *a = [[NSArray alloc] initWithObjects:s1, s2, nil];

To release everything, all I have to do is call [a release]. This will decrement the retain count of a to 0 causing its dealloc function to be called. [NSArray dealloc] overloads [NSObject dealloc] to iterate through its contents and call release on each object. This causes dealloc to be called on each item within the array, and everything gets deallocated. Of course with ARC you don't have to do anything - just alloc and let the runtime deal with when things get deallocated.

So yeah, long story short, having to loop through an array and manually free anything which might have been allocated feels weird to me (but does make sense).

 

[filfat]

Thank you for clarifying. This is different to what I'm used to in Obj-C, even using manual reference counting. Take for example:

NSString *s1 = [[NSString alloc] initWithString:@"Hello];
NSString *s2 = [[NSString alloc] initWithString:@"World];
NSArray *a = [[NSArray alloc] initWithObjects:s1, s2, nil];

To release everything, all I have to do is call [a release]. This will decrement the retain count of a to 0 causing its dealloc function to be called. [NSArray dealloc] overloads [NSObject dealloc] to iterate through its contents and call release on each object. This causes dealloc to be called on each item within the array, and everything gets deallocated. Of course with ARC you don't have to do anything - just alloc and let the runtime deal with when things get deallocated.

So yeah, long story short, having to loop through an array and manually free anything which might have been allocated feels weird to me (but does make sense).

Super OT,
but Objective C looks like someone combined C# with a ini config file