LinuxQuestions.org - How does the cdev_alloc memory get freed up?

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- - How does the cdev_alloc memory get freed up? (https://www.linuxquestions.org/questions/linux-kernel-70/how-does-the-cdev_alloc-memory-get-freed-up-629661/)

nabramovitz

03-21-2008 10:40 AM

How does the cdev_alloc memory get freed up?

The cdev_alloc function uses kmalloc to allocate the cdev structure. Looking at the st and sg drivers, the cdev_del function does not seem to free the allocated memory with kmfree. I would not expect the cdev_del to free the memory since the cdev_del can also be used with cdev_init which uses preallocated memory.

So, if you use cdev_alloc, should kmfree be called to free the memory? If not, how does this allocated memory gets freed up when cdev_del is called?

ashok449

03-25-2008 03:03 AM

cdev_alloc — allocate a cdev structure

look at below example

Code:

static struct cdev *a_cdev;

a_cdev = cdev_alloc();

this has nothing to do with cdev_del()

cdev_add() and cdev_del() are like kmalloc(), kfree()

cdev_add()

Code:

cdev_add — add a char device to the system 

int cdev_add ( struct cdev *  p,  

  dev_t  dev,  

  unsigned  count);

cdev_del()

Code:

cdev_del — remove a cdev from the system 



void cdev_del ( struct cdev *  p);

Hope this helps you...

nabramovitz

03-28-2008 07:04 PM

cdev_alloc

Unfortunately, it does not because to create a cdev structure you are suppose to use either cdev_alloc or cdev_init. The cdev_alloc allocates the storage dynamically while the cdev_init is meant for statically defined structures. The cdev_add makes the character device available to the kernel and to users. The cdev_del call makes the character device unavailable for future use. If you use the cdev_alloc does cdev_del somehow know that the dynamic memory needs to be freed? I do not see how that would happen looking at the cdev_del code. What am I missing here? There does not seem be a cdev_free call which is what I would have expected.

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