CODEWITHSUNDEEP
linuxlinux-kernellinux-device-driver
hakunamatata
hakunamatata

Reputation: 127

Why need container_of when kmalloc already returns the pointer to memory location?

kmalloc() returns a pointer to the memory location allocated during initialization and if one makes the struct containing cdev point to it, why would one need to do container_of in the file operations open call to get the address of struct containing cdev again ?

Upvotes: 0

Views: 367

Answers (1)

Nemanja Boric
Nemanja Boric

Reputation: 22157

I presume you're referring to something like this:

http://www.cs.uni.edu/~diesburg/courses/dd/code/scull/pipe.c

static int scull_p_open(struct inode *inode, struct file *filp)
{
    struct scull_pipe *dev;

    dev = container_of(inode->i_cdev, struct scull_pipe, cdev);
    // ...

And kmalloc is used like this:

    scull_p_devices = kmalloc(scull_p_nr_devs * sizeof(struct scull_pipe), GFP_KERNEL);

Where struct scull_pipe is:

struct scull_pipe {
        wait_queue_head_t inq, outq;       /* read and write queues */
        char *buffer, *end;                /* begin of buf, end of buf */
        int buffersize;                    /* used in pointer arithmetic */
        char *rp, *wp;                     /* where to read, where to write */
        int nreaders, nwriters;            /* number of openings for r/w */
        struct fasync_struct *async_queue; /* asynchronous readers */
        struct semaphore sem;              /* mutual exclusion semaphore */
        struct cdev cdev;                  /* Char device structure */
};

The reason for using container_of is that in scull_p_open callback you don't have pointer to struct scull_pipe instance, but you have access to cdev member of struct scull_pipe structure (through inode->i_cdev). In order to get the container address of cdev (in other words address of struct scull_pipe instance), you need to use container_of:

struct scull_pipe *dev;

dev = container_of(inode->i_cdev, struct scull_pipe, cdev);

Upvotes: 3

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