What You Will Learn

• Exactly how the kernel invokes your driver’s open() function
• Why the open() function signature must match the kernel’s expectation precisely
• How to safely allocate memory inside a driver method
• What nonseekable_open() does and when you should call it
• How to log context information usefully during open()

Prerequisites

• A working misc character driver with a file_operations table already registered
• Basic familiarity with kernel memory allocation (kmalloc family)
• A disposable Linux VM for testing kernel modules

What Happens When a Process Calls open()

When a user-space program calls open("/dev/mydevice", O_RDWR), that call does not go anywhere near your driver directly. It is intercepted by the kernel’s Virtual File System layer, which resolves the device node, finds the file_operations table attached to it, and then invokes whichever function you placed in the .open slot.

Two structures are handed to your function: an inode, which represents the device file on the filesystem, and a file pointer, which represents this particular open instance. If the same device is opened twice by two different processes, each gets its own file structure but they usually share the same inode.

The Correct Function Signature

Your open function’s signature has to match precisely what the kernel expects for the .open member of file_operations, otherwise you will get a compiler warning when you assign it into the table:

static int my_open(struct inode *inode, struct file *filp)
{
    /* driver-specific setup goes here */
    return 0;
}

Returning 0 tells the VFS that the open succeeded. The kernel itself takes care of returning a valid file descriptor number to user space — your driver never deals with file descriptor numbers directly.

Allocating a Per-Open Buffer Safely

It is common for an open() handler to allocate a small buffer that will be used later during read/write operations, or simply to hold a pathname for logging. Always check the return value of any kernel allocation function, since it can legitimately fail:

static int my_open(struct inode *inode, struct file *filp)
{
    char *path_buf = kzalloc(PATH_MAX, GFP_KERNEL);

    if (!path_buf)
        return -ENOMEM;

    /* use path_buf as needed, then release it if it
     * is only needed temporarily within this function
     */
    kfree(path_buf);

    return 0;
}

GFP_KERNEL is the correct allocation flag here because open() runs in normal process context and is allowed to sleep while the kernel looks for free memory. Never use GFP_KERNEL inside code that runs in interrupt context; that is a different topic covered later in this course.

Logging the Calling Process Context

A genuinely useful thing to print during open() is which process is opening your device. The kernel exposes the currently running task through the current macro, giving you access to its name and PID directly, without needing any special helper macro:

pr_info("mydevice: opened by process \"%s\" (pid %d)\n",
         current->comm, current->pid);

This single line is extremely valuable during debugging on a busy system, since it immediately tells you which application is talking to your driver at any given moment.

Marking the Device as Non-Seekable

Many simple drivers — sensors, message queues, control interfaces — do not have a meaningful concept of a file position. For these, you should explicitly disable seeking by calling nonseekable_open() from within your open handler:

static int my_open(struct inode *inode, struct file *filp)
{
    pr_info("mydevice: opened by process \"%s\" (pid %d)\n",
             current->comm, current->pid);

    return nonseekable_open(inode, filp);
}

Internally, this clears the seek and positioned-read/write mode bits on the file structure, so any later call to lseek(), pread(), or pwrite() against your device fails cleanly with -ESPIPE instead of silently appearing to succeed. This is important: without it, a confused application might assume seeking works, when your driver’s read and write methods never actually honored a file position in the first place.

Note: for drivers where read and write are used simultaneously by the same process in a producer/consumer style (true stream devices), the modern kernel API stream_open() is the more correct choice, since it avoids a position-lock deadlock that nonseekable_open() does not fully solve for that specific pattern. For a straightforward request/response style device, nonseekable_open() remains perfectly correct and is what you will see in the majority of simple drivers.

Common Mistakes to Avoid

• Not checking kzalloc()’s return value: memory allocation can fail under pressure; always handle it.
• Forgetting nonseekable_open() on position-less devices: leaves your driver silently accepting seek calls that make no sense.
• Doing heavy or blocking work in open(): keep open() fast; move expensive setup to a background workqueue if needed.
• Mismatched function signature: always match the exact prototype from your target kernel’s fs.h.

Best Practices

• Keep open() short and predictable; avoid long blocking operations.
• Free any temporary buffers before returning, and only keep permanent per-file state in filp->private_data if you need it across read/write calls.
• Always return a proper negative errno on failure, never a positive number.
• Log enough context (process name, PID) to make debugging painless without flooding the kernel log on every open.

Security Considerations

Because open() is the very first point of contact with your driver, it is a natural place to enforce access checks beyond the standard file permission bits, for example refusing to open the device more than once at a time if your hardware genuinely cannot support concurrent access. Never trust that a process opening your device has good intentions just because the file permissions allowed the open() call to reach your driver.

Frequently Asked Questions

1. What does the open() method actually receive from the kernel?
It receives a pointer to the inode representing the device file and a pointer to the file structure representing this specific open instance.

2. Why does my driver need nonseekable_open()?
It tells the kernel that lseek, pread and pwrite do not make sense for this device, so those calls fail cleanly instead of behaving unpredictably.

3. Can I allocate memory inside open()?
Yes, using GFP_KERNEL, since open() runs in a context that is allowed to sleep. Always check the returned pointer for NULL.

4. What is the difference between nonseekable_open() and stream_open()?
stream_open() is designed for devices where read and write may need to run concurrently on the same file without deadlocking on the file position lock; nonseekable_open() is simpler and suits most request/response style devices.

5. How do I find out which process opened my device?
Use the current macro, which points to the task_struct of the currently running process, and read its comm and pid fields.

6. Should open() ever return a positive number?
No. Return 0 on success or a negative errno value (such as -ENOMEM or -EBUSY) on failure. The file descriptor itself is managed entirely by the kernel.

Summary & Key Takeaways

• The VFS calls your driver’s open() function through the file_operations table whenever a process opens your device node.
• The function signature must match the kernel’s expected prototype exactly.
• Use GFP_KERNEL for allocations inside open(), since this context is allowed to sleep.
• Call nonseekable_open() for devices with no meaningful file position; consider stream_open() for true concurrent stream devices.

Conclusion

The open() method is deceptively small, but it sets the tone for everything else your driver does: how it logs, how it validates access, and how it prepares state for reads and writes to come. Once your open() handler is solid, you are ready to implement the read() and write() methods, which is where the real data movement between user space and your driver begins — and that is exactly what we cover in the next lecture of this free Linux kernel development course.