Clarify ioctl third argument as untyped pointer

[wurrrrrrrrrr]

This pull request updates the documentation for the ioctl function, correcting the description of its third parameter. The original text incorrectly described it as being of type long. It is actually an untyped pointer to memory, traditionally declared as char *argp before void * was valid in C.

Upstream reference:https://man7.org/linux/man-pages/man2/ioctl.2.html

This change improves technical accuracy and helps developers understand how to properly pass data—particularly pointers to structures—from user space to the kernel using ioctl.

Summary by Bito

This pull request revises the ioctl function documentation to clarify the nature of the third argument. It corrects a common misconception by specifying that the parameter is a memory pointer rather than a numeric value. The updated explanation improves clarity for developers using ioctl interfaces in kernel modules and drivers.

Unit tests added: False

Estimated effort to review (1–5, lower is better): 2

[bito-code-review]

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[visitorckw]

This pull request updates the documentation for the ioctl function, correcting the description of its third parameter. The original text incorrectly described it as being of type long. It is actually an untyped pointer to memory, traditionally declared as char *argp before void * was valid in C.

Upstream reference:https://man7.org/linux/man-pages/man2/ioctl.2.html

Thanks for the PR.

However, IIUC, the man page you linked describes the C library's ioctl wrapper, not the kernel's system call interface.

In the Linux kernel, the third argument is of type unsigned long. If historically it used to be a different type, could you clarify when that changed (e.g., from which kernel version)?

Also, IIUC, while the third argument is often used as a pointer, it can also simply carry other types of data, such as an integer representing a command or a value.

[wurrrrrrrrrr]

Thanks for the detailed explanation!

I see — the man page I initially referred to describes the libc wrapper, where the third argument is traditionally documented as char *argp for historical reasons (before void * was introduced in C).
In the kernel syscall interface, the third argument is actually an unsigned long, which can carry either a pointer or a value depending on the ioctl command.

Thanks again for pointing this out.