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Asked: 2026-06-18T02:27:43+00:00

I have a Linux standard header file e.g. /usr/src/linux-headers-3.2.0-35/include/linux/usbdevice_fs.h which contain define statements as

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I have a Linux standard header file e.g.

/usr/src/linux-headers-3.2.0-35/include/linux/usbdevice_fs.h

which contain define statements as follows:

#define USBDEVFS_SUBMITURB32       _IOR('U', 10, struct usbdevfs_urb32)
#define USBDEVFS_DISCARDURB        _IO('U', 11)
#define USBDEVFS_REAPURB           _IOW('U', 12, void *)

What does ‘_IOR’, ‘_IO’ and ‘_IOW’ mean? What value is actually given e.g. to USBDEVFS_DISCARDURB?

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1 Answer

  1. Editorial Team

    They define ioctl numbers, based on ioctl function and input parameters.
    The are defined in kernel, in include/asm-generic/ioctl.h.

    You need to include <linux/ioctl.h> (or linux/asm-generic/ioctl.h) in your program. Before including
    /usr/src/linux-headers-3.2.0-35/include/linux/usbdevice_fs.h

    You can’t “precompile” this values (e.g. USBDEVFS_DISCARDURB), because they can be different on other platforms. For example, you are developing your code on plain old x86, but then someone will try to use it on x86_64/arm/mips/etc. So you should always include kernel’s ioctl.h to make sure, you are using right values.

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