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How do I know whether a disk does a READ or a WRITE?

This is a discussion on How do I know whether a disk does a READ or a WRITE? within the Linux Programming forums, part of the Platform Specific Boards category; There's struct request , which represents a request by the hard disk. The question is, how can I get info ...

  1. #1
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    How do I know whether a disk does a READ or a WRITE?

    There's struct request, which represents a request by the hard disk. The question is, how can I get info on whether the disk does a read or a write?

    Your help will be much appreciated!

  2. #2
    C++ Witch laserlight's Avatar
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    As far as I know, there is no struct request in the C standard library. I am going to hazard a guess as to the context that you did not provide: *moved to Linux programming forum*
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  3. #3
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    Good call, laserlight.

    The struct request defined in include/linux/blkdev.h file in the Linux kernel sources has a field unsigned int cmd_flags (line 102 or so). If (cmd_flags & REQ_WRITE) is true the request is a write request, otherwise it is a read request.

    This is documented in the comments for the enum rq_flag_bits type in the include/linux/blk_types.h file (line 140 or so) in the kernel sources.
    Xpl0ReRChR likes this.

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    Thank you Nominal Animal, you have been most helpful!

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