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Checking if a file descriptor is valid

This is a discussion on Checking if a file descriptor is valid within the Networking/Device Communication forums, part of the General Programming Boards category; Hey, does anybody know how to check whether the file descriptor is valid?...

  1. #1
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    Checking if a file descriptor is valid

    Hey, does anybody know how to check whether the file descriptor is valid?

  2. #2
    int x = *((int *) NULL); Cactus_Hugger's Avatar
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    You might need to clarify that question a bit...

    Functions like open() will return a certain value (-1) for failure. Otherwise, the easiest way is that it's up to you to keep track of file descriptors.

    Alternatively, if you're in Linux, you could read your process's dir in /proc, but again, this all depends - what are you wanting to do?
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    there are two ways to achieve this:

    1. check for the return value when returned from open() system call.

    Code:
    int fdesc;
    
    if(fdesc = open(//arguments) == -1)
    {
        perror("open");
    }

    2. the other way is ioctl() system calls and check the inode file structure.

    Hope i answered your query...

  4. #4
    Cat without Hat CornedBee's Avatar
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    To just see if a file descriptor is valid you could use this:
    Code:
    int is_valid_fd(int fd)
    {
        return fcntl(fd, F_GETFL) != -1 || errno != EBADF;
    }
    fcntl(GETFL) is probably the cheapest and least likely to fail operation you can perform on a file descriptor, so I've chosen it. In particular, the specification suggests that it cannot be interrupted by signals, nor is it affected by any sort of lock held anywhere.
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    CornedBee

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