IOException when reading few bytes from /dev/input

Question

When trying to read input_events from /dev/input/event16 I noticed that the size of the buffer I'm reading into may cause an exception. Here is the code I wrote:

    public static void main(String[] args) throws IOException{
        FileInputStream is = new FileInputStream("/dev/input/event16");
        byte[] three_bytes = new byte[3];
        byte[] twentyfour_bytes = new byte[24];
        is.read(three_bytes); // fails
        is.read(twentyfour_bytes); // does not fail

    }

My initial experiments suggest that the buffer needs capacity for at least one full input_event struct. But I could not find out why.

The problem is the line is.read(three_bytes); causes the follwoing exception:

Exception in thread "main" java.io.IOException: Invalid argument
        at java.base/java.io.FileInputStream.readBytes(Native Method)
        at java.base/java.io.FileInputStream.read(FileInputStream.java:249)
        at main.Test.main(Test.java:11)

I would like to figure out why the line is.read(three_bytes); throws the exception while is.read(twentyfour_bytes); reads the data as expected

I'm not sure if I understand you correctly. Do you want to programmatically get the size of /dev/input/event16? — zhh
– zhh, Commented Jun 8, 2021 at 8:49
It's still not clear to me what you are going to find out. It seems you can read the file with a 24 bytes array, why not just use the larger array? — zhh
– zhh, Commented Jun 8, 2021 at 9:03

Stephen C · Accepted Answer · 2021-06-16 07:24:51Z

I would like to figure out why the line is.read(three_bytes); throws the exception while is.read(twentyfour_bytes); reads the data as expected.

For a start, /dev/input/event16 is not a regular file. It is a device file, and device files often don't behave like regular files.

In this case, the /dev/input/event* device files are for reading events from input devices. When you perform a read syscall on them, they will return one or more complete events. These are binary data whose format is given by the following C struct:

struct input_event {    
    struct timeval time;    
    unsigned short type;
    unsigned short code;
    unsigned int value; 
};

The size of that struct is (presumably) 24 bytes on a typical 64bit Linux system, though.

My initial experiments suggest that the buffer needs capacity for at least one full input_event struct. But I could not find out why.

The behavior of the event devices is documented in Linux Input drivers v1.0, section 5. It states that a read will always give a whole number of input_event structs.

It follows that the read syscall provides a buffer that is less than sizeof(input_event), the kernel cannot return anything. Apparently, this causes the read syscall to fail with errno value EINVAL. (IMO, this is a reasonable design choice, and consistent with the documented meaning of EINVAL.)

So, in Java, when you call read(three_bytes), that will map to a read syscall with a read size of 3 bytes which fails; see above. The syscall failure is signaled to the Java application by throw an IOException.

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