CODEWITHSUNDEEP
linux-kernellinux-device-driver
Prajosh Premdas
Prajosh Premdas

Reputation: 988

Read system call gives wrong count size?

I have created a misc driver and has made a sample read function like this 

static ssize_t test_read(struct file *file, char __user *buffer,
        size_t count, loff_t *ppos)
{
    pr_info("Count arg : %d\n",count);

    return ret;
}

I now try to read the device using a userspace code as shown below

uint64_t read_buff;
fread(&read_buff, sizeof(read_buff), 1, fp)

The dmesg log I get is 

[ 1593.273163] Count arg : 4096

I was expecting it to be of the size of uint64_t. Could anybody point me why I get an unexpected value?

Upvotes: 0

Views: 139

Answers (1)

Oleksii Shmalko
Oleksii Shmalko

Reputation: 3778

Seems that fread() tries to buffer some data for userland. I found source code of one fread() that buffers data (in __srefill()). So, it's OK for fread() to do so.

If you want to avoid such unexpected results, lower one level down and work with read() function in userland.

Upvotes: 3

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