Question

I write a linux char device driver to simulate a file. The data is stored in an array and I want to implement a “read-file”-handler…

static ssize_t data_read(struct file *f, char __user *buf, size_t count, loff_t *f_pos){
char *msg_pointer;
int bytes_read = 0;

if(vault.storage==NULL)
    return -EFAULT;

msg_pointer = vault.storage + *f_pos;

while (count && (*f_pos < vault.size) ) {
    put_user(*(msg_pointer++), buf++);
    count--;
    bytes_read++;
    ++*f_pos;
}

return bytes_read;
}

vault.storage is a pointer to a kmalloc-creation. If I test the code by copying with dd it works as expected, but when I want to open the file with C

if((fp_data = open("/dev/vault0", O_RDWR)) < 0){
    perror("could not open file.\n");
}

err = write(fp_data, "ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890", 36);

if (err < 0){
    perror("failed to write to sv \n");
} 

read(fp_data, buffer, 36);
read(fp_data, buffer, 36);

the first read-command returns 4.. the second 0 – how is this possible?

Answer 1

write performed on a file is not guaranteed to write all the bytes requested atomically … that is only reserved for a pipe or FIFO when the requested write-amount is less than PIPE_BUF in size. For instance, write can be interrupted by a signal after writing some bytes, and there will be other instances where write will not output the full number of requested bytes before returning. Therefore you should be testing the number of bytes written before reading back any information into a buffer to make sure you are attempting to read-back the same number of bytes written.