5.3.5. The read and write system calls

• read and write data to device

• return values:
  • = count: the requested amount of bytes were read/written
  • < count: count bytes were read/written (no error!)
  • = 0: read: EOF, write: unspecified (old: equals EAGAIN)
  • < 0: error

• stdio library handles partial reads/writes

• carefull with userspace pointers!
  • could be unmapped in kernel space
  • could be swapped out
  • could be invalid!

API

read and write file operations:

ssize_t read(struct file *filp, char __user *buff, size_t count, loff_t *f_pos);
ssize_t write(struct file *filp, const char __user *buff, size_t count, loff_t *f_pos);

userspace access:

unsigned long copy_to_user(void __user *to, const void *from, unsigned long count);
unsigned long copy_from_user(void *to, const void __user *from, unsigned long count);