The Unix I/O Interface to the User

• Devices are considered to be part of the file system - mapped as special files : /dev/tty1 or /dev/lpr (the special case : /dev/null)
• Device independence :

cat file.txt > /dev/tty
cat file.txt > /dev/lpr
cat file.au > /dev/audio

• I/O Classes :
  Characters (unstructured) : files and devices
  Block (structured) : files
  Socket (message) : network interface
• I/O Calls are synchronous

fd = open(filename, mode)
close(fd)

nbytesread = read(fd,buffer,nbytes)
nbyteswritten = write(fd, buffer, nbytes)
pos = lseek(fd,offset,offsettype)

• where : fd : index to a file descriptor and mode : 0,1 or 2 (for read, write or read/write)
• offsettype : relative or absolute (bytes or 512 byte blocks)
• How are requests made to perform activities on devices ? How are these requests managed ?
• Answer: Need a File Manager process which performs these activities

Requests are made to the file manager via a request queue

  
Figure 3: Filemanager

Each descriptor within the request queue is as follows :

  
Figure 4: FileMan request queue

What additional information is needed for an open request ?