CPS 346 & 444/544 Lecture notes: Low-level I/O


Coverage: [UPE] §2.7 (pp. 65-70) and §§7.1-7.2 (pp. 201-214), and [USP] §2.4 (pp. 26-29) and §§4.1-4.6 (pp. 91-128)

Overview

  • file descriptors
    • 0 (STDIN_FILENO): stdin
    • 1 (STDOUT_FILENO): stdout
    • 2 (STDERR_FILENO): stderr
  • file descriptor table
    • resides in user program area
    • entry removed per close
  • system file table
    • resides in kernel area
    • shared by all processes
    • one entry per active open; fork?
    • each entry contains an offset for the open file
    • entry deleted when pointer count becomes 0
  • in-memory inode table
    • resides in kernel area
    • entry deleted when pointer count becomes 0
flushed (i.e., write called) when full



(regenerated from [USP] Fig. 4.2, p. 120)

Context switching (library vs. system calls)

  • develop iterative versions of UNIX cat
    • (verion 1: catv1.c) using only standard I/O and library functions getchar/putchar
    • (verion 2: catv2.c) using only standard I/O and library functions fread/fwrite with a buffer of size 1
    • (verion 3: catv3.c) using the system calls read/write with a buffer of size 1
  • compile (to cat1, cat2, and cat3) and time these programs (using /usr/bin/time) on a large input stream redirecting the output to the system's trashcan (/dev/null); what do you notice?
  • experimentation
    • modify versions 2 (catv2.c) and 3 (catv3.c) so that they take the buffer size as a command-line argument and call these version 4 (catv4.c) and version 5 (catv5.c), respectively
    • increase the buffer size
    • what do you notice?
    • at what buffer size does the performance begin to remain constant?
    • now time the UNIX cat command. what can you infer?

Self-exercise: code the UNIX cp command

  • here's a solution (courtesy William Kimball, CPS 445, Winter 2006), which, by default (i.e., without requiring the -p option), only preserves the owner's permissions on the source in the target; how to copy while preserving the permissions of group and other?
  • here's another solution (courtesy Matt Mize, CPS 445, Winter 2006), which does not preserve permissions (i.e., it behaves like cp without any options)

Restart library

  • [USP] example 2.8 on p. 29: close
  • [USP] example 2.9 on p. 29: r_close

Character and block special files in UNIX

  • files which represent hardware devices are located in /dev
  • character special file 
    $ ls -l /devices/pci@1e,600000/ide@d/dad@0,0:a,raw
crw-r-----   1 root     sys      136,  8 Feb 19 17:47 /devices/pci@1e,600000/ide@d/dad@0,0:a,raw
  • block special file 
    $ ls -l /devices/pci@1e,600000/ide@d/dad@0,0:a
brw-r-----   1 root     sys      136,  8 Feb 19 02:33 /devices/pci@1e,600000/ide@d/dad@0,0:a
  • read and write devices just as we do files
  • for more information, see Chapter 7: `Device and Network Interfaces' of the UNIX manual ($ man -s 7 intro)

I/O recap

  • what is the difference between the library function and a system call? UNIX as opposed to C
  • what is the difference between a FILE* and a file descriptor?
    • UNIX as opposed to C
    • both referred to as a handle
    • former involves another level of indirection (handle to handle)
  • read, write, open, close
  • hint on opening files: open(path, O_WRONLY | O_CREAT | O_TRUNC, mode) = creat(const char *path, mode_t mode) because former is so common

select and poll

  • handling I/O from multiple sources
  • non-blocking I/O
  • select: organizes parameters by type of condition (e.g., read ready or writeready)
  • poll: organizes parameters by file descriptor

References

    [UPE] B.W. Kernighan and R. Pike. The UNIX Programming Environment. Prentice Hall, Upper Saddle River, NJ, Second edition, 1984.
    [USP] K.A. Robbins and S. Robbins. UNIX Systems Programming: Concurrency, Communication, and Threads. Prentice Hall, Upper Saddle River, NJ, Second edition, 2003
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