February 1, 2006

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Code Solution to Problem 10 of Exam 1 tested in Win32

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• buffer overflow problem
• setting last element of buffer to null character ('\0') is necessary, even in Win32
• code courtesy William Kimball

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Importance of Using Appropriate Types

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char c;

while ((c = getchar()) != EOF) {
   ...
}

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I/O Recap

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• read, write, open, close
• 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)
• self-exercise: code the UNIX cp command
  • here's one solution, courtesy William Kimball, 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, which does not preserve permissions (i.e., it behaves like cp without any options)

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UNIX I/O Data Structures

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• 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

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Buffered Output

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• flushed (i.e., write called) when full
• fflush and setvbuf
• disk i/o vs. terminal i/o
• full buffering vs. line buffering
  • stdout
• scanf flushes stdout buffer
• a return from main causes the buffers to be flushed
• printf followed by a fork
  • with and without a newline

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Redirection

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• setup for free by the shell
• stdout to a file
  • open disk file
  • copy file descriptor for disk file into entry for stdout
    • use dup2
      • involves an implicit close on the target of the copy
  • close disk file using its file descriptor

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Examples

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• open followed by fork
  • consequences?
• fork followed by open
  • consequences?

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Other Things Briefly Mentioned

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