January 25, 2006

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Process Wrap-up

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• fork
  • called once, returns twice
  • initial value of
    • child's data is parent's data at time of fork
    • child's PC is instruction immediately following fork
  • child and parent have same
    • user id
    • group id
    • process group id
    • root directory
    • current working directory
    • signal handling settings
    • file mode creation mask
  • they differ in
    • process id
    • parent process id
  • child and parent share
    • data
    • open files
      • typically parent closes file after fork to avoid sharing file
      • though can be used as a communication mechanism
    • do parent and child share program text in main memory?
  • they do not share
    • file descriptors
      • child has its own copies of parent's file descriptors
  • when is it convenient to have exact copies of parent process in memory?
    • consider a network server process; might copy itself to handle a request
  • how is fork executed?
• how is execlp executed?
• UNIX init, login, and command shell processes
• background processes
  • call setsid to prevent the process from receiving any signals as a result of a ctrl-c from a controlling terminal
  • interesting questions
    • (courtesy Matt Mize) since parent does not wait for a background process, does the init process wait for it?
    • (courtesy Matt Mize) does running a process in the background change the parent pid of the background process?
    • can we pass an & to the execl family of functions to run the swapped in process in the background?

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Variable Argument Lists in C

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• use ellipses in prototype/header
  • e.g., int printf (char *fmt, ...)
  • means number and type may vary across calls
  • ellipses must come at the end
• to step through arguments
  • declare variable of type va_list in function
    • va_list ap;
  • initialize ap with va_start
    • va_start (ap);
  • then call va_arg with ap and a datatype to retrieve a value of that type
    • va_arg (ap, int);
  • call va_end with ap after all arguments have been processed, but before the function returns
    • va_end (ap);
  • if variable types are used, use a switch to control the particular call made to va_arg
• typically pass the number of variable arguments as a parameter to the function
• necessary macro, datatypes, and functions are declared in stdarg.h
• another use: operator node in a parse tree (with a variable number of operands)
  • see pp. 21, 23 of Thomas Niemann's Compact Guide to Lex and Yacc
• other things discussed
  • unions

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Introduction to I/O in UNIX

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• file descriptors
  • 0 - stdin
  • 1 - stdout
  • 2 - stderr
• develop iterative versions of UNIX cat
  1. using only standard i/o and library functions getchar/putchar
  2. using only standard i/o and library functions fread/fwrite with a buffer of size 1
  3. using the system calls read/write with a buffer of size 1
• compile and time these programs (using time) on a large input stream redirecting the output to the system's trashcan (/dev/null)
  • what do you notice?
• self-experimentation
  • modify versions 2 and 3 to so that they take the buffer size as a command-line argument
  • 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?