Perl under the Hood

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Perl under the Hood
The University of North Carolina at Chapel Hill

• COMP 144 Programming Language Concepts
• Spring 2003

David Stotts Feb 4
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How does the Interpreter work?

• Perl gives program access to its symbol tables
• Variables are either global or lexical
• Names can refer to data and nondata
• spud, _at_spud, spud spud, spud (filehandle), all
  are the name spud used for different things
• Symbol table is a hash table with separate areas
  for each kind of thing
• A typeglob is a multi-tail pointer pile for
  each name in the hash table

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Symbol table structure
Typeglob ( spud )
Scalar value ( spud )
spud
List value (array) (_at_spud)
arf
Hash value (spud)
Code value (spud)
foo3
Filehandle (spud)
Format (ignore this)
Typeglob ( arf )
Symbol table (hashed)
Typeglob ( foo3 )
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Eval the Interpreter itself

• A Perl script can directly run a copy of its own
  interpreter
• One way theeval function
• while (line ltgt)
• str . line slarp in all lines and
  concat them
• eval str now run the interpreter on it
• str c a b
• a 10 b 15
• eval str treats value of str as
  code and executes it
• print c\n prints 25

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Interpreter (cont.)

• Check this out
• while (defined(s ltgt))
• result eval s
• if ( _at_ )
• print Invalid input string\n s
• else
• print result, \n
• Interactive Perl interpreter voila

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References

• S. Srinivasan, Advanced Perl Programming,
  OReilly, 1997