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Storage Classes extern and static					[TOC]
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---- VIDEO ------------------------------
https://www.youtube.com/embed/5BFV85znXyQ
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Exercise: Lexer for the _abc_ Compiler
======================================
We have to apply some improvements to our compiler project that we started in
__Session 13, Page 4__:

- The lexer should also give as information about the line number and column in
  which a token was found. It should also be possible to retrieve the current
  token directly. And yes, we use here global variables declared in a header
  (in the end only _lexer.h_ will declare a global variable, and just one).

  The header was therefore modified as follows (we clearly need _struct_ and
  _enum_ soon):

  :import: session14/abc/lexer.h

- This is the modified test program. Just a line of code was added to also print
  the token position:

  :import: session14/abc/xtest_lexer.c

- In _lexer.c_ all functions and global variables that are not declared in the
  header should be declared as static.
  
---- BOX -----------------------------------------------------------------------
  
*Of course the real deal here is:* Change the implementation in _lexer.c_ so
that after calling _getToken()_ the values of the global variables declared in
~lexer.h~ are correct.

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Some Example
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You can use a file like this to test you lexer:

---- CODE (file=test_lexter.in) ------------------------------------------------
a = 5;
b = 42;
c = (a + b) *2;
123 0123 0xaB12 abc +-/*%^()
--------------------------------------------------------------------------------

Then with ~xtest_test~ you should get:

---- CODE (type=txt) -----------------------------------------------------------
theon$ ./xtest_lexer < test_lexer.in
1.1: IDENTIFIER
1.3: EQUAL
1.5: DEC_LITERAL
1.6: SEMICOLON
2.1: IDENTIFIER
2.3: EQUAL
2.5: DEC_LITERAL
2.7: SEMICOLON
3.1: IDENTIFIER
3.3: EQUAL
3.5: LPAREN
3.6: IDENTIFIER
3.8: PLUS
3.10: IDENTIFIER
3.11: RPAREN
3.13: ASTERISK
3.14: DEC_LITERAL
3.15: SEMICOLON
4.1: DEC_LITERAL
4.5: OCT_LITERAL
4.10: HEX_LITERAL
4.17: IDENTIFIER
4.21: PLUS
4.22: MINUS
4.23: SLASH
4.24: ASTERISK
4.25: PERCENT
4.26: BAD_TOKEN
4.27: LPAREN
4.28: RPAREN
--------------------------------------------------------------------------------

Once we double checked that the lexer is handling lines and columns correctly we
can save the output in a file:

---- CODE (type=txt) -----------------------------------------------------------
theon$ ./xtest_lexer < test_lexer.in > test_lexer.ref.out
--------------------------------------------------------------------------------

Later we can use in the makefile a target _check_ so that _make check_ simply
comares in furture the result with this trusted output. Basically this target
will do in this case a _diff_:

---- CODE (type=txt) -----------------------------------------------------------
theon$ ./xtest_lexer < test_lexer.in > test_lexer.out
theon$ diff test_lexer.out test_lexer.ref.out
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:links: Session 13, Page 4 -> doc:session13/page04