Prosjekt del II: Semantikkanalysator

Goal

You are going to make a parser recognising valid C- programs, checking whether the program is type correct, and outputting the syntax tree and symbol table for the program. If the program is invalid, an error message is to be printed.

You are to do the exercise in groups of two. (If there is an odd number of students, you should ask us. We accept one, but only one group of three.)

Deadline

You have to hand in your answers no later than Friday 26th Oct 2001 at 10:15 am. You may hand in to lecturer Jean Blair at the lecture, or in advance to group leader Ole Kåre Endresen.

Contents

The answers handed in must include:

• Print of the source code.
• Print of documentation
• Path to a directory on the departemnet system, where we can find all the classes we need to test your program.

The program

The program is to read source code for a C- program from standard in, and print the abstract syntax tree and symbol table to standard out after the program having finished. If errors are found in the source code, the program should report the error to standard err. We give some examples of what the output may look like (see below).

Feilrapportering

• When the parser finds errors during syntax analysis, it should determine the type and cause of the errors as exactly as possible.
• Each reference should be reported with reference to line number and column number in the source file.
• The parser should attempt to recover after an error.

We require the program to continue parsing after some errors:

• Errors in expression. Encountering for instance

       if ( a <= b <= c ) {
         foobar ( a + b ) ;
       }
     

  the program shall report an error in the condition, and continue parsing to analyse the statement block.
• Multiple declarations. If you write

       int foobar ;
       void foobar ( int b ) { ... } ;
     

  in the same block, the program should report that foobar is already defined, but continue. Each multiple declaration should cause but one error message, so if you call foobar both as a function and as a variable after this multiple declaration, the program should not report another error.

Symbol table

• All symbols in the program must be reported after analysis, not only the global ones.
• For each symbol, the program must report name and type.
• Remember that parameters to functions work as local variables.

In the next handin, you will need more attributes for functions and local variables, on top of those required here. It may be wise to take this into account even at this point.

To handle local variables, you may use a stack of symbol tables. When you come to a compound statement, you add a new table on top of the stack, and when you reach the end, you pop the stack. Each node in the syntax tree may keep a reference to the symbol table stack which is valid in this particular node. Thus the symbol tables are available from the syntax tree, even if the parser routines throws them away. To print the symbol table, you may travers the syntax tree and print the top-most table in each node ending a scope.

Type checking

• All variables and functions must be declared before use.
• Function calls must have the right number of parameters, which must be of correct type.
• Assignments must be of compatible types (integers in practice).
• All programs must have a main function which takes no parameters and returns int.

Examples

A first example with syntax tree output.

Fibonacci

Two programs computing Fibonacci numbers:

1. #nor# Rekursivt: #eng# Recursive: fibrec.c-
2. #nor# Med tabellar: #eng# With arrays: fibarr.c-

Documentation

• Describe the structure of the program.
• Emphasise the parts requiring special consideration or where you had to make priorities.
• Avoid repeating descriptions, even when your code is repeated.
• Do not include general text about program analysis and restrict yourselves to your own code.
• Keep the documentation short. It is supposed to be easier reading than the source code is.
• If you use several classes, each of them must be described, and you have to tell how they are supposed to interact.
• You may use UML to describe part of the system if you desire.
• Make a running example where you describe how analysis and type checking is performed in real, on a simple sample of source code (perhaps with type mismatch). Include the source code which is used in this example. (If you plan the example ahead of time, it may help you in the implementation of the program.)
• The running sample may very well be used to illustrate the construction of the program.
• Make a print of a test run for your running example. You may want to use the script command to make this. (You find documentation with man script.) You may add more examples.

Hint

• Start with the simple things, and expand the program as you go.
• Write documentation and source code in parallel. Afterwards everything appears simple or forgotten.
• Start early. This takes time.
• Do not wait till the topics are covered in lecture, lest you lose time and fail to meet the deadline.
• There is no one way to solve the problem.
• Discuss the exercise with your partner and with other students, perhaps on the newsgroup.