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Computer Engineering 175
Phase III: Symbol Table Construction
“Be angry when you will, it shall have scope.”
Shakespeare, Julius Caesar, Act IV
1 Overview
In this assignment, you will augment your parser to construct a symbol table for the Simple C language. This
assignment is worth 20% of your project grade. Your program is due at 11:59 pm, Sunday, February 12th.
2 Semantic Checking
To perform most semantic checks, your compiler must record information about an identifier (such as its type) at
the time of its declaration and then lookup that information when the identifier is used. The symbol table is the
central repository for all such information.
In Simple C, a type consists of a type specifier (char, int, long, or void) along with optional declarators (“function returning T,” “array of T,” and “pointer to T”). To manage identifiers, Simple C uses static nested scoping:
scopes can be nested hierarchically and when an identifier is used after being declared, the closest or innermost
declaration is used.
A local variable or parameter may be declared at most once in a scope. However, a global variable or function
may be declared multiple times in the global scope so long as all of the declarations are identical (including any
parameter types and array lengths). However, a function may be defined only once. The same identifier may be
used to declare different objects in different scopes.
3 Semantic Rules
3.1 Translation units
translation-unit → ϵ
| global-declaration translation-unit
| function-definition translation-unit
The scope of the translation unit (i.e., file) begins at the top of the file before any global-declaration orfunctiondefinition, and persists until the end of the file.
3.2 Function definitions
function-definition → specifier pointers id ( parameters ) { declarations statements }
specifier → int
| char
| long
| void
pointers → ϵ
| * pointers
The function is both declared and defined in the current translation unit. The scope of the function begins
immediately after the identifier and persists until the end of statements. The type of the function is “function
returning T,” where T has a specifier ofspecifier along with any pointer declarators specified as part of pointers. The
function must not have been previous defined [E1] and any previous declaration must be identical [E2], ignoring
the parameters as any previous declaration will have an unspecified parameter list.
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3.3 Parameters
parameters → void
| parameter-list
parameter-list → parameter
| parameter , parameter-list
parameter → specifier pointers id
Each parameter is declared in the current scope, and must not have been previously declared in the current
scope [E3]. The type of the parameter is that of specifier along with any pointer declarators specified as part of
pointers. A parameter of type void alone is illegal [E5].
3.4 Declarations
global-declaration → specifier global-declarator-list ;
global-declarator-list → global-declarator
| global-declarator , global-declarator-list
global-declarator → pointers id
| pointers id ( )
| pointers id [ num ]
declarations → ϵ
| declaration declarations
declaration → specifier declarator-list ;
declarator-list → declarator
| declarator , declarator-list
declarator → pointers id
| pointers id [ num ]
Each variable is declared in the current scope. If the variable is a global variable, then any previous declaration
must be identical [E2]. If the variable is a local variable, then the variable must not be previously declared in
the current scope [E3]. The type of the variable is that of specifier along with any specified pointer and array
declarators. A variable of type void or “array of void” is illegal [E5]. Variables of type “pointer to void” or “array of
pointer to void” are examples of legal declarations.
Each function is declared in the global scope. The type of the function is “function returning T,” where T has a
specifier of specifier along with any pointer declarators specified as part of pointers. Any previous declaration must
be identical [E2], ignoring any parameters specified as part of a previous function definition.
3.5 Statements
statement → { declarations statements }
The scope of the block begins before the declarations and persists until the end of the statements.
3.6 Expressions
primary-expression → id ( expression-list )
| id ( )
| id
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The identifier must be declared in the current scope or in an enclosing scope [E4]. Note that you are only
checking if the identifier in a function call expression is declared, and that the number and types of the arguments
are irrelevant for this assignment.
4 Assignment
You will design and implement a symbol table for Simple C by augmenting your parser, using the given rules as a
guide. You will only be given syntactically legal programs as input. Your compiler should indicate any errors by
writing the appropriate error messages to the standard error:
E1. redefinition of ’name ’
E2. conflicting types for ’name ’
E3. redeclaration of ’name ’
E4. ’name ’ undeclared
E5. ’name ’ has type void
Each error message must be prefixed with the line number in the form “line number: ”. The line number may
vary slightly from the examples. Any messages written to the standard output will be ignored. A function definition always replaces any previous declaration or definition, even if erroneous. However, an erroneous redeclaration of a function or variable is discarded and the original declaration kept. Note that global objects cannot yield
error [E3] only [E2], and that local objects cannot yield error [E2] only [E3]. If multiple error messages apply, issue
only the first error message listed.
5 Hints
The Standard Template Library provides several useful data structures. You will probably find it easiest to model
the nesting of scopes using a stack. (Either an explicit stack can be used, or each scope can maintain a pointer
to its enclosing scope, thus forming a linked-list of scopes.) Each scope itself can be implemented using a map
that associates the name of an identifier to information that includes its type. Such an object is called a symbol,
which usually has a name, type, and any other necessary information. A type can be modeled as a specifier, the
number of levels of indirection due to pointer declarators, and a declarator (scalar, array, or function). Develop
classes for types (Type.cpp), symbols (Symbol.cpp), and scopes (Scope.cpp). Finally, develop a separate module
(checker.cpp) for performing the semantic checks.
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