This document discusses semantic analysis of C programs. It describes the stages of compiler design and focuses on the semantic analysis phase. The goals of semantic analysis are to check if the code forms sensible instructions, functions always return values, there are no unreachable statements, return types match function headers, and expressions are compatible with datatypes. The design of the system uses LEX, YACC, and Python to perform semantic analysis on an input C program and detect any semantic errors. Examples of semantic errors related to return values, unreachable code, and expression compatibility with datatypes are provided. Semantic analysis is important as it makes code more meaningful, flexible, and error-free while enabling more potential for optimization.

1. Semantic Analysis of a C program
Done under the guidance of Prof. Kiran P
Department of Computer Science
PES Institute of Technology
Done by
Aakar Tripathi - 01
Chandramouli S Sastry - 46
Darshan SJ - 51
Debarati Das - 52

2. Stages in Compiler Design
We Work on this stage !

3. Semantic Analysis Phase
Semantics of a language provide meaning to its constructs, like tokens and syntax
structure and help in interpreting symbols, their types, and their relations with each
other.
Semantic analysis is that phase in Compiler Design where we delve deep to check
whether the code we have written forms a sensible set of instructions in the
programming language.
This project aims at finding semantic errors when the given input is a C program.

4. Questions Pertaining to Semantic Analysis

5. The Questions We Answer :
Semantic
Analysis
Function
always
returns some
value
Unreachable
Statements
Function
Return Type
is compatible
with Header
Expression
Compatibility
with
Datatypes

6. Assumptions made in Design
1. The given code is syntactically valid (parenthesis matched, function call
statements are correct and so on.)
2. We assume that the only type of control structures used is the if condition.
3. The function name is of the form func[a-z0-9]*
4. Declarations can only be of the form Type ID; That is, multiple declarations on a
single line is not allowed.
5. Return statements can be of the form return ID; It shouldn’t have any
expressions.

7. Design of the System
LEX YACC
PYTHON Program
Input C Program
Semantic Errors

8. Compatibility of Returned Value with Return Type
int func1(int c)
{
int x;
float y;
if(c==1)
return x;
else
return y;
//y is of type float. But return type should be
int.
}

9. Handling No Unreachable Statements and Function
should always return some value. int func1(int a,int b,int c)
{
if(c==1)
return a+b;
else if(c==2)
return b-a;
else if(c==3)
return a-b;
//semantic error->nothing
may be returned
}
**************************************************
int func2(int a,int b,int c)
{
if(c==1)
return a+b;
else
return b-a;
//following statements are unreachable
printf(“here”);
if(c==3)
return a-b;

10. Example
int func1(int x)
{
if(x==1)
{
return x;
}
else
{
if(x==2)
{
return x;
}
}
}

12. 4. Expression Type Compatibility
int funcabc(int a,float b,double c)
{
int x;
x=10.5;//error
int y;
y=a+b+c+10.03f+100.505;//error
printf("%ds%d",x,y);
}

13. Need For Semantic Analysis
1. Code becomes more meaningful
2. Code has more potential for
Optimisation
3. Code becomes flexible and ERROR
free.