What a compiler is

ACompressed recap

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People write programs in high-level languages; processors run only machine code, so programs must be translated. A compiler translates the whole program once into a target program; an interpreter executes the source directly every run. In C, the compiler sits in a toolchain: preprocessor → compiler → assembler → linker → loader. Inside, the compiler has six phases: lexical, syntax, semantic (front end) and intermediate code, optimization, code generation (back end), all sharing a symbol table and an error handler. Each kind of error is caught by a specific phase, and some errors are invisible to every phase.

BMerged rule card

0.1.1 Programs, machine code and why we translate; compiler vs interpreter

• Compiler: whole source → equivalent target program, before running; reports source errors at compile time.
• Interpreter: source + input → results directly, statement by statement; no target program.
• Compiled code is usually faster; interpreted code is easier to change and test.
• Break-even: compile when the number of runs n > C / (i − r).
• Target can be machine code, assembly-like text, bytecode or another high-level language.

0.1.2 The toolchain: preprocessor, compiler, assembler, linker, loader

• Order: preprocessor → compiler → assembler → linker → loader.
• Files: .c → .i → .s → .o → executable.
• Preprocessor: #include, #define, textual only.
• Assembler output: relocatable object code with unresolved external references.
• Linker: joins objects + libraries, resolves references → executable; “undefined reference” errors.
• Loader (OS): copies into memory, relocates if needed, starts execution.

0.1.3 The phases of a compiler; symbol table; error handler; which phase catches which error

• Phases: lexical → syntax → semantic → intermediate code → optimizer → code generator.
• Front end (analysis) = first three, language-dependent; back end (synthesis) = last three, machine-dependent.
• Symbol table: one entry per name (type, location, …), shared by all phases.
• Errors: illegal character → lexical; shape/brackets/semicolon → syntax; undeclared/types/argument count → semantic; input-dependent → run time.
• A misspelled keyword is an identifier to the scanner.
• m languages × n machines: m × n compilers vs m + n parts.

CInterleaved GATE-style questions

Q1MCQ1 markToolchain

An “undefined reference to sqroot” message comes from the:

• (A) preprocessor
• (B) compiler’s syntax analyzer
• (C) linker
• (D) loader

Q2MCQ1 markError phase

int x = 3 @ 4; in C. The first phase to report it is:

• (A) lexical analysis
• (B) syntax analysis
• (C) semantic analysis
• (D) run time

Q3MSQ1 markCompiler vs interpreter

Which are true?

• (A) A compiler can translate one high-level language into another.
• (B) An interpreter finds every syntax error before the program starts running.
• (C) Compiled machine code depends on the processor family.
• (D) Java uses both compilation (to bytecode) and interpretation.

Q4NAT1 markMacro

Given #define DBL(x) x+x, what is 3*DBL(4)?

Numerical answer: ________

Q5MCQ1 markPhases

Which phase produces machine-independent code with temporaries such as t1, t2?

• (A) lexical analysis
• (B) semantic analysis
• (C) intermediate code generation
• (D) code generation

Q6MCQ2 marksError phase

Consider int a; float b; a = b + ;. Which phase reports the error?

• (A) lexical
• (B) syntax
• (C) semantic
• (D) none, it compiles

Q7NAT1 markBreak-even

Compiling takes 10 s, each compiled run 1 s, each interpreted run 3 s. Smallest number of runs where compiling is strictly cheaper in total?

Numerical answer: ________

Q8MSQ2 marksSymbol table

Which pieces of information are typically stored in a symbol table entry?

• (A) the name’s type
• (B) where the name was declared
• (C) the memory location assigned to it
• (D) the number of times the program will be run