Types of Errors in C

Every programmer encounters errors — they're a natural part of learning and development. Understanding different types of errors helps you debug faster and write more reliable code. In this tutorial, we'll explore all error types in C with practical examples.

Why Understanding Errors Matters

•Faster debugging: Know where to look when something goes wrong
•Better code: Avoid common mistakes before they happen
•Confidence: Error messages become helpful, not scary

Classification of Errors

When Are Errors Detected?

Compile Time

While compiling

Syntax Errors•Semantic Errors

Link Time

Combining files

Linker Errors

Runtime

While running

Runtime Errors•Logical Errors

Earlier detection = Easier to fix ✓

Syntax Errors

Grammar mistakes

Missing ; or )

Easy ✓

Semantic Errors

Meaning mistakes

Wrong type used

Medium

Linker Errors

Missing definitions

undefined reference

Medium

Runtime Errors

Crashes while running

Segmentation fault

Hard

Logical Errors

Wrong output

Bug in algorithm

Hardest ✗

Unreachable Code

Dead code warning

Code after return

Easy ✓

Error Type	When Detected	Detected By	Difficulty to Fix
Syntax Error	Compile Time	Compiler	Easy
Semantic Error	Compile Time	Compiler	Medium
Linker Error	Link Time	Linker	Medium
Runtime Error	Execution	OS/Program	Hard
Logical Error	Output Analysis	Programmer	Hardest
Unreachable Code	Compile Time (Warning)	Compiler	Easy

1. Syntax Errors (Compile-Time Errors)

Syntax errors occur when you violate the grammar rules of C. The compiler cannot understand your code and refuses to compile it. These are the easiest to fix because the compiler tells you exactly where the problem is.

Common Syntax Errors:

Missing Semicolon

missing_semicolon.c

1#include <stdio.h>
2
3int main() {
4    int x = 5    // ERROR: Missing semicolon!
5    printf("x = %d", x);
6    return 0;
7}

error: expected ';' before 'printf'

Undeclared Variable

undeclared_variable.c

1#include <stdio.h>
2
3int main() {
4    y = 10;  // ERROR: 'y' was never declared!
5    printf("%d", y);
6    return 0;
7}

error: 'y' undeclared (first use in this function)

Mismatched Brackets

mismatched_brackets.c

1#include <stdio.h>
2
3int main() {
4    if (5 > 3) {
5        printf("Five is greater");
6    // ERROR: Missing closing brace!
7    return 0;
8}

error: expected declaration or statement at end of input

Correct Version

correct_syntax.c

1#include <stdio.h>
2
3int main() {
4    int x = 5;  // Semicolon added
5    int y;      // Variable declared
6    y = 10;
7    
8    if (5 > 3) {
9        printf("Five is greater");
10    }  // Closing brace present
11    
12    return 0;
13}

2. Semantic Errors

Semantic errors occur when the syntax is correct but the meaning doesn't make sense. It's like saying "I ate the color blue" — grammatically correct but meaningless.

Type Mismatch

type_mismatch.c

1#include <stdio.h>
2
3int main() {
4    int num = "Hello";  // Assigning string to int!
5    
6    float price = 10.5;
7    int *ptr = price;   // Assigning float to pointer!
8    
9    return 0;
10}

error: incompatible types when initializing type 'int' using type 'char *'

Wrong Number of Arguments

wrong_args.c

1#include <stdio.h>
2
3void greet(char *name, int age) {
4    printf("Hello %s, age %d\n", name, age);
5}
6
7int main() {
8    greet("Alice");  // Missing second argument!
9    return 0;
10}

error: too few arguments to function 'greet'

3. Linker Errors

Linker errors occur after compilation, when the linker tries to combine object files. Usually caused by missing function definitions or multiple definitions.

Undefined Reference

undefined_ref.c

1#include <stdio.h>
2
3void sayHello();  // Declaration only, no definition!
4
5int main() {
6    sayHello();  // Linker can't find the function body!
7    return 0;
8}

error: undefined reference to 'sayHello'

Fix: Add Function Definition

linker_fixed.c

1#include <stdio.h>
2
3void sayHello();  // Declaration
4
5int main() {
6    sayHello();
7    return 0;
8}
9
10// Function definition added
11void sayHello() {
12    printf("Hello, World!\n");
13}

4. Runtime Errors

Runtime errors occur while the program is running. The code compiles successfully, but crashes or behaves unexpectedly during execution. These are harder to debug because they depend on specific conditions.

Division by Zero

division_zero.c

1#include <stdio.h>
2
3int main() {
4    int a = 10;
5    int b = 0;
6    
7    int result = a / b;  // CRASH! Division by zero!
8    
9    printf("Result: %d\n", result);
10    return 0;
11}

Runtime Error: Floating point exception (core dumped)

Array Out of Bounds

array_bounds.c

1#include <stdio.h>
2
3int main() {
4    int arr[5] = {1, 2, 3, 4, 5};
5    
6    // Valid indices: 0, 1, 2, 3, 4
7    printf("%d\n", arr[10]);  // Index 10 doesn't exist!
8    
9    arr[100] = 999;  // Writing to invalid memory!
10    
11    return 0;
12}

Runtime Error: Segmentation fault (core dumped)

Null Pointer Dereference

null_pointer.c

1#include <stdio.h>
2#include <stdlib.h>
3
4int main() {
5    int *ptr = NULL;
6    
7    *ptr = 100;  // CRASH! Can't write to NULL address!
8    
9    return 0;
10}

Runtime Error: Segmentation fault (core dumped)

How to Prevent Runtime Errors

prevent_runtime.c

1#include <stdio.h>
2#include <stdlib.h>
3
4int main() {
5    int a = 10;
6    int b = 0;
7    
8    // Check before dividing
9    if (b != 0) {
10        int result = a / b;
11        printf("Result: %d\n", result);
12    } else {
13        printf("Error: Cannot divide by zero!\n");
14    }
15    
16    // Check array bounds
17    int arr[5] = {1, 2, 3, 4, 5};
18    int index = 3;
19    if (index >= 0 && index < 5) {
20        printf("arr[%d] = %d\n", index, arr[index]);
21    }
22    
23    // Check malloc result
24    int *data = malloc(sizeof(int) * 10);
25    if (data != NULL) {
26        data[0] = 5;
27        free(data);
28    } else {
29        printf("Memory allocation failed!\n");
30    }
31    
32    return 0;
33}

5. Logical Errors

Logical errors are the hardest to find. The program compiles and runs without crashing, but produces wrong results. The compiler can't help you — you must trace through the logic yourself.

Off-by-One Error

off_by_one.c

1#include <stdio.h>
2
3int main() {
4    int sum = 0;
5    
6    // Goal: Sum numbers 1 to 5 (should be 15)
7    for (int i = 1; i < 5; i++) {  // Should be i <= 5
8        sum += i;
9    }
10    
11    printf("Sum of 1-5: %d\n", sum);  // Prints 10, not 15!
12    
13    return 0;
14}

Output: Sum of 1-5: 10
Expected: Sum of 1-5: 15

Using = Instead of ==

assignment_vs_comparison.c

1#include <stdio.h>
2
3int main() {
4    int x = 5;
5    
6    // BUG: This ASSIGNS 10 to x, doesn't compare!
7    if (x = 10) {  // Should be x == 10
8        printf("x is 10\n");  // This always prints!
9    }
10    
11    printf("x is now: %d\n", x);  // x is now 10!
12    
13    return 0;
14}

Output: x is 10
x is now: 10
Bug: x was changed from 5 to 10!

Debugging Tips for Logical Errors

1.Add printf() statements to trace variable values
2.Use a debugger (gdb) to step through code line by line
3.Test with simple, known inputs first
4.Check boundary conditions (first, last, empty, max)
5.Read your code out loud — sometimes you'll catch the bug

6. Unreachable Code Error

Unreachable code is code that will never execute due to the control flow. The compiler detects this and issues a warning or error.

Common Causes:

Code After Return Statement

unreachable_return.c

1#include <stdio.h>
2
3int main() {
4    printf("Hello!\n");
5    
6    return 0;  // Function exits here!
7    
8    // UNREACHABLE - Never executed!
9    printf("Goodbye!\n");
10}

Warning: statement is unreachable

Code After Break Statement

unreachable_break.c

1#include <stdio.h>
2
3int main() {
4    int value = 2;
5    
6    for (;;) {  // Infinite loop
7        if (value == 2) {
8            break;  // Exit loop immediately!
9            
10            // UNREACHABLE - Never executed!
11            printf("This never prints\n");
12        }
13    }
14    
15    return 0;
16}

Code After Continue Statement

unreachable_continue.c

1#include <stdio.h>
2
3int main() {
4    for (int i = 0; i < 5; i++) {
5        continue;  // Jump to next iteration!
6        
7        // UNREACHABLE - Never executed!
8        printf("Iteration %d\n", i);
9    }
10    
11    printf("Loop complete\n");
12    return 0;
13}

Always-False Conditions

unreachable_condition.c

1#include <stdio.h>
2#include <math.h>
3
4int main() {
5    // sqrt(2) is about 1.414, never greater than 5
6    double x = sqrt(2);
7    
8    if (x > 5) {  // 1.414 > 5 is ALWAYS false!
9        // UNREACHABLE
10        x++;
11    }
12    
13    // 2 + 1 = 3, never equals 4
14    int n = 2 + 1;
15    
16    if (n == 4) {  // 3 == 4 is ALWAYS false!
17        // UNREACHABLE
18        printf("Equal\n");
19    }
20    
21    return 0;
22}

!Code Pitfalls: Common Mistakes & What to Watch For

Copied code can categorize and explain error types but often fails to teach the proactive mindset needed to prevent them in the first place.

If you search online "what are the types of errors in C?", it will give you a textbook answer: syntax, semantic, runtime, logical, linker errors. However, it rarely emphasizes that the *order* of detection matters hugely for debugging effort (compile-time is cheap, runtime is expensive) or that defensive programming habits can prevent most runtime and logical errors before they occur.

The Trap: Online sources provide reactive information—what errors *are*—rather than proactive strategies—how to *avoid* them. They might not emphasize the importance of compiler warnings (`-Wall -Wextra`), static analyzers, or coding conventions that prevent common mistakes. Learning about errors without learning prevention is like studying diseases without studying hygiene.

The Reality: The best programmers aren't those who can fix all errors; they're those who write code that rarely has errors. Use resources to understand error types, but develop habits that prevent them: enable all compiler warnings, initialize all variables, check all return values, and use static analysis tools regularly.

Frequently Asked Questions

Q:What's the easiest type of error to fix?

A: Syntax errors! The compiler tells you exactly what's wrong and on which line. Read the error message, fix the typo or missing semicolon, and recompile.

Q:Why do I get "undefined reference" errors?

A: Linker can't find the function definition. Either you forgot to link a library (-lm for math), misspelled a function name, or didn't include the source file in compilation.

Q:How do I find logical errors?

A: These are hardest because the program runs fine but gives wrong results. Use a debugger to step through code, add printf statements to trace values, and test with known inputs.

Q:Should I treat warnings as errors?

A: Yes! Use -Werror to make warnings fail compilation. Most warnings indicate real bugs — fix them before they become runtime errors.

Summary: Error Detection Checklist

Compile-Time Errors

Check for missing semicolons
Declare variables before use
Match all brackets
Include required headers

Linker Errors

Define all declared functions
Link required libraries (-lm)
Check for typos in function names
Include all source files in build

Runtime Errors

Check divisor is not 0 before dividing
Validate array indices
Check malloc return value
Initialize pointers before use

Logical Errors

Use == for comparison, not =
Check loop boundaries (off-by-one)
Verify operator precedence
Test with edge cases

Best Practices to Minimize Errors

1.Compile with -Wall -Wextra flags
2.Fix warnings — they often indicate bugs
3.Test incrementally as you write code

4.Use a debugger (gdb) for runtime issues
5.Initialize all variables
6.Always check return values

Why Understanding Errors Matters

•Faster debugging: Know where to look when something goes wrong
•Better code: Avoid common mistakes before they happen
•Confidence: Error messages become helpful, not scary

Classification of Errors

When Are Errors Detected?

Compile Time

While compiling

Syntax Errors•Semantic Errors

Link Time

Combining files

Linker Errors

Runtime

While running

Runtime Errors•Logical Errors

Earlier detection = Easier to fix ✓

Syntax Errors

Grammar mistakes

Missing ; or )

Easy ✓

Semantic Errors

Meaning mistakes

Wrong type used

Medium

Linker Errors

Missing definitions

undefined reference

Medium

Runtime Errors

Crashes while running

Segmentation fault

Hard

Logical Errors

Wrong output

Bug in algorithm

Hardest ✗

Unreachable Code

Dead code warning

Code after return

Easy ✓

Error Type	When Detected	Detected By	Difficulty to Fix
Syntax Error	Compile Time	Compiler	Easy
Semantic Error	Compile Time	Compiler	Medium
Linker Error	Link Time	Linker	Medium
Runtime Error	Execution	OS/Program	Hard
Logical Error	Output Analysis	Programmer	Hardest
Unreachable Code	Compile Time (Warning)	Compiler	Easy

1. Syntax Errors (Compile-Time Errors)

Common Syntax Errors:

Missing Semicolon

missing_semicolon.c

1#include <stdio.h>
2
3int main() {
4    int x = 5    // ERROR: Missing semicolon!
5    printf("x = %d", x);
6    return 0;
7}

error: expected ';' before 'printf'

Undeclared Variable

undeclared_variable.c

1#include <stdio.h>
2
3int main() {
4    y = 10;  // ERROR: 'y' was never declared!
5    printf("%d", y);
6    return 0;
7}

error: 'y' undeclared (first use in this function)

Mismatched Brackets

mismatched_brackets.c

1#include <stdio.h>
2
3int main() {
4    if (5 > 3) {
5        printf("Five is greater");
6    // ERROR: Missing closing brace!
7    return 0;
8}

error: expected declaration or statement at end of input

Correct Version

correct_syntax.c

1#include <stdio.h>
2
3int main() {
4    int x = 5;  // Semicolon added
5    int y;      // Variable declared
6    y = 10;
7    
8    if (5 > 3) {
9        printf("Five is greater");
10    }  // Closing brace present
11    
12    return 0;
13}

2. Semantic Errors

Semantic errors occur when the syntax is correct but the meaning doesn't make sense. It's like saying "I ate the color blue" — grammatically correct but meaningless.

Type Mismatch

type_mismatch.c

1#include <stdio.h>
2
3int main() {
4    int num = "Hello";  // Assigning string to int!
5    
6    float price = 10.5;
7    int *ptr = price;   // Assigning float to pointer!
8    
9    return 0;
10}

error: incompatible types when initializing type 'int' using type 'char *'

Wrong Number of Arguments

wrong_args.c

1#include <stdio.h>
2
3void greet(char *name, int age) {
4    printf("Hello %s, age %d\n", name, age);
5}
6
7int main() {
8    greet("Alice");  // Missing second argument!
9    return 0;
10}

error: too few arguments to function 'greet'

3. Linker Errors

Linker errors occur after compilation, when the linker tries to combine object files. Usually caused by missing function definitions or multiple definitions.

Undefined Reference

undefined_ref.c

1#include <stdio.h>
2
3void sayHello();  // Declaration only, no definition!
4
5int main() {
6    sayHello();  // Linker can't find the function body!
7    return 0;
8}

error: undefined reference to 'sayHello'

Fix: Add Function Definition

linker_fixed.c

1#include <stdio.h>
2
3void sayHello();  // Declaration
4
5int main() {
6    sayHello();
7    return 0;
8}
9
10// Function definition added
11void sayHello() {
12    printf("Hello, World!\n");
13}

4. Runtime Errors

Division by Zero

division_zero.c

1#include <stdio.h>
2
3int main() {
4    int a = 10;
5    int b = 0;
6    
7    int result = a / b;  // CRASH! Division by zero!
8    
9    printf("Result: %d\n", result);
10    return 0;
11}

Runtime Error: Floating point exception (core dumped)

Array Out of Bounds

array_bounds.c

1#include <stdio.h>
2
3int main() {
4    int arr[5] = {1, 2, 3, 4, 5};
5    
6    // Valid indices: 0, 1, 2, 3, 4
7    printf("%d\n", arr[10]);  // Index 10 doesn't exist!
8    
9    arr[100] = 999;  // Writing to invalid memory!
10    
11    return 0;
12}

Runtime Error: Segmentation fault (core dumped)

Null Pointer Dereference

null_pointer.c

1#include <stdio.h>
2#include <stdlib.h>
3
4int main() {
5    int *ptr = NULL;
6    
7    *ptr = 100;  // CRASH! Can't write to NULL address!
8    
9    return 0;
10}

Runtime Error: Segmentation fault (core dumped)

How to Prevent Runtime Errors

prevent_runtime.c

1#include <stdio.h>
2#include <stdlib.h>
3
4int main() {
5    int a = 10;
6    int b = 0;
7    
8    // Check before dividing
9    if (b != 0) {
10        int result = a / b;
11        printf("Result: %d\n", result);
12    } else {
13        printf("Error: Cannot divide by zero!\n");
14    }
15    
16    // Check array bounds
17    int arr[5] = {1, 2, 3, 4, 5};
18    int index = 3;
19    if (index >= 0 && index < 5) {
20        printf("arr[%d] = %d\n", index, arr[index]);
21    }
22    
23    // Check malloc result
24    int *data = malloc(sizeof(int) * 10);
25    if (data != NULL) {
26        data[0] = 5;
27        free(data);
28    } else {
29        printf("Memory allocation failed!\n");
30    }
31    
32    return 0;
33}

5. Logical Errors

Off-by-One Error

off_by_one.c

1#include <stdio.h>
2
3int main() {
4    int sum = 0;
5    
6    // Goal: Sum numbers 1 to 5 (should be 15)
7    for (int i = 1; i < 5; i++) {  // Should be i <= 5
8        sum += i;
9    }
10    
11    printf("Sum of 1-5: %d\n", sum);  // Prints 10, not 15!
12    
13    return 0;
14}

Output: Sum of 1-5: 10
Expected: Sum of 1-5: 15

Using = Instead of ==

assignment_vs_comparison.c

1#include <stdio.h>
2
3int main() {
4    int x = 5;
5    
6    // BUG: This ASSIGNS 10 to x, doesn't compare!
7    if (x = 10) {  // Should be x == 10
8        printf("x is 10\n");  // This always prints!
9    }
10    
11    printf("x is now: %d\n", x);  // x is now 10!
12    
13    return 0;
14}

Output: x is 10
x is now: 10
Bug: x was changed from 5 to 10!

Debugging Tips for Logical Errors

1.Add printf() statements to trace variable values
2.Use a debugger (gdb) to step through code line by line
3.Test with simple, known inputs first
4.Check boundary conditions (first, last, empty, max)
5.Read your code out loud — sometimes you'll catch the bug

6. Unreachable Code Error

Unreachable code is code that will never execute due to the control flow. The compiler detects this and issues a warning or error.

Common Causes:

Code After Return Statement

unreachable_return.c

1#include <stdio.h>
2
3int main() {
4    printf("Hello!\n");
5    
6    return 0;  // Function exits here!
7    
8    // UNREACHABLE - Never executed!
9    printf("Goodbye!\n");
10}

Warning: statement is unreachable

Code After Break Statement

unreachable_break.c

1#include <stdio.h>
2
3int main() {
4    int value = 2;
5    
6    for (;;) {  // Infinite loop
7        if (value == 2) {
8            break;  // Exit loop immediately!
9            
10            // UNREACHABLE - Never executed!
11            printf("This never prints\n");
12        }
13    }
14    
15    return 0;
16}

Code After Continue Statement

unreachable_continue.c

1#include <stdio.h>
2
3int main() {
4    for (int i = 0; i < 5; i++) {
5        continue;  // Jump to next iteration!
6        
7        // UNREACHABLE - Never executed!
8        printf("Iteration %d\n", i);
9    }
10    
11    printf("Loop complete\n");
12    return 0;
13}

Always-False Conditions

unreachable_condition.c

1#include <stdio.h>
2#include <math.h>
3
4int main() {
5    // sqrt(2) is about 1.414, never greater than 5
6    double x = sqrt(2);
7    
8    if (x > 5) {  // 1.414 > 5 is ALWAYS false!
9        // UNREACHABLE
10        x++;
11    }
12    
13    // 2 + 1 = 3, never equals 4
14    int n = 2 + 1;
15    
16    if (n == 4) {  // 3 == 4 is ALWAYS false!
17        // UNREACHABLE
18        printf("Equal\n");
19    }
20    
21    return 0;
22}

!Code Pitfalls: Common Mistakes & What to Watch For

Copied code can categorize and explain error types but often fails to teach the proactive mindset needed to prevent them in the first place.

Frequently Asked Questions

Q:What's the easiest type of error to fix?

A: Syntax errors! The compiler tells you exactly what's wrong and on which line. Read the error message, fix the typo or missing semicolon, and recompile.

Q:Why do I get "undefined reference" errors?

A: Linker can't find the function definition. Either you forgot to link a library (-lm for math), misspelled a function name, or didn't include the source file in compilation.

Q:How do I find logical errors?

A: These are hardest because the program runs fine but gives wrong results. Use a debugger to step through code, add printf statements to trace values, and test with known inputs.

Q:Should I treat warnings as errors?

A: Yes! Use -Werror to make warnings fail compilation. Most warnings indicate real bugs — fix them before they become runtime errors.

Summary: Error Detection Checklist

Compile-Time Errors

Check for missing semicolons
Declare variables before use
Match all brackets
Include required headers

Linker Errors

Define all declared functions
Link required libraries (-lm)
Check for typos in function names
Include all source files in build

Runtime Errors

Check divisor is not 0 before dividing
Validate array indices
Check malloc return value
Initialize pointers before use

Logical Errors

Use == for comparison, not =
Check loop boundaries (off-by-one)
Verify operator precedence
Test with edge cases

Best Practices to Minimize Errors

1.Compile with -Wall -Wextra flags
2.Fix warnings — they often indicate bugs
3.Test incrementally as you write code

4.Use a debugger (gdb) for runtime issues
5.Initialize all variables
6.Always check return values

What You Will Learn

Why Understanding Errors Matters

Classification of Errors

When Are Errors Detected?

Syntax Errors

Semantic Errors

Linker Errors

Runtime Errors

Logical Errors

Unreachable Code

1. Syntax Errors (Compile-Time Errors)

Common Syntax Errors:

Missing Semicolon

Undeclared Variable

Mismatched Brackets

Correct Version

2. Semantic Errors

Type Mismatch

Wrong Number of Arguments

3. Linker Errors

Undefined Reference

Fix: Add Function Definition

4. Runtime Errors

Division by Zero

Array Out of Bounds

Null Pointer Dereference

How to Prevent Runtime Errors

5. Logical Errors

Off-by-One Error

Using = Instead of ==

Debugging Tips for Logical Errors

6. Unreachable Code Error

Common Causes:

Code After Return Statement

Code After Break Statement

Code After Continue Statement

Always-False Conditions

!Code Pitfalls: Common Mistakes & What to Watch For

Copied code can categorize and explain error types but often fails to teach the proactive mindset needed to prevent them in the first place.

Frequently Asked Questions

Q:What's the easiest type of error to fix?

Q:Why do I get "undefined reference" errors?

Q:How do I find logical errors?

Q:Should I treat warnings as errors?

Summary: Error Detection Checklist

Compile-Time Errors

Linker Errors

Runtime Errors

Logical Errors

Best Practices to Minimize Errors

Test Your Knowledge

Related Tutorials

Basic Syntax in C

Common Beginner Mistakes in C

Enumerations in C

Have Feedback?

What You Will Learn

Why Understanding Errors Matters

Classification of Errors

When Are Errors Detected?

Syntax Errors

Semantic Errors

Linker Errors

Runtime Errors

Logical Errors

Unreachable Code

1. Syntax Errors (Compile-Time Errors)

Common Syntax Errors:

Missing Semicolon

Undeclared Variable

Mismatched Brackets

Correct Version

2. Semantic Errors

Type Mismatch

Wrong Number of Arguments

3. Linker Errors

Undefined Reference

Fix: Add Function Definition

4. Runtime Errors

Division by Zero