Runtime Errors and Exceptions in C

Unlike C++ or Java, C has no built-in exception handling mechanism like try-catch. Runtime errors in C manifest as undefined behavior or program crashes, often resulting in system signals likeSIGFPE or SIGSEGV.

Why Runtime Errors are Dangerous

•Silent failures: Sometimes produce garbage values instead of crashing
•Security vulnerabilities: Buffer overflows can be exploited by attackers
•Hard to reproduce: May only occur with specific inputs or memory states
•No stack trace: Unlike Java/Python, C doesn't show where the error occurred

Common Runtime Errors in C

Error Type	Signal	Description
Division by Zero	SIGFPE	Dividing an integer by zero causes floating-point exception
Segmentation Fault	SIGSEGV	Accessing memory outside allocated space (NULL pointer, out of bounds)
Stack Overflow	SIGSEGV	Infinite recursion or very large local arrays exhaust stack space
Buffer Overflow	Undefined	Writing beyond array bounds corrupts memory
Memory Leak	None	Allocated memory never freed, eventually exhausting system memory
Double Free	SIGABRT	Calling free() twice on the same pointer
Format String Error	Undefined	Wrong format specifier in printf/scanf (%d for long long)

1. Division by Zero

Dividing an integer by zero is undefined behavior in C. Most systems generate a SIGFPE (Floating Point Exception) signal and terminate the program immediately.

Code That Crashes

division_by_zero.c

1#include <stdio.h>
2
3int main() {
4    int a = 5;
5    int b = 0;
6    
7    // RUNTIME ERROR: Division by zero!
8    int result = a / b;
9    
10    printf("Result: %d\n", result);  // Never reached
11    return 0;
12}

Runtime Error: Floating point exception (core dumped)

✓Safe Version with Check

safe_division.c

1#include <stdio.h>
2
3int main() {
4    int a = 5;
5    int b = 0;
6    
7    // ✓ Always check before dividing
8    if (b != 0) {
9        int result = a / b;
10        printf("Result: %d\n", result);
11    } else {
12        printf("Error: Cannot divide by zero!\n");
13    }
14    
15    return 0;
16}

Output: Error: Cannot divide by zero!

Note: Float Division by Zero

Dividing a float or double by zero doesn't crash — it produces inf (infinity) ornan (not a number). Still, you should avoid it!

2. Segmentation Fault (SIGSEGV)

A segmentation fault occurs when your program tries to access memory it doesn't have permission to access. This is the most common runtime error in C.

Visual: Memory Access

Valid Access ✓

arr[0]

arr[1]

arr[2]

arr[3]

arr[4]

Your array

Invalid ✗

arr[5]

arr[6]

arr[7]

arr[...]

arr[10]

Not your memory!

Accessing memory outside your array causes SIGSEGV

Cause 2.1: NULL Pointer Dereference

Dereferencing NULL

null_pointer.c

1#include <stdio.h>
2
3int main() {
4    int *ptr = NULL;  // Pointer to nothing
5    
6    // CRASH: Trying to read from address 0x0
7    printf("%d\n", *ptr);
8    
9    return 0;
10}

Runtime Error: Segmentation fault (core dumped)

✓Always Check for NULL

safe_pointer.c

1#include <stdio.h>
2#include <stdlib.h>
3
4int main() {
5    int *ptr = malloc(sizeof(int));
6    
7    // ✓ Check if allocation succeeded
8    if (ptr != NULL) {
9        *ptr = 42;
10        printf("Value: %d\n", *ptr);
11        free(ptr);
12    } else {
13        printf("Memory allocation failed!\n");
14    }
15    
16    return 0;
17}

Cause 2.2: Array Out of Bounds

Accessing Invalid Index

array_bounds.c

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

Runtime Error: Segmentation fault (core dumped)
(or garbage value / silent corruption)

✓Validate Array Indices

safe_array.c

1#include <stdio.h>
2
3#define SIZE 5
4
5int main() {
6    int array[SIZE] = {1, 2, 3, 4, 5};
7    int index = 10;
8    
9    // ✓ Check bounds before accessing
10    if (index >= 0 && index < SIZE) {
11        printf("array[%d] = %d\n", index, array[index]);
12    } else {
13        printf("Error: Index %d out of bounds (0-%d)\n", 
14               index, SIZE - 1);
15    }
16    
17    return 0;
18}

Cause 2.3: Using Freed Memory

Use After Free

use_after_free.c

1#include <stdio.h>
2#include <stdlib.h>
3
4int main() {
5    int *ptr = malloc(sizeof(int));
6    *ptr = 42;
7    
8    free(ptr);  // Memory returned to system
9    
10    // DANGEROUS: ptr still points to freed memory!
11    printf("%d\n", *ptr);  // Undefined behavior
12    *ptr = 100;             // May crash or corrupt memory
13    
14    return 0;
15}

✓Set to NULL After Free

safe_free.c

1#include <stdio.h>
2#include <stdlib.h>
3
4int main() {
5    int *ptr = malloc(sizeof(int));
6    *ptr = 42;
7    printf("Before free: %d\n", *ptr);
8    
9    free(ptr);
10    ptr = NULL;  // ✓ Prevent accidental use
11    
12    // Now any access will clearly crash (NULL deref)
13    // instead of silent corruption
14    if (ptr != NULL) {
15        printf("%d\n", *ptr);
16    }
17    
18    return 0;
19}

3. Stack Overflow

Stack overflow happens when the call stack exceeds its limit, usually due to infinite recursion or very large local arrays.

Infinite Recursion

infinite_recursion.c

1#include <stdio.h>
2
3void infinite() {
4    printf("Calling myself...\n");
5    infinite();  // No base case - never stops!
6}
7
8int main() {
9    infinite();
10    return 0;
11}

Runtime Error: Segmentation fault (core dumped)
(Stack exhausted after ~1 million calls)

Huge Local Array

large_array.c

1#include <stdio.h>
2
3int main() {
4    // 40MB on stack (typical stack is 1-8MB)
5    int huge[10000000];
6    
7    huge[0] = 1;
8    printf("%d\n", huge[0]);
9    
10    return 0;
11}

✓Use Heap for Large Arrays

heap_array.c

1#include <stdio.h>
2#include <stdlib.h>
3
4int main() {
5    // ✓ Allocate on heap instead
6    int *huge = malloc(10000000 * sizeof(int));
7    
8    if (huge != NULL) {
9        huge[0] = 1;
10        printf("%d\n", huge[0]);
11        free(huge);
12    } else {
13        printf("Failed to allocate memory\n");
14    }
15    
16    return 0;
17}

4. Memory Leaks

A memory leak occurs when you allocate memory but never free it. The program keeps consuming more memory until it runs out.

Memory Leak

memory_leak.c

1#include <stdlib.h>
2
3void process() {
4    int *data = malloc(1000 * sizeof(int));
5    // ... use data ...
6    
7    // LEAK: Forgot to free!
8    // Memory is lost when function returns
9}
10
11int main() {
12    for (int i = 0; i < 1000000; i++) {
13        process();  // Each call leaks memory!
14    }
15    return 0;  // Program may crash from memory exhaustion
16}

✓Always Free Allocated Memory

no_leak.c

1#include <stdlib.h>
2
3void process() {
4    int *data = malloc(1000 * sizeof(int));
5    if (data == NULL) return;
6    
7    // ... use data ...
8    
9    free(data);  // ✓ Memory freed
10}
11
12int main() {
13    for (int i = 0; i < 1000000; i++) {
14        process();  // No leak!
15    }
16    return 0;
17}

5. Double Free

Freeing Twice

double_free.c

1#include <stdlib.h>
2
3int main() {
4    int *ptr = malloc(sizeof(int));
5    *ptr = 42;
6    
7    free(ptr);
8    free(ptr);  // CRASH: Double free!
9    
10    return 0;
11}

Runtime Error: double free or corruption (fasttop)

✓Set Pointer to NULL After Free

safe_double_free.c

1#include <stdlib.h>
2
3int main() {
4    int *ptr = malloc(sizeof(int));
5    *ptr = 42;
6    
7    free(ptr);
8    ptr = NULL;  // ✓ Mark as freed
9    
10    // Safe: free(NULL) does nothing
11    free(ptr);  // No crash!
12    
13    return 0;
14}

6. Format String Errors

Using the wrong format specifier in printf() orscanf() causes undefined behavior.

Wrong Format Specifiers

format_error.c

1#include <stdio.h>
2
3int main() {
4    long long bigNum = 9223372036854775807LL;
5    
6    // Using %d for long long (should be %lld)
7    printf("Number: %d\n", bigNum);  // Wrong output!
8    
9    // Missing argument
10    printf("Values: %d %d\n", 42);  // Undefined!
11    
12    // Wrong type
13    int x = 42;
14    printf("Address: %s\n", x);  // Crash likely!
15    
16    return 0;
17}

✓Correct Format Specifiers

correct_format.c

1#include <stdio.h>
2
3int main() {
4    int num = 42;
5    long long bigNum = 9223372036854775807LL;
6    double pi = 3.14159;
7    char ch = 'A';
8    char *str = "Hello";
9    
10    printf("int:       %d\n", num);       // ✓
11    printf("long long: %lld\n", bigNum);  // ✓
12    printf("double:    %f\n", pi);        // ✓
13    printf("char:      %c\n", ch);        // ✓
14    printf("string:    %s\n", str);       // ✓
15    printf("pointer:   %p\n", (void*)str);// ✓
16    
17    return 0;
18}

Error Handling Techniques in C

Since C has no try-catch, we use manual error checking with return codes, errno, and helper functions.

Method 1: Check Return Values

return_values.c

1#include <stdio.h>
2#include <stdlib.h>
3
4int main() {
5    FILE *file = fopen("data.txt", "r");
6    
7    // ✓ Check if fopen succeeded
8    if (file == NULL) {
9        printf("Error: Could not open file\n");
10        return 1;
11    }
12    
13    // ✓ Check if malloc succeeded
14    int *data = malloc(100 * sizeof(int));
15    if (data == NULL) {
16        printf("Error: Memory allocation failed\n");
17        fclose(file);
18        return 1;
19    }
20    
21    // Use file and data...
22    
23    free(data);
24    fclose(file);
25    return 0;
26}

Method 2: Using errno and perror()

errno_example.c

1#include <stdio.h>
2#include <errno.h>
3#include <string.h>
4
5int main() {
6    FILE *file = fopen("nonexistent.txt", "r");
7    
8    if (file == NULL) {
9        // Method 1: perror() - prints description automatically
10        perror("fopen failed");
11        
12        // Method 2: strerror() - get error string
13        printf("Error code: %d\n", errno);
14        printf("Error message: %s\n", strerror(errno));
15        
16        return 1;
17    }
18    
19    fclose(file);
20    return 0;
21}

Output:
fopen failed: No such file or directory
Error code: 2
Error message: No such file or directory

Method 3: setjmp/longjmp (Advanced)

setjmp_example.c

1#include <stdio.h>
2#include <setjmp.h>
3
4jmp_buf jump_buffer;
5
6void dangerous_operation() {
7    printf("Doing something risky...\n");
8    
9    // Simulate an error
10    longjmp(jump_buffer, 1);  // Jump back with error code 1
11    
12    printf("This never runs\n");
13}
14
15int main() {
16    // setjmp returns 0 first time, non-zero when longjmp called
17    int error = setjmp(jump_buffer);
18    
19    if (error == 0) {
20        // Normal execution
21        dangerous_operation();
22    } else {
23        // Error handling (jumped here from longjmp)
24        printf("Error occurred! Code: %d\n", error);
25    }
26    
27    printf("Program continues...\n");
28    return 0;
29}

Output:
Doing something risky...
Error occurred! Code: 1
Program continues...

!Code Pitfalls: Common Mistakes & What to Watch For

Copied code often generates C code that compiles successfully but is riddled with potential runtime errors like null pointer dereferences, division by zero, or buffer overflows.

If you search online to "read numbers from a file and calculate their average," it might produce code that opens a file, reads data, and divides. However, it frequently omits crucial checks: What if `fopen()` fails? What if the file is empty (division by zero)? What if the input is malformed? copied code often handles the "happy path" but ignores edge cases that cause crashes.

The Trap: Online sources are trained on code that "works" for typical inputs. They don't inherently understand C's lack of runtime safety—no exceptions, no garbage collection, no bounds checking. They might use `malloc()` without checking for `NULL`, dereference pointers without validation, or perform arithmetic without overflow checks. The code compiles, so it *seems* correct.

The Reality: In C, you are responsible for all runtime safety. Compiling without errors is only the first step; surviving runtime is the real challenge. Always review copied code for potential runtime errors, add defensive checks, and use tools like Valgrind and AddressSanitizer to catch issues the compiler can't.

Frequently Asked Questions

Q:What is a "Segmentation Fault"?

A: Your program tried to access memory it doesn't have permission to use — usually a NULL pointer, freed memory, or array out of bounds. The OS kills the process to prevent damage.

Q:Why does my program crash only sometimes?

A: Undefined behavior! Memory bugs often depend on random factors like memory layout. Use Valgrind or ASan to find the bug — they make it reproducible.

Q:How do I prevent division by zero?

A: Always check before dividing:if (divisor != 0). For user input, validate and reject zero values. Return an error code or use a default value.

Q:What's the best way to debug runtime errors?

A: 1) Compile with -g flag. 2) Run in GDB to see where it crashes. 3) Use Valgrind/ASan for memory bugs. 4) Add printf statements to narrow down the problem area.

Best Practices Summary

✓ Always Do

✓ Check divisor ≠ 0 before division
✓ Check pointers for NULL
✓ Validate array indices
✓ Check malloc/fopen return values
✓ Free all allocated memory
✓ Set pointers to NULL after free
✓ Use correct format specifiers
✓ Add base case in recursion

✗ Never Do

✗ Divide without checking for zero
✗ Dereference NULL pointers
✗ Access arrays out of bounds
✗ Use memory after free()
✗ Forget to free() allocated memory
✗ Call free() twice on same pointer
✗ Use wrong printf/scanf specifiers
✗ Create infinite recursion

Debugging Tools for Runtime Errors

Valgrind (Linux/Mac)

valgrind ./program

Detects memory leaks, invalid access, use after free

AddressSanitizer (GCC/Clang)

gcc -fsanitize=address program.c

Catches buffer overflows, stack issues

GDB Debugger

gdb ./program

Step through code, inspect variables at crash

Compiler Warnings

gcc -Wall -Wextra -Werror

Catch potential issues at compile time

Why Runtime Errors are Dangerous

•Silent failures: Sometimes produce garbage values instead of crashing
•Security vulnerabilities: Buffer overflows can be exploited by attackers
•Hard to reproduce: May only occur with specific inputs or memory states
•No stack trace: Unlike Java/Python, C doesn't show where the error occurred

Common Runtime Errors in C

Error Type	Signal	Description
Division by Zero	SIGFPE	Dividing an integer by zero causes floating-point exception
Segmentation Fault	SIGSEGV	Accessing memory outside allocated space (NULL pointer, out of bounds)
Stack Overflow	SIGSEGV	Infinite recursion or very large local arrays exhaust stack space
Buffer Overflow	Undefined	Writing beyond array bounds corrupts memory
Memory Leak	None	Allocated memory never freed, eventually exhausting system memory
Double Free	SIGABRT	Calling free() twice on the same pointer
Format String Error	Undefined	Wrong format specifier in printf/scanf (%d for long long)

1. Division by Zero

Dividing an integer by zero is undefined behavior in C. Most systems generate a SIGFPE (Floating Point Exception) signal and terminate the program immediately.

Code That Crashes

division_by_zero.c

1#include <stdio.h>
2
3int main() {
4    int a = 5;
5    int b = 0;
6    
7    // RUNTIME ERROR: Division by zero!
8    int result = a / b;
9    
10    printf("Result: %d\n", result);  // Never reached
11    return 0;
12}

Runtime Error: Floating point exception (core dumped)

✓Safe Version with Check

safe_division.c

1#include <stdio.h>
2
3int main() {
4    int a = 5;
5    int b = 0;
6    
7    // ✓ Always check before dividing
8    if (b != 0) {
9        int result = a / b;
10        printf("Result: %d\n", result);
11    } else {
12        printf("Error: Cannot divide by zero!\n");
13    }
14    
15    return 0;
16}

Output: Error: Cannot divide by zero!

Note: Float Division by Zero

Dividing a float or double by zero doesn't crash — it produces inf (infinity) ornan (not a number). Still, you should avoid it!

2. Segmentation Fault (SIGSEGV)

A segmentation fault occurs when your program tries to access memory it doesn't have permission to access. This is the most common runtime error in C.

Visual: Memory Access

Valid Access ✓

arr[0]

arr[1]

arr[2]

arr[3]

arr[4]

Your array

Invalid ✗

arr[5]

arr[6]

arr[7]

arr[...]

arr[10]

Not your memory!

Accessing memory outside your array causes SIGSEGV

Cause 2.1: NULL Pointer Dereference

Dereferencing NULL

null_pointer.c

1#include <stdio.h>
2
3int main() {
4    int *ptr = NULL;  // Pointer to nothing
5    
6    // CRASH: Trying to read from address 0x0
7    printf("%d\n", *ptr);
8    
9    return 0;
10}

Runtime Error: Segmentation fault (core dumped)

✓Always Check for NULL

safe_pointer.c

1#include <stdio.h>
2#include <stdlib.h>
3
4int main() {
5    int *ptr = malloc(sizeof(int));
6    
7    // ✓ Check if allocation succeeded
8    if (ptr != NULL) {
9        *ptr = 42;
10        printf("Value: %d\n", *ptr);
11        free(ptr);
12    } else {
13        printf("Memory allocation failed!\n");
14    }
15    
16    return 0;
17}

Cause 2.2: Array Out of Bounds

Accessing Invalid Index

array_bounds.c

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

Runtime Error: Segmentation fault (core dumped)
(or garbage value / silent corruption)

✓Validate Array Indices

safe_array.c

1#include <stdio.h>
2
3#define SIZE 5
4
5int main() {
6    int array[SIZE] = {1, 2, 3, 4, 5};
7    int index = 10;
8    
9    // ✓ Check bounds before accessing
10    if (index >= 0 && index < SIZE) {
11        printf("array[%d] = %d\n", index, array[index]);
12    } else {
13        printf("Error: Index %d out of bounds (0-%d)\n", 
14               index, SIZE - 1);
15    }
16    
17    return 0;
18}

Cause 2.3: Using Freed Memory

Use After Free

use_after_free.c

1#include <stdio.h>
2#include <stdlib.h>
3
4int main() {
5    int *ptr = malloc(sizeof(int));
6    *ptr = 42;
7    
8    free(ptr);  // Memory returned to system
9    
10    // DANGEROUS: ptr still points to freed memory!
11    printf("%d\n", *ptr);  // Undefined behavior
12    *ptr = 100;             // May crash or corrupt memory
13    
14    return 0;
15}

✓Set to NULL After Free

safe_free.c

1#include <stdio.h>
2#include <stdlib.h>
3
4int main() {
5    int *ptr = malloc(sizeof(int));
6    *ptr = 42;
7    printf("Before free: %d\n", *ptr);
8    
9    free(ptr);
10    ptr = NULL;  // ✓ Prevent accidental use
11    
12    // Now any access will clearly crash (NULL deref)
13    // instead of silent corruption
14    if (ptr != NULL) {
15        printf("%d\n", *ptr);
16    }
17    
18    return 0;
19}

3. Stack Overflow

Stack overflow happens when the call stack exceeds its limit, usually due to infinite recursion or very large local arrays.

Infinite Recursion

infinite_recursion.c

1#include <stdio.h>
2
3void infinite() {
4    printf("Calling myself...\n");
5    infinite();  // No base case - never stops!
6}
7
8int main() {
9    infinite();
10    return 0;
11}

Runtime Error: Segmentation fault (core dumped)
(Stack exhausted after ~1 million calls)

Huge Local Array

large_array.c

1#include <stdio.h>
2
3int main() {
4    // 40MB on stack (typical stack is 1-8MB)
5    int huge[10000000];
6    
7    huge[0] = 1;
8    printf("%d\n", huge[0]);
9    
10    return 0;
11}

✓Use Heap for Large Arrays

heap_array.c

1#include <stdio.h>
2#include <stdlib.h>
3
4int main() {
5    // ✓ Allocate on heap instead
6    int *huge = malloc(10000000 * sizeof(int));
7    
8    if (huge != NULL) {
9        huge[0] = 1;
10        printf("%d\n", huge[0]);
11        free(huge);
12    } else {
13        printf("Failed to allocate memory\n");
14    }
15    
16    return 0;
17}

4. Memory Leaks

A memory leak occurs when you allocate memory but never free it. The program keeps consuming more memory until it runs out.

Memory Leak

memory_leak.c

1#include <stdlib.h>
2
3void process() {
4    int *data = malloc(1000 * sizeof(int));
5    // ... use data ...
6    
7    // LEAK: Forgot to free!
8    // Memory is lost when function returns
9}
10
11int main() {
12    for (int i = 0; i < 1000000; i++) {
13        process();  // Each call leaks memory!
14    }
15    return 0;  // Program may crash from memory exhaustion
16}

✓Always Free Allocated Memory

no_leak.c

1#include <stdlib.h>
2
3void process() {
4    int *data = malloc(1000 * sizeof(int));
5    if (data == NULL) return;
6    
7    // ... use data ...
8    
9    free(data);  // ✓ Memory freed
10}
11
12int main() {
13    for (int i = 0; i < 1000000; i++) {
14        process();  // No leak!
15    }
16    return 0;
17}

5. Double Free

Freeing Twice

double_free.c

1#include <stdlib.h>
2
3int main() {
4    int *ptr = malloc(sizeof(int));
5    *ptr = 42;
6    
7    free(ptr);
8    free(ptr);  // CRASH: Double free!
9    
10    return 0;
11}

Runtime Error: double free or corruption (fasttop)

✓Set Pointer to NULL After Free

safe_double_free.c

1#include <stdlib.h>
2
3int main() {
4    int *ptr = malloc(sizeof(int));
5    *ptr = 42;
6    
7    free(ptr);
8    ptr = NULL;  // ✓ Mark as freed
9    
10    // Safe: free(NULL) does nothing
11    free(ptr);  // No crash!
12    
13    return 0;
14}

6. Format String Errors

Using the wrong format specifier in printf() orscanf() causes undefined behavior.

Wrong Format Specifiers

format_error.c

1#include <stdio.h>
2
3int main() {
4    long long bigNum = 9223372036854775807LL;
5    
6    // Using %d for long long (should be %lld)
7    printf("Number: %d\n", bigNum);  // Wrong output!
8    
9    // Missing argument
10    printf("Values: %d %d\n", 42);  // Undefined!
11    
12    // Wrong type
13    int x = 42;
14    printf("Address: %s\n", x);  // Crash likely!
15    
16    return 0;
17}

✓Correct Format Specifiers

correct_format.c

1#include <stdio.h>
2
3int main() {
4    int num = 42;
5    long long bigNum = 9223372036854775807LL;
6    double pi = 3.14159;
7    char ch = 'A';
8    char *str = "Hello";
9    
10    printf("int:       %d\n", num);       // ✓
11    printf("long long: %lld\n", bigNum);  // ✓
12    printf("double:    %f\n", pi);        // ✓
13    printf("char:      %c\n", ch);        // ✓
14    printf("string:    %s\n", str);       // ✓
15    printf("pointer:   %p\n", (void*)str);// ✓
16    
17    return 0;
18}

Error Handling Techniques in C

Since C has no try-catch, we use manual error checking with return codes, errno, and helper functions.

Method 1: Check Return Values

return_values.c

1#include <stdio.h>
2#include <stdlib.h>
3
4int main() {
5    FILE *file = fopen("data.txt", "r");
6    
7    // ✓ Check if fopen succeeded
8    if (file == NULL) {
9        printf("Error: Could not open file\n");
10        return 1;
11    }
12    
13    // ✓ Check if malloc succeeded
14    int *data = malloc(100 * sizeof(int));
15    if (data == NULL) {
16        printf("Error: Memory allocation failed\n");
17        fclose(file);
18        return 1;
19    }
20    
21    // Use file and data...
22    
23    free(data);
24    fclose(file);
25    return 0;
26}

Method 2: Using errno and perror()

errno_example.c

1#include <stdio.h>
2#include <errno.h>
3#include <string.h>
4
5int main() {
6    FILE *file = fopen("nonexistent.txt", "r");
7    
8    if (file == NULL) {
9        // Method 1: perror() - prints description automatically
10        perror("fopen failed");
11        
12        // Method 2: strerror() - get error string
13        printf("Error code: %d\n", errno);
14        printf("Error message: %s\n", strerror(errno));
15        
16        return 1;
17    }
18    
19    fclose(file);
20    return 0;
21}

Output:
fopen failed: No such file or directory
Error code: 2
Error message: No such file or directory

Method 3: setjmp/longjmp (Advanced)

setjmp_example.c

1#include <stdio.h>
2#include <setjmp.h>
3
4jmp_buf jump_buffer;
5
6void dangerous_operation() {
7    printf("Doing something risky...\n");
8    
9    // Simulate an error
10    longjmp(jump_buffer, 1);  // Jump back with error code 1
11    
12    printf("This never runs\n");
13}
14
15int main() {
16    // setjmp returns 0 first time, non-zero when longjmp called
17    int error = setjmp(jump_buffer);
18    
19    if (error == 0) {
20        // Normal execution
21        dangerous_operation();
22    } else {
23        // Error handling (jumped here from longjmp)
24        printf("Error occurred! Code: %d\n", error);
25    }
26    
27    printf("Program continues...\n");
28    return 0;
29}

Output:
Doing something risky...
Error occurred! Code: 1
Program continues...

!Code Pitfalls: Common Mistakes & What to Watch For

Copied code often generates C code that compiles successfully but is riddled with potential runtime errors like null pointer dereferences, division by zero, or buffer overflows.

Frequently Asked Questions

Q:What is a "Segmentation Fault"?

A: Your program tried to access memory it doesn't have permission to use — usually a NULL pointer, freed memory, or array out of bounds. The OS kills the process to prevent damage.

Q:Why does my program crash only sometimes?

A: Undefined behavior! Memory bugs often depend on random factors like memory layout. Use Valgrind or ASan to find the bug — they make it reproducible.

Q:How do I prevent division by zero?

A: Always check before dividing:if (divisor != 0). For user input, validate and reject zero values. Return an error code or use a default value.

Q:What's the best way to debug runtime errors?

A: 1) Compile with -g flag. 2) Run in GDB to see where it crashes. 3) Use Valgrind/ASan for memory bugs. 4) Add printf statements to narrow down the problem area.

Best Practices Summary

✓ Always Do

✓ Check divisor ≠ 0 before division
✓ Check pointers for NULL
✓ Validate array indices
✓ Check malloc/fopen return values
✓ Free all allocated memory
✓ Set pointers to NULL after free
✓ Use correct format specifiers
✓ Add base case in recursion

✗ Never Do

✗ Divide without checking for zero
✗ Dereference NULL pointers
✗ Access arrays out of bounds
✗ Use memory after free()
✗ Forget to free() allocated memory
✗ Call free() twice on same pointer
✗ Use wrong printf/scanf specifiers
✗ Create infinite recursion

Debugging Tools for Runtime Errors

Valgrind (Linux/Mac)

valgrind ./program

Detects memory leaks, invalid access, use after free

AddressSanitizer (GCC/Clang)

gcc -fsanitize=address program.c

Catches buffer overflows, stack issues

GDB Debugger

gdb ./program

Step through code, inspect variables at crash

Compiler Warnings

gcc -Wall -Wextra -Werror

Catch potential issues at compile time

What You Will Learn

Why Runtime Errors are Dangerous

Common Runtime Errors in C

1. Division by Zero

Code That Crashes

✓Safe Version with Check

Note: Float Division by Zero

2. Segmentation Fault (SIGSEGV)

Visual: Memory Access

Cause 2.1: NULL Pointer Dereference

Dereferencing NULL

✓Always Check for NULL

Cause 2.2: Array Out of Bounds

Accessing Invalid Index

✓Validate Array Indices

Cause 2.3: Using Freed Memory

Use After Free

✓Set to NULL After Free

3. Stack Overflow

Infinite Recursion

Huge Local Array

✓Use Heap for Large Arrays

4. Memory Leaks

Memory Leak

✓Always Free Allocated Memory

5. Double Free

Freeing Twice

✓Set Pointer to NULL After Free

6. Format String Errors

Wrong Format Specifiers

✓Correct Format Specifiers

Error Handling Techniques in C

Method 1: Check Return Values

Method 2: Using errno and perror()

Method 3: setjmp/longjmp (Advanced)

!Code Pitfalls: Common Mistakes & What to Watch For

Copied code often generates C code that compiles successfully but is riddled with potential runtime errors like null pointer dereferences, division by zero, or buffer overflows.

Frequently Asked Questions

Q:What is a "Segmentation Fault"?

Q:Why does my program crash only sometimes?

Q:How do I prevent division by zero?

Q:What's the best way to debug runtime errors?

Best Practices Summary

✓ Always Do

✗ Never Do

Debugging Tools for Runtime Errors

Valgrind (Linux/Mac)

AddressSanitizer (GCC/Clang)

GDB Debugger

Compiler Warnings

Test Your Knowledge

Related Tutorials

Types of Errors in C

Bit Fields in Structures

Graphics Programming with graphics.h

Have Feedback?

What You Will Learn

Why Runtime Errors are Dangerous

Common Runtime Errors in C

1. Division by Zero

Code That Crashes

✓Safe Version with Check

Note: Float Division by Zero

2. Segmentation Fault (SIGSEGV)

Visual: Memory Access

Cause 2.1: NULL Pointer Dereference

Dereferencing NULL

✓Always Check for NULL

Cause 2.2: Array Out of Bounds

Accessing Invalid Index

✓Validate Array Indices

Cause 2.3: Using Freed Memory

Use After Free

✓Set to NULL After Free

3. Stack Overflow

Infinite Recursion

Huge Local Array

✓Use Heap for Large Arrays

4. Memory Leaks

Memory Leak