Error Handling and Debugging in C

Error handling is a critical aspect of any robust C program. C provides several mechanisms for error handling and debugging, including return codes, errno, perror(), strerror(), and debugging tools like GDB.

1. Error Codes and Return Values

In C, functions typically indicate success or failure through return values. A function may return 0 for success and a non-zero value (often -1) for failure. These return values can be checked to handle errors gracefully.

Example: Checking Return Values

#include <stdio.h>
 
int divide(int a, int b) {
    if (b == 0) {
        return -1; // Error: Division by zero
    }
    return a / b;
}
 
int main() {
    int result = divide(10, 0);
    if (result == -1) {
        printf("Error: Division by zero\n");
    } else {
        printf("Result: %d\n", result);
    }
    return 0;
}

Explanation:

• The divide() function returns -1 if the denominator is zero, signaling an error.
• The error is handled by checking the return value in the main() function.

Output:

Error: Division by zero

2. errno, perror(), and strerror()

In addition to return values, C provides the errno variable, which stores error codes set by system calls and library functions when an error occurs. The perror() and strerror() functions are used to display human-readable error messages.

Using errno

errno is a global variable that stores the error code when a function encounters an error.

#include <stdio.h>
#include <errno.h>
#include <string.h>
 
int main() {
    FILE *file = fopen("non_existent_file.txt", "r");
 
    if (file == NULL) {
        // Print the error code and message
        printf("Error code: %d\n", errno);
        printf("Error message: %s\n", strerror(errno));
    }
 
    return 0;
}

Explanation:

• fopen() fails because the file does not exist, setting errno to the corresponding error code.
• strerror(errno) returns a human-readable string describing the error.

Output:

Error code: 2
Error message: No such file or directory

Using perror()

perror() is a simpler way to print an error message directly based on errno.

#include <stdio.h>
 
int main() {
    FILE *file = fopen("non_existent_file.txt", "r");
 
    if (file == NULL) {
        // Print the error message using perror
        perror("File error");
    }
 
    return 0;
}

Explanation:

• perror() automatically prints an error message corresponding to the current value of errno.

Output:

File error: No such file or directory

3. Debugging with GDB

GDB (GNU Debugger) is a powerful tool for debugging C programs. It allows you to step through your code, inspect variables, set breakpoints, and more.

Basic GDB Commands

1. Compile with debugging symbols: Add the -g flag when compiling.

   gcc -g program.c -o program

2. Start GDB: Run GDB on your executable.

   gdb ./program

3. Set a breakpoint: Pause execution at a specific line.

   (gdb) break main

4. Run the program:

   (gdb) run

5. Step through the code: Use next to execute the next line of code.

   (gdb) next

6. Inspect variables: Print the value of variables.

   (gdb) print var_name

7. Continue execution: Resume program execution.

   (gdb) continue

8. Quit GDB:

   (gdb) quit

Example: Using GDB to Debug a Segmentation Fault

Consider the following buggy code:

#include <stdio.h>
 
int main() {
    int *ptr = NULL;
    *ptr = 10;  // Dereferencing a null pointer (causes segmentation fault)
 
    return 0;
}

Debugging Steps:

1. Compile the code with debugging symbols:

   gcc -g segfault.c -o segfault

2. Run the program with GDB:

   gdb ./segfault

3. Set a breakpoint at main():

   (gdb) break main

4. Run the program:

   (gdb) run

5. Step through the code using next and identify the line causing the segmentation fault.

6. Inspect the value of ptr to see that it is NULL.

GDB Output Example:

Program received signal SIGSEGV, Segmentation fault.
0x00000000004004c7 in main () at segfault.c:6
6       *ptr = 10;
(gdb) print ptr
$1 = (int *) 0x0

Explanation:

• GDB shows that the segmentation fault occurred at line 6 (*ptr = 10).
• Inspecting ptr with print ptr reveals that ptr is NULL, which causes the crash.

4. Diagram for Error Handling and Debugging

5. Recap

• Error Codes and Return Values: Functions indicate success or failure with return values.
• errno: Stores error codes set by functions when errors occur.
• perror() and strerror(): Functions for printing human-readable error messages.
• GDB: A powerful tool for debugging C programs.

These tools and techniques provide a structured way to handle errors and debug issues in C programs, ensuring better code reliability and maintainability.