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C Error Handling

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⚠️ C Error Handling

Unlike some modern languages, C does not have built-in exception handling (like try-catch).
Instead, C uses several alternative mechanisms for detecting and managing errors manually.

🔹 Methods of Error Handling in C

Method Used For
Return values Function failure signals
errno Global error reporting
perror() / strerror() Human-readable error messages
Assertions (assert) Debugging and sanity checks
setjmp() / longjmp() Manual exception-like behavior

✅ 1. Error Handling using Return Values

Functions often return special values like -1, NULL, or 0 to indicate failure.

Example:

#include <stdio.h>

int divide(int a, int b) {
if (b == 0)
return -1; // error
return a / b;
}

int main() {
int res = divide(10, 0);

if (res == -1)
printf(“Error: Division by zero!\n”);
else
printf(“Result = %d\n”, res);

return 0;
}

✅ 2. Error Handling Using errno

errno is a global variable set by some library functions when an error occurs.

Include:

#include <errno.h>
#include <string.h>
#include <stdio.h>

Example:

#include <stdio.h>
#include <errno.h>
#include <string.h>int main() {
FILE *f = fopen(“missing.txt”, “r”);

if (f == NULL) {
printf(“Error: %s\n”, strerror(errno));
}

return 0;
}

✅ 3. perror() and strerror()

Function Purpose
perror("message") Prints a system error message with context
strerror(errno) Returns error message as a string

Example:

FILE *f = fopen("data.txt", "r");

if (!f) {
perror(“File open failed”);
}

Output example:

File open failed: No such file or directory

✅ 4. Using assert() (Debugging Only)

Stops the program if a condition is false.

#include <assert.h>
#include <stdio.h>int main() {
int x = -5;
assert(x >= 0); // program stops here
printf(“x = %d\n”, x);
return 0;
}

⚠️ Assertions are removed in production builds using:

gcc -DNDEBUG program.c

✅ 5. setjmp() and longjmp() (Manual Exception Handling)

Used to jump to a saved execution point — similar to exceptions.

#include <stdio.h>
#include <setjmp.h>jmp_buf buffer;

void testError() {
printf(“Error occurred! Jumping…\n”);
longjmp(buffer, 1);
}

int main() {

if (setjmp(buffer) == 0) {
printf(“Normal execution.\n”);
testError();
} else {
printf(“Recovered from error.\n”);
}

return 0;
}

🧪 Real Example: Safe File Opening

#include <stdio.h>
#include <errno.h>
#include <string.h>int main() {
FILE *file = fopen(“input.txt”, “r”);

if (file == NULL) {
perror(“Failed to open file”);
return 1; // handle error
}

printf(“File opened successfully.\n”);
fclose(file);

return 0;
}

🧠 Best Practices

✔ Always check return values (especially for I/O and pointer operations)
✔ Use errno, perror(), and strerror() for readable errors
✔ Use assert() in debugging, not production
✔ Use setjmp() and longjmp() only for advanced cases

🏁 Summary Table

Technique Best For
Return codes Simple functions
errno System/library errors
perror() / strerror() User-friendly error messages
assert() Debug/testing
setjmp() / longjmp() Advanced error control

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Sanjit Sinha — Web Developer | PHP • Laravel • CodeIgniter • MySQL • Bootstrap Founder, CodeCapsule — Student projects & practical coding guides. Email: info@codecapsule.in • Website: CodeCapsule.in

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