Error Handling in C (errno, perror, strerror)

What Causes Runtime Errors?

• Opening a non-existing file
• Division by zero (undefined)
• Invalid memory access
• File read/write failures
• Insufficient memory for malloc()
• Invalid system-level operations

errno – Global Error Indicator

errno is a global integer variable defined in <errno.h>. It stores the error code of the last failed operation.

• Value becomes non-zero when an error occurs
• Each error has a symbolic name (EACCES, ENOENT, etc.)
• Must check only after a function fails

perror() – Prints Error Message

perror() prints a human-readable error message based on the current value of errno.

• Prototype: void perror(const char *msg);
• Adds a custom message before the system message

strerror() – Returns Error Message String

strerror(errno) returns the error message as a string so you can print it manually.

• Prototype: char* strerror(int errnum);
• Useful in formatted output

Common Error Codes (errno)

• ENOENT → No such file or directory
• EACCES → Permission denied
• ENOMEM → Not enough memory
• EBADF → Bad file descriptor
• EINVAL → Invalid argument
• EEXIST → File already exists

Examples – All in One Block

1. Using errno and perror()

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

int main() {

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

    if(f == NULL){
        perror("Error Opening File");
    }

    return 0;
}

2. Using strerror() with errno

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

int main() {

    FILE *f = fopen("no.txt", "r");

    if(!f){
        printf("Failed: %s", strerror(errno));
    }

    return 0;
}

3. malloc() Failure Detection

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

int main() {

    int *p = malloc(100000000000L);

    if(!p){
        printf("Memory Error: %s", strerror(errno));
    }

    return 0;
}

4. fopen() Permission Error

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

int main() {

    FILE *fp = fopen("/root/system.txt", "r");

    if(!fp){
        perror("Access Denied");
    }

    return 0;
}

5. Custom Error Logging

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

int main() {

    FILE *f = fopen("unknown.bin", "rb");

    if(f == NULL){
        printf("Error Code: %d\n", errno);
        printf("Message: %s", strerror(errno));
    }

    return 0;
}

When Should You Use These?

• File opening / reading / writing
• Memory allocation
• Invalid arguments in system-level functions
• Network or device operations
• Multi-file projects using extern

Practice Questions

1. What is errno? When does it change?
2. Difference between perror() and strerror()
3. Write a program to detect file opening failure.
4. Explain three common errno values.
5. Why does C not have exception handling like C++?

Practice Task