Error Handling During File Operations in C

File operations are a common task in C programming, but they can encounter various errors that need to be handled gracefully. Proper error handling ensures that your program can handle unexpected situations, such as missing files or insufficient permissions, without crashing. In this article, we will learn how to handle some common errors during file operations in C.

Here are some common errors that can occur during file operations:

Failure to check for errors then the program may behave abnormally therefore an unchecked error may result in premature termination for the program or incorrect output.

Error Handling Techniques

Below are some standard error handling techniques:

1. File Not Found Error

A missing file can occur when opening a file in read mode (r) or append mode (a). Use fopen() and check for NULL. If it is, the error message can be printed using perror() function.

#include <stdio.h>

int main() {
    FILE *file = fopen("data.txt", "r");
  
  	// Check if the file is opened/found
    if (file == NULL) {
        perror("Error");
        return 1;
    }

    // Perform file operations...
    fclose(file);
    return 0;
}

Output

Error: No such file or directory

Explanation: After attempting to read from the file, ferror() checks if there was a read error. If ferror() returns a non-zero value, it indicates that there was an error during the read operation.

2. Handle Permission Denied Error

If the file exists but the program lacks the required permissions, fopen() will fail. We can change the perror() output to “permission deined” as shown in the below snippet.

FILE *file = fopen("/restricted/file.txt", "w");
if (file == NULL) {
    perror("Permission denied");
}

3. Handle Disk Full Error

When writing to a file, ensure the disk has enough space. Errors during writing can be detected using ferror().

#include <stdio.h>

int main() {
    FILE *fptr = fopen("output.txt", "r");
    if (fptr == NULL) {
        perror("Error opening file");
        return 1;
    }
    
    fprintf(fptr, "Writing to file");
    if (ferror(fptr)) {
        perror("Error writing to file");
    }
    
    fclose(fptr);
    return 0;
}

Output

Error writing to file: Permission Denied

4. Handle File Already Exists

When creating a new file with fopen() in w mode, the existing file will be overwritten. To avoid this, check if the file exists first.

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
int main() {
    const char *filename = "example.txt";
    FILE *fptr;

    // Try to open the file in write mode
    fptr = fopen(filename, "wx");

    if (fptr == NULL) {
      
        // Check if the error is due to file already existing
        if (errno == EEXIST)
            perror("Error");
        return EXIT_FAILURE;
    }

    // If we reach here, the file was created successfully
    fprintf(fptr, "This is a new file.\n");
    fclose(fptr);
    printf("File '%s' created successfully.\n", filename);

    return EXIT_SUCCESS;
}

Output

Error: File exists

5. Handle Invalid File Pointer

Always verify that the file pointer is not NULL before performing operations like reading or writing.

FILE *file = NULL;
if (file == NULL) {
    printf("Invalid file pointer. File operations cannot proceed.\n");
}

6. Handle End-of-File (EOF)

When reading a file, reaching the end is not an error but requires proper handling to avoid unexpected results.

#include <stdio.h>
int main() {
    FILE *file = fopen("example.txt", "r");
    if (file == NULL) {
        perror("Error opening file");
        return 1;
    }

  	// Check for eof while reading
    char ch;
    while ((ch = fgetc(file)) != EOF)
        putchar(ch);

  	// Use feof() to make sure EOF occurred or not
    if (feof(file))
        printf("End of file reached.");
    else if (ferror(file))
        printf("Error reading the file.");

    fclose(file);
}

Output

This is a new file.
End of file reached.

7. Handle File Not Open

Always check if the file pointer is valid before performing any operations.

#include <stdio.h>
int main() {
    FILE *file = fopen("example.txt", "r");
    if (file == NULL) {
        printf("File could not be opened.\n");
    } else {
        printf("File opened successfully.\n");
        fclose(file);
    }
    
    return 0;
}

Output

File could not be opened.

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Article Tags :

• C Language
• C Programs
• C++
• C++ Programs
• C Error Handling Programs
• C-File Handling
• cpp-file-handling
• File Handling

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