C ferror() Function | Check for Errors in a File Stream

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Introduction

The ferror() function in C is a standard library function that checks the error indicator for the specified stream. It is part of the C standard library (stdio.h) and is commonly used to detect errors during input and output operations.

ferror() Function Syntax

The syntax for the ferror() function is as follows:

int ferror(FILE *stream);

Parameters:

• stream: A pointer to a FILE object that specifies the stream to be checked.

Returns:

• The function returns a non-zero value if the error indicator is set for the specified stream. Otherwise, it returns 0.

Examples

Checking for Errors After Reading a File

To demonstrate how to use ferror() to check for errors after reading a file, we will write a simple program.

Example

#include <stdio.h>

int main() {
    FILE *file;
    int ch;

    // Open the file for reading
    file = fopen("example.txt", "r");
    if (file == NULL) {
        printf("Error: Could not open file for reading.\n");
        return 1;
    }

    // Read characters until the end of the file
    while ((ch = fgetc(file)) != EOF) {
        putchar(ch);
    }

    // Check if an error occurred
    if (ferror(file)) {
        printf("\nError occurred while reading the file.\n");
    } else if (feof(file)) {
        printf("\nEnd of file reached.\n");
    }

    // Close the file
    fclose(file);

    return 0;
}

Output (assuming example.txt contains the text "Hello, World!"):

Hello, World!
End of file reached.

Checking for Errors After Writing to a File

This example shows how to use ferror() to check for errors after writing to a file.

Example

#include <stdio.h>

int main() {
    FILE *file;
    const char *text = "Hello, World!";

    // Open the file for writing
    file = fopen("output.txt", "w");
    if (file == NULL) {
        printf("Error: Could not open file for writing.\n");
        return 1;
    }

    // Write text to the file
    if (fputs(text, file) == EOF) {
        printf("Error occurred while writing to the file.\n");
    }

    // Check if an error occurred
    if (ferror(file)) {
        printf("Error occurred while writing to the file.\n");
    } else {
        printf("Writing to the file was successful.\n");
    }

    // Close the file
    fclose(file);

    return 0;
}

Output:

Writing to the file was successful.

Real-World Use Case

Reading Data from a File with Error Handling

In real-world applications, the ferror() function can be used to handle errors gracefully while reading data from a file.

Example

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

int main() {
    FILE *file;
    char buffer[256];

    // Open the file for reading
    file = fopen("data.txt", "r");
    if (file == NULL) {
        printf("Error: Could not open file for reading: %s\n", strerror(errno));
        return 1;
    }

    // Read lines from the file
    while (fgets(buffer, sizeof(buffer), file) != NULL) {
        printf("%s", buffer);
    }

    // Check if an error occurred
    if (ferror(file)) {
        printf("Error occurred while reading the file: %s\n", strerror(errno));
    } else if (feof(file)) {
        printf("\nEnd of file reached.\n");
    }

    // Close the file
    fclose(file);

    return 0;
}

Output (assuming data.txt contains multiple lines of text):

First line of data
Second line of data
Third line of data
End of file reached.

Conclusion

The ferror() function is useful for determining whether an error has occurred during input or output operations on a specified stream. It helps in managing I/O operations by allowing you to handle error conditions appropriately.