How to Properly Close a File in C

Properly Close a File in C

To properly close a file in C, you need to use the appropriate file closing functions such as fclose() for standard file pointers and close() for low-level file descriptors. This ensures that buffered data is written to disk and system resources are released.

---

Example 1: Basic File Closing using fclose()

In this example, we will open a file in read mode and then properly close it using the fclose() function.

main.c

#include <stdio.h>

int main() {
    FILE *fp = fopen("example.txt", "r");
    if (fp == NULL) {
        printf("Failed to open file.\n");
        return 1;
    }
    // File operations go here

    fclose(fp);
    printf("File closed successfully.\n");
    return 0;
}

Explanation:

1. We declare a file pointer fp of type FILE * and open example.txt in read mode using fopen().
2. If the file fails to open (fp == NULL), an error message is printed and the program exits.
3. After performing any necessary file operations, we call fclose(fp) to properly close the file.
4. Finally, we print a confirmation message indicating that the file has been closed successfully.

Output:

File closed successfully.

Example 2: File Closing with Error Checking

In this example, we open a file in write mode, write some data into it, and then close it using fclose() while checking the return value to handle any errors during the file closing process.

main.c

#include <stdio.h>

int main() {
    FILE *fp = fopen("data.txt", "w");
    if (fp == NULL) {
        printf("Error opening file.\n");
        return 1;
    }
    // Write data to the file
    fprintf(fp, "Hello, World!\n");
    
    // Attempt to close the file and check for errors
    if (fclose(fp) == 0) {
        printf("File closed successfully.\n");
    } else {
        printf("Error closing file.\n");
    }
    return 0;
}

Explanation:

1. We declare a file pointer fp and open data.txt in write mode using fopen().
2. If fopen() fails (fp == NULL), an error message is printed and the program terminates.
3. Data is written to the file using fprintf().
4. We call fclose(fp) and check its return value: a return value of 0 indicates the file closed successfully; otherwise, an error occurred.

Output:

File closed successfully.

Example 3: Closing a File Descriptor using close()

In this example, we use low-level file I/O by opening a file using open() and then properly closing it using the close() function.

main.c

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>

int main() {
    int fd = open("log.txt", O_RDONLY);
    if (fd == -1) {
        perror("Error opening file");
        return 1;
    }
    // File operations go here

    // Attempt to close the file descriptor and check for errors
    if (close(fd) == 0) {
        printf("File descriptor closed successfully.\n");
    } else {
        perror("Error closing file descriptor");
    }
    return 0;
}

Explanation:

1. We include the necessary headers: <fcntl.h> for the open() function and <unistd.h> for the close() function.
2. An integer variable fd is declared to store the file descriptor returned by open().
3. We open log.txt in read-only mode; if fd is -1, it indicates an error in opening the file.
4. After performing any file operations, we call close(fd) and check its return value to ensure the file descriptor is properly closed.

Output:

File descriptor closed successfully.

Conclusion

Properly closing a file in C is crucial for ensuring data integrity and freeing system resources. Whether using fclose() for file pointers or close() for file descriptors, always include error checking to handle any issues that may arise during the file closing process.