C Write to Files – Output Data to Text and Binary Files in C

Introduction – How to Write to Files in C?

In C programming, writing to files enables a program to store data persistently in external files. Whether you’re saving logs, exporting reports, or storing raw binary data, C provides standard functions like fprintf(), fputs(), and fwrite() to perform file output operations.

In this guide, you’ll learn:

• How to open files for writing
• Write to text files using fprintf() and fputs()
• Write to binary files using fwrite()
• Handle common writing errors and close files properly

Opening Files for Writing

Before writing to a file, it must be opened in write mode using fopen(). Writing modes:

FILE *fp = fopen("output.txt", "w");
if (fp == NULL) {
    perror("Error opening file");
}

🖊️ Writing to Text Files

fprintf() – Formatted Output

FILE *fp = fopen("data.txt", "w");
fprintf(fp, "Name: %s\nAge: %d\n", "Alice", 30);
fclose(fp);

• Behaves like printf() but writes to a file
• Supports format specifiers (%s, %d, %f, etc.)

fputs() – Write String to File

FILE *fp = fopen("log.txt", "a");
fputs("Log entry: Program started.\n", fp);
fclose(fp);

• Writes a string (without format control)
• Faster and simpler than fprintf() for plain strings

Writing to Binary Files

fwrite() – Write Raw Data

FILE *fp = fopen("data.bin", "wb");
int numbers[] = {10, 20, 30};
fwrite(numbers, sizeof(int), 3, fp);
fclose(fp);

Syntax:

fwrite(pointer, size_of_each_element, number_of_elements, file_pointer);

• Writes raw memory as-is into the file
• Ideal for compact storage and structured records

Error Handling While Writing

Always verify that the file is opened and written successfully:

FILE *fp = fopen("file.txt", "w");
if (fp == NULL) {
    perror("Failed to open file");
    return 1;
}

if (fprintf(fp, "Hello") < 0) {
    printf("Write failed!\n");
}

fclose(fp);

Use Cases

Best Practices & Tips

Best Practice:
Always close the file using fclose() after writing to ensure buffers are flushed and file descriptors released.

Tip:
Use "a" or "ab" mode to preserve existing data and append new content.

Pitfall:
Opening a file in "w" or "wb" overwrites its content if it already exists.

Summary – Recap & Next Steps

Writing to files in C is a core part of building persistent, user-driven applications. Whether using formatted text or binary output, you can control how data is saved and organized externally.

Key Takeaways:

• Use fopen("file.txt", "w") or "wb" to write files
• fprintf() and fputs() are for text output
• fwrite() is used for binary output
• Always check file opening success and close files properly

Real-World Relevance:

Used in loggers, data exporters, binary storage tools, report generation, and embedded device state storage.

Frequently Asked Questions (FAQ)

How do I write to a text file in C?

Use fopen() with "w" or "a" mode, then write using fprintf() or fputs().

How can I write binary data to a file?

Use fopen() with "wb" mode and write with fwrite().

What happens if I open an existing file in "w" mode?

It gets overwritten, and all previous content is lost.

Is fprintf() safer than fputs()?

fprintf() offers format control, while fputs() is simpler for plain strings. Both are safe if used correctly.

Can I write multiple types of data in one file?

Yes. Use fprintf() for formatted text or fwrite() with structs for binary.