Writing to files in C - Step-by-Step Execution

Concept Flow - Writing to files

Start

↓

Open file with fopen

↓

Check if file opened

Yes No↓

Write data

↓

Close file with fclose

↓

End

The program opens a file, checks if it opened successfully, writes data if yes, then closes the file.

Execution Sample

C

#include <stdio.h>

int main() {
  FILE *f = fopen("data.txt", "w");
  if (f == NULL) return 1;
  fprintf(f, "Hello\n");
  fclose(f);
  return 0;
}

This code opens a file named data.txt for writing, writes "Hello" with a newline, then closes the file.

Execution Table

Step	Action	Evaluation	Result
1	Call fopen("data.txt", "w")	File exists or created	Returns file pointer (f)
2	Check if f == NULL	File opened?	Yes, file opened successfully
3	Call fprintf(f, "Hello\n")	Write string to file	String "Hello\n" written
4	Call fclose(f)	Close file	File closed
5	Return 0	Program ends	Success

💡 Program ends after closing the file and returning 0

Variable Tracker

Variable	Start	After fopen	After fprintf	After fclose	Final
f	undefined	file pointer (valid)	file pointer (valid)	file pointer (valid)	file pointer (valid)

Key Moments - 2 Insights

Why do we check if fopen returns NULL?

Because fopen returns NULL if the file cannot be opened or created. Checking prevents writing to an invalid file pointer (see execution_table step 2).

What happens if we forget to call fclose?

The file may not save properly or resources stay open. fclose ensures data is written and file is closed (see execution_table step 4).

Visual Quiz - 3 Questions

Look at the execution table, what does fopen return if the file opens successfully?

A An integer 0

B A valid file pointer

C NULL

D A string with filename

💡 Hint

Check execution_table row 1 under Result

At which step does the program write data to the file?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Look at execution_table Action column for writing

If fopen returns NULL, what should the program do?

A Return or handle error

B Continue writing to file

C Close the file

D Ignore and proceed

💡 Hint

See execution_table step 2 and key_moments about fopen check

Concept Snapshot

Writing to files in C:
- Use fopen(filename, mode) to open/create file
- Check if fopen returns NULL (means error)
- Use fprintf or fwrite to write data
- Always fclose to save and close file
- Return 0 on success

Full Transcript

This example shows how to write to a file in C. First, fopen opens the file in write mode. We check if fopen returns NULL to ensure the file opened correctly. If yes, fprintf writes the string "Hello" with a newline to the file. Then fclose closes the file to save changes. Finally, the program returns 0 to indicate success.