Opening and closing files - Step-by-Step Execution

Concept Flow - Opening and closing files

Start

↓

Call fopen()

↓

Check if file opened?

No→Handle error

Yes↓

Use file pointer

↓

Call fclose()

↓

End

Open a file with fopen(), check success, use the file, then close it with fclose().

Execution Sample

C

#include <stdio.h>

int main() {
  FILE *fp = fopen("test.txt", "r");
  if (fp == NULL) return 1;
  fclose(fp);
  return 0;
}

This code opens a file named "test.txt" for reading, checks if it opened successfully, then closes it.

Execution Table

Step	Action	Evaluation	Result
1	Call fopen("test.txt", "r")	File exists?	Returns FILE pointer or NULL
2	Check if fp == NULL	fp is not NULL	File opened successfully
3	Use file pointer (not shown here)	N/A	File ready for operations
4	Call fclose(fp)	Close file	File closed
5	Return 0	Program ends	Success exit

💡 Program ends after closing the file successfully

Variable Tracker

Variable	Start	After fopen	After fclose	Final
fp	undefined	FILE pointer (not NULL)	Still FILE pointer	No longer used after fclose

Key Moments - 2 Insights

Why do we check if fopen returns NULL?

Because fopen returns NULL if the file cannot be opened (see execution_table step 2). Checking prevents errors when using the file pointer.

What happens if we forget to call fclose?

The file remains open, which can cause resource leaks. fclose (step 4) properly releases the file handle.

Visual Quiz - 3 Questions

Look at the execution table, what does fopen return if the file does not exist?

A A valid FILE pointer

B An integer 0

C NULL

D A boolean false

💡 Hint

Check step 1 and 2 in the execution_table where fopen returns NULL if file can't be opened.

At which step is the file actually closed?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the action column in execution_table step 4 where fclose is called.

If fopen returns NULL, what should the program do next?

A Call fclose immediately

B Handle the error and avoid using the file pointer

C Use the file pointer anyway

D Return 0 and continue

💡 Hint

Refer to execution_table step 2 where checking for NULL prevents errors.

Concept Snapshot

Opening a file: FILE *fp = fopen("filename", "mode");
Check if fp == NULL to ensure success.
Use fp for file operations.
Close file with fclose(fp) to free resources.
Always check fopen result before using the file.

Full Transcript

This example shows how to open and close files in C. First, fopen is called with the filename and mode. If fopen returns NULL, the file could not be opened, so the program should handle this error. If fopen succeeds, it returns a FILE pointer used to access the file. After finishing file operations, fclose is called to close the file and release resources. Forgetting to close files can cause problems. Always check fopen's return value before using the file pointer.