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.

Practice

1. What does the fopen function do in C programming?

easy

A. It closes an open file stream.

B. It opens a file and returns a pointer to the file stream.

C. It reads data from a file.

D. It writes data to a file.

Solution

1. Step 1: Understand the purpose of fopen

   The fopen function is used to open a file and returns a pointer to the file stream if successful.

2. Step 2: Differentiate fopen from other file functions

   Functions like fclose close files, fread reads data, and fwrite writes data. Only fopen opens files.

3. Final Answer:

   It opens a file and returns a pointer to the file stream. -> Option B

4. Quick Check:

   fopen = open file pointer [OK]

Hint: fopen opens files, fclose closes them [OK]

Common Mistakes:

• Confusing fopen with fclose
• Thinking fopen reads or writes data
• Not knowing fopen returns NULL on failure

2. Which of the following is the correct syntax to open a file named "data.txt" for writing in C?

easy

A. FILE *fp = fopen("data.txt", "w");

B. FILE *fp = fopen("data.txt", "r");

C. FILE *fp = open("data.txt", "w");

D. FILE fp = fopen("data.txt", "w");

Solution

1. Step 1: Identify correct fopen syntax

   The correct syntax uses FILE * pointer and fopen with filename and mode as strings.

2. Step 2: Check mode for writing

   Mode "w" opens the file for writing. "r" is for reading, and open is not a standard C function for files.

3. Final Answer:

   FILE *fp = fopen("data.txt", "w"); -> Option A

4. Quick Check:

   fopen + "w" = open for writing [OK]

Hint: Use FILE * and fopen with mode "w" to write [OK]

Common Mistakes:

• Using open instead of fopen
• Declaring FILE variable instead of pointer
• Using wrong mode like "r" for writing

3. What will be the output of the following C code snippet?

FILE *fp = fopen("nonexistent.txt", "r");
if (fp == NULL) {
    printf("File not found\n");
} else {
    printf("File opened successfully\n");
    fclose(fp);
}

medium

A. File opened successfully

B. No output

C. File not found

D. Compilation error

Solution

1. Step 1: Understand fopen with "r" mode on nonexistent file

   Opening a file for reading with "r" mode returns NULL if the file does not exist.

2. Step 2: Check the if condition

   The code checks if fp == NULL, then prints "File not found". Since file does not exist, this condition is true.

3. Final Answer:

   File not found -> Option C

4. Quick Check:

   fopen returns NULL if file missing [OK]

Hint: fopen returns NULL if file missing in "r" mode [OK]

Common Mistakes:

• Assuming fopen creates file in "r" mode
• Not checking for NULL before using file pointer
• Expecting "File opened successfully" always

4. Identify the error in the following code snippet:

FILE *fp;
fp = fopen("file.txt", "w");
printf("File opened\n");
// Missing fclose here

medium

A. fopen mode "w" is incorrect

B. File pointer is not checked for NULL

C. printf statement is incorrect

D. fclose is missing to close the file

Solution

1. Step 1: Check fopen usage and mode

   The fopen call with mode "w" is correct to open file for writing.

2. Step 2: Identify missing fclose

   The code opens the file but never closes it with fclose, which can cause resource leaks.

3. Final Answer:

   fclose is missing to close the file -> Option D

4. Quick Check:

   Always fclose after fopen [OK]

Hint: Always close files with fclose after opening [OK]

Common Mistakes:

• Ignoring fclose causes resource leaks
• Not checking fopen return value
• Thinking printf affects file handling

5. You want to append text to an existing file "log.txt" safely. Which code snippet correctly opens the file, checks for errors, writes "Hello\n", and closes the file?

hard

A. FILE *fp = fopen("log.txt", "a");
if (fp == NULL) {
perror("Error opening file");
} else {
fprintf(fp, "Hello\n");
fclose(fp);
}

B. FILE *fp = fopen("log.txt", "w");
if (fp != NULL) {
fprintf(fp, "Hello\n");
fclose(fp);
}

C. FILE *fp = fopen("log.txt", "r");
fprintf(fp, "Hello\n");
fclose(fp);

D. FILE *fp = fopen("log.txt", "a");
fprintf(fp, "Hello\n");
// no fclose

Solution

1. Step 1: Choose correct mode for appending

   Mode "a" opens file for appending without erasing existing content.

2. Step 2: Check for NULL and handle errors

   Always check if fopen returns NULL before writing. Use perror to show error if NULL.

3. Step 3: Write and close file properly

   Use fprintf to write text and fclose to close file to avoid leaks.

4. Final Answer:

   FILE *fp = fopen("log.txt", "a"); if (fp == NULL) { perror("Error opening file"); } else { fprintf(fp, "Hello\n"); fclose(fp); } -> Option A

5. Quick Check:

   Append mode + check NULL + fclose = safe file write [OK]

Hint: Use "a" mode, check NULL, fclose after writing [OK]

Common Mistakes:

• Using "w" mode erases file instead of appending
• Not checking fopen return value before writing
• Forgetting to close file with fclose