File Handling in C

01What is File Handling?

?Why Do We Need Files?

Think about what happens when you close your program — all your variables vanish! Files solve this problem.

Variables (RAM)

Disappear when program ends

Lost on power failure

✓Very fast access

int score = 100; // GONE!

Files (Disk)

✓Persist after program ends

✓Survive power failure

Slower access

scores.txt → 100 // SAVED!

•

Save Games

Player progress

•

Logs

Error tracking

⚙️

Config Files

User settings

•

Data Export

CSV, reports

Understanding FILE* — The Bookmark Analogy

Imagine a FILE* as a smart bookmark in a book:

data.txt

File on Disk

FILE *fp

Your Bookmark

Your Bookmark Tracks:

Position — which line you're reading
Mode — reading or writing?
Buffer — cached data for speed
Status — any errors? end of file?

The 3-Step Process:

fopen() → Get bookmark

read/write → Use bookmark

fclose() → Return bookmark

Always check if fopen() returns NULL! It means the file couldn't be opened (doesn't exist, no permission, disk full, etc.)

Text Files

• Human-readable content
• Each line ends with newline
• Examples: .txt, .csv, .html
• Use: fgets, fputs, fprintf, fscanf

Binary Files

• Raw bytes, not readable
• Exact memory representation
• Examples: .exe, .bin, .dat
• Use: fread, fwrite

Function	Purpose	Header
fopen()	Open a file	<stdio.h>
fclose()	Close a file	<stdio.h>
fgets() / fputs()	Read/write string (line)	<stdio.h>
fgetc() / fputc()	Read/write single char	<stdio.h>
fprintf() / fscanf()	Formatted read/write	<stdio.h>
fread() / fwrite()	Binary read/write	<stdio.h>
fseek() / ftell()	File position	<stdio.h>

02The FILE Structure

What is FILE?

FILE is a structure defined in <stdio.h>. It contains information about the file being accessed. You never access its members directly - you always use FILE * (a pointer) and functions like fopen, fread, etc.

The FILE structure contains (simplified):

struct FILE {

char *buffer; /* Data buffer */

int bufferSize; /* Size of buffer */

char *currentPos; /* Current position in buffer */

int fileDescriptor; /* OS file handle */

int mode; /* Read/write mode */

int eof; /* End-of-file indicator */

int error; /* Error indicator */

};

How file operations work:

Your Program

↓ FILE * pointer ↓

FILE Structure (Buffer)

↓ System Calls ↓

Operating System

↓ Hardware ↓

Disk (HDD/SSD)

Why Buffering?

Disk operations are slow. The buffer collects data and writes/reads in larger chunks, making file operations much faster.

03Opening & Closing Files

fopen() Syntax

FILE *fopen(const char *filename, const char *mode);

• filename: Path to the file
• mode: How to open (read, write, append, etc.)
• Returns: FILE pointer, or NULL if failed

open_close.c

1#include <stdio.h>
2
3int main() {
4    // Open file for reading
5    FILE *file = fopen("data.txt", "r");
6    
7    // Always check if file opened!
8    if (file == NULL) {
9        printf("Error: Cannot open file!\n");
10        return 1;
11    }
12    
13    printf("File opened successfully!\n");
14    
15    // Always close when done!
16    fclose(file);
17    
18    return 0;
19}

Output

File opened successfully!

Always Close Files!

Not closing files can cause: data loss (buffer not flushed), resource leaks, and file locks. Always use fclose()!

04File Modes

"r"

Read Only

Opens existing file

Starts at beginning

Fails if missing

"w"

Write Only

Creates or overwrites

ERASES old content!

✓ Creates if missing

"a"

Append Only

➕ Adds to end

✓ Keeps old content

✓ Creates if missing

Combined Modes (Read + Write)

"r+"

Read & Write

File must exist

"w+"

Read & Write

Erases content!

"a+"

Read & Append

Write at end only

Mode	Read	Write	Create	Truncate	Position
"r"	✓	✗	✗	✗	Start
"w"	✗	✓	✓	✓	Start
"a"	✗	✓	✓	✗	End
"r+"	✓	✓	✗	✗	Start
"w+"	✓	✓	✓	✓	Start
"a+"	✓	✓	✓	✗	End

Text vs Binary Mode

Add "b" to any mode for binary files:"rb","wb","ab", etc.

Text Mode (default)

"r", "w", "a"

• Human-readable content
• Newline translations (Windows: \r\n ↔ \n)
• .txt, .csv, .json, .html

Binary Mode

"rb", "wb", "ab"

• Raw bytes, exact copy
• No translations
• .exe, .png, .mp3, structs

05Reading Text Files

Method 1: fgetc() - Read Character by Character

read_char.c

1#include <stdio.h>
2
3int main() {
4    FILE *file = fopen("data.txt", "r");
5    if (file == NULL) return 1;
6    
7    int ch;
8    while ((ch = fgetc(file)) != EOF) {
9        putchar(ch);  // Print character
10    }
11    
12    fclose(file);
13    return 0;
14}

Method 2: fgets() - Read Line by Line (Recommended)

read_line.c

1#include <stdio.h>
2
3int main() {
4    FILE *file = fopen("data.txt", "r");
5    if (file == NULL) return 1;
6    
7    char line[256];
8    
9    // fgets reads until newline or buffer full
10    while (fgets(line, sizeof(line), file) != NULL) {
11        printf("%s", line);  // line includes \n
12    }
13    
14    fclose(file);
15    return 0;
16}

Method 3: fscanf() - Formatted Reading

read_formatted.c

1#include <stdio.h>
2
3int main() {
4    FILE *file = fopen("students.txt", "r");
5    if (file == NULL) return 1;
6    
7    char name[50];
8    int age;
9    float grade;
10    
11    // File contains: "Alice 20 3.8"
12    while (fscanf(file, "%s %d %f", name, &age, &grade) == 3) {
13        printf("Name: %s, Age: %d, Grade: %.1f\n", name, age, grade);
14    }
15    
16    fclose(file);
17    return 0;
18}

Output

Name: Alice, Age: 20, Grade: 3.8

06Writing Text Files

Method 1: fputc() - Write Character

write_char.c

1#include <stdio.h>
2
3int main() {
4    FILE *file = fopen("output.txt", "w");
5    if (file == NULL) return 1;
6    
7    fputc('H', file);
8    fputc('i', file);
9    fputc('!', file);
10    fputc('\n', file);
11    
12    fclose(file);
13    printf("Written to output.txt\n");
14    return 0;
15}

Method 2: fputs() - Write String

write_string.c

1#include <stdio.h>
2
3int main() {
4    FILE *file = fopen("output.txt", "w");
5    if (file == NULL) return 1;
6    
7    fputs("Hello, World!\n", file);
8    fputs("This is line 2\n", file);
9    
10    fclose(file);
11    return 0;
12}

Method 3: fprintf() - Formatted Writing (Most Used)

write_formatted.c

1#include <stdio.h>
2
3int main() {
4    FILE *file = fopen("report.txt", "w");
5    if (file == NULL) return 1;
6    
7    char name[] = "Alice";
8    int age = 25;
9    float score = 95.5;
10    
11    // Just like printf, but to file!
12    fprintf(file, "Name: %s\n", name);
13    fprintf(file, "Age: %d\n", age);
14    fprintf(file, "Score: %.1f\n", score);
15    
16    fclose(file);
17    printf("Report saved!\n");
18    return 0;
19}

report.txt

Name: Alice

Age: 25

Score: 95.5

07Binary Files

When to Use Binary?

Use binary for structs, arrays, images, or any raw data. Binary is faster and more compact than text, but not human-readable.

fwrite() and fread() Syntax

fwrite(ptr, size, count, file);

fread(ptr, size, count, file);

• ptr: Pointer to data
• size: Size of each element
• count: Number of elements
• file: FILE pointer

Write and Read a Struct

binary_struct.c

1#include <stdio.h>
2
3struct Student {
4    char name[50];
5    int age;
6    float grade;
7};
8
9int main() {
10    struct Student s1 = {"Alice", 20, 3.8};
11    
12    // Write struct to binary file
13    FILE *file = fopen("student.dat", "wb");
14    if (file == NULL) return 1;
15    
16    fwrite(&s1, sizeof(struct Student), 1, file);
17    fclose(file);
18    
19    // Read struct back
20    struct Student s2;
21    file = fopen("student.dat", "rb");
22    if (file == NULL) return 1;
23    
24    fread(&s2, sizeof(struct Student), 1, file);
25    fclose(file);
26    
27    printf("Name: %s, Age: %d, Grade: %.1f\n", 
28           s2.name, s2.age, s2.grade);
29    
30    return 0;
31}

Output

Name: Alice, Age: 20, Grade: 3.8

Write and Read Array

binary_array.c

1#include <stdio.h>
2
3int main() {
4    int arr[] = {10, 20, 30, 40, 50};
5    int n = 5;
6    
7    // Write array
8    FILE *file = fopen("numbers.bin", "wb");
9    fwrite(arr, sizeof(int), n, file);
10    fclose(file);
11    
12    // Read array
13    int loaded[5];
14    file = fopen("numbers.bin", "rb");
15    fread(loaded, sizeof(int), n, file);
16    fclose(file);
17    
18    for (int i = 0; i < n; i++) {
19        printf("%d ", loaded[i]);
20    }
21    
22    return 0;
23}

Output

10 20 30 40 50

08File Positioning

The Position Indicator (Cursor)

Every file has an invisible cursor that tracks where you are in the file.

data.txt contents:

HelloWorld

0123456789

↑ Position = 5 (ftell returns 5)

SEEK_SET

From beginning

fseek(f, 5, SEEK_SET)

→ Go to byte 5

SEEK_CUR

From current

fseek(f, 3, SEEK_CUR)

→ Move 3 bytes forward

SEEK_END

From end

fseek(f, -2, SEEK_END)

→ 2 bytes before end

Function	Purpose	Syntax
fseek()	Move to position	fseek(file, offset, origin)
ftell()	Get current position	long pos = ftell(file)
rewind()	Go to start	rewind(file)

fseek() Origin Constants

Constant	Value	Meaning
SEEK_SET	0	Beginning of file
SEEK_CUR	1	Current position
SEEK_END	2	End of file

file_position.c

1#include <stdio.h>
2
3int main() {
4    FILE *file = fopen("data.txt", "r");
5    if (file == NULL) return 1;
6    
7    // Get file size
8    fseek(file, 0, SEEK_END);  // Go to end
9    long size = ftell(file);   // Get position = size
10    printf("File size: %ld bytes\n", size);
11    
12    // Go back to start
13    rewind(file);  // Same as fseek(file, 0, SEEK_SET)
14    
15    // Read first character
16    int ch = fgetc(file);
17    printf("First char: %c\n", ch);
18    
19    // Skip 5 characters from current position
20    fseek(file, 5, SEEK_CUR);
21    ch = fgetc(file);
22    printf("6th char: %c\n", ch);
23    
24    fclose(file);
25    return 0;
26}

09Error Handling

Function	Purpose
feof(file)	Returns non-zero if end-of-file reached
ferror(file)	Returns non-zero if error occurred
clearerr(file)	Clear error and EOF flags
perror("msg")	Print error message

error_handling.c

1#include <stdio.h>
2
3int main() {
4    FILE *file = fopen("missing.txt", "r");
5    
6    if (file == NULL) {
7        perror("Error opening file");
8        return 1;
9    }
10    
11    char buffer[100];
12    while (fgets(buffer, sizeof(buffer), file) != NULL) {
13        printf("%s", buffer);
14    }
15    
16    // Check why loop ended
17    if (feof(file)) {
18        printf("\nEnd of file reached.\n");
19    }
20    if (ferror(file)) {
21        printf("\nError reading file!\n");
22    }
23    
24    fclose(file);
25    return 0;
26}

Output (if file missing)

Error opening file: No such file or directory

!Code Pitfalls: Common Mistakes & What to Watch For

These are the most common mistakes that trip up beginners. Study them carefully to avoid hours of debugging!

Not Checking fopen() Return

FILE *f = fopen("x.txt", "r");

fprintf(f, "Hi"); // Crash if NULL!

FILE *f = fopen("x.txt", "r");

if (f == NULL) return 1;

Forgetting to Close File

Always fclose(file) to flush buffer and release resources.

Using "w" Instead of "a"

"w" erases existing content! Use "a" to append without erasing.

Wrong Mode for Binary

Always use "rb"/"wb" for binary files. Text mode can corrupt binary data!

12Frequently Asked Questions

Q:What's the difference between text and binary mode?

A: Text mode translates newlines (Windows uses \r\n, Unix uses \n) and may alter certain characters. Binary mode ("rb"/"wb") reads/writes bytes exactly as-is. Use text for .txt, .csv; use binary for images, executables, structs.

Q:What happens if I don't close a file?

A: Buffered data might not be written, the file remains locked, and you waste system resources. The OS closes files at program exit, but for long-running programs, you can run out of file descriptors. Always fclose() when done!

Q:How do I check if a file exists before opening?

A: Try opening it and check for NULL:if (fopen("file.txt", "r") == NULL). Or use access() from <unistd.h>. Note: there's always a race condition — the file could be deleted between checking and opening.

Q:What's fseek and ftell used for?

A: fseek() moves the file position to read/write at specific locations. ftell() tells you the current position. Common pattern to get file size: seek to end, ftell, seek back to start.

Q:Should I use fread/fwrite or fprintf/fscanf?

A: Use fprintf/fscanf for human-readable text (config files, logs). Use fread/fwrite for binary data (images, raw structs, efficiency). Binary is faster and more compact but not human-readable or portable across architectures.

12Summary

Complete File I/O Workflow

fopen()

Open

NULL check

Verify

read/write

Use

fclose()

Text File Functions

fgets(buf, n, file)read line

fputs(str, file)write string

fprintf(file, fmt, ...)formatted write

fscanf(file, fmt, ...)formatted read

Binary File Functions

fread(ptr, size, n, file)read bytes

fwrite(ptr, size, n, file)write bytes

fseek(file, offset, origin)move cursor

ftell(file)get position

Mode Cheat Sheet

"r" = read

"w" = write (erase!)

"a" = append

+ = add read+write

b = binary mode

✓Safe File Handling Template

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

if (file == NULL) {

perror("Error");

return 1;

}

// ... read or write ...

fclose(file);

Key Takeaways

✓FILE* = your bookmark/handle to a file
✓Always check if fopen() returns NULL
✓Always close files with fclose()

✓Text files: use fgets, fputs, fprintf
✓Binary files: use fread, fwrite with "rb"/"wb"
"w" mode erases existing content!

01What is File Handling?

?Why Do We Need Files?

Think about what happens when you close your program — all your variables vanish! Files solve this problem.

Variables (RAM)

Disappear when program ends

Lost on power failure

✓Very fast access

int score = 100; // GONE!

Files (Disk)

✓Persist after program ends

✓Survive power failure

Slower access

scores.txt → 100 // SAVED!

•

Save Games

Player progress

•

Logs

Error tracking

⚙️

Config Files

User settings

•

Data Export

CSV, reports

Understanding FILE* — The Bookmark Analogy

Imagine a FILE* as a smart bookmark in a book:

data.txt

File on Disk

FILE *fp

Your Bookmark

Your Bookmark Tracks:

Position — which line you're reading
Mode — reading or writing?
Buffer — cached data for speed
Status — any errors? end of file?

The 3-Step Process:

fopen() → Get bookmark

read/write → Use bookmark

fclose() → Return bookmark

Always check if fopen() returns NULL! It means the file couldn't be opened (doesn't exist, no permission, disk full, etc.)

Text Files

• Human-readable content
• Each line ends with newline
• Examples: .txt, .csv, .html
• Use: fgets, fputs, fprintf, fscanf

Binary Files

• Raw bytes, not readable
• Exact memory representation
• Examples: .exe, .bin, .dat
• Use: fread, fwrite

Function	Purpose	Header
fopen()	Open a file	<stdio.h>
fclose()	Close a file	<stdio.h>
fgets() / fputs()	Read/write string (line)	<stdio.h>
fgetc() / fputc()	Read/write single char	<stdio.h>
fprintf() / fscanf()	Formatted read/write	<stdio.h>
fread() / fwrite()	Binary read/write	<stdio.h>
fseek() / ftell()	File position	<stdio.h>

02The FILE Structure

What is FILE?

The FILE structure contains (simplified):

struct FILE {

char *buffer; /* Data buffer */

int bufferSize; /* Size of buffer */

char *currentPos; /* Current position in buffer */

int fileDescriptor; /* OS file handle */

int mode; /* Read/write mode */

int eof; /* End-of-file indicator */

int error; /* Error indicator */

};

How file operations work:

Your Program

↓ FILE * pointer ↓

FILE Structure (Buffer)

↓ System Calls ↓

Operating System

↓ Hardware ↓

Disk (HDD/SSD)

Why Buffering?

Disk operations are slow. The buffer collects data and writes/reads in larger chunks, making file operations much faster.

03Opening & Closing Files

fopen() Syntax

FILE *fopen(const char *filename, const char *mode);

• filename: Path to the file
• mode: How to open (read, write, append, etc.)
• Returns: FILE pointer, or NULL if failed

open_close.c

1#include <stdio.h>
2
3int main() {
4    // Open file for reading
5    FILE *file = fopen("data.txt", "r");
6    
7    // Always check if file opened!
8    if (file == NULL) {
9        printf("Error: Cannot open file!\n");
10        return 1;
11    }
12    
13    printf("File opened successfully!\n");
14    
15    // Always close when done!
16    fclose(file);
17    
18    return 0;
19}

Output

File opened successfully!

Always Close Files!

Not closing files can cause: data loss (buffer not flushed), resource leaks, and file locks. Always use fclose()!

04File Modes

"r"

Read Only

Opens existing file

Starts at beginning

Fails if missing

"w"

Write Only

Creates or overwrites

ERASES old content!

✓ Creates if missing

"a"

Append Only

➕ Adds to end

✓ Keeps old content

✓ Creates if missing

Combined Modes (Read + Write)

"r+"

Read & Write

File must exist

"w+"

Read & Write

Erases content!

"a+"

Read & Append

Write at end only

Mode	Read	Write	Create	Truncate	Position
"r"	✓	✗	✗	✗	Start
"w"	✗	✓	✓	✓	Start
"a"	✗	✓	✓	✗	End
"r+"	✓	✓	✗	✗	Start
"w+"	✓	✓	✓	✓	Start
"a+"	✓	✓	✓	✗	End

Text vs Binary Mode

Add "b" to any mode for binary files:"rb","wb","ab", etc.

Text Mode (default)

"r", "w", "a"

• Human-readable content
• Newline translations (Windows: \r\n ↔ \n)
• .txt, .csv, .json, .html

Binary Mode

"rb", "wb", "ab"

• Raw bytes, exact copy
• No translations
• .exe, .png, .mp3, structs

05Reading Text Files

Method 1: fgetc() - Read Character by Character

read_char.c

1#include <stdio.h>
2
3int main() {
4    FILE *file = fopen("data.txt", "r");
5    if (file == NULL) return 1;
6    
7    int ch;
8    while ((ch = fgetc(file)) != EOF) {
9        putchar(ch);  // Print character
10    }
11    
12    fclose(file);
13    return 0;
14}

Method 2: fgets() - Read Line by Line (Recommended)

read_line.c

1#include <stdio.h>
2
3int main() {
4    FILE *file = fopen("data.txt", "r");
5    if (file == NULL) return 1;
6    
7    char line[256];
8    
9    // fgets reads until newline or buffer full
10    while (fgets(line, sizeof(line), file) != NULL) {
11        printf("%s", line);  // line includes \n
12    }
13    
14    fclose(file);
15    return 0;
16}

Method 3: fscanf() - Formatted Reading

read_formatted.c

1#include <stdio.h>
2
3int main() {
4    FILE *file = fopen("students.txt", "r");
5    if (file == NULL) return 1;
6    
7    char name[50];
8    int age;
9    float grade;
10    
11    // File contains: "Alice 20 3.8"
12    while (fscanf(file, "%s %d %f", name, &age, &grade) == 3) {
13        printf("Name: %s, Age: %d, Grade: %.1f\n", name, age, grade);
14    }
15    
16    fclose(file);
17    return 0;
18}

Output

Name: Alice, Age: 20, Grade: 3.8

06Writing Text Files

Method 1: fputc() - Write Character

write_char.c

1#include <stdio.h>
2
3int main() {
4    FILE *file = fopen("output.txt", "w");
5    if (file == NULL) return 1;
6    
7    fputc('H', file);
8    fputc('i', file);
9    fputc('!', file);
10    fputc('\n', file);
11    
12    fclose(file);
13    printf("Written to output.txt\n");
14    return 0;
15}

Method 2: fputs() - Write String

write_string.c

1#include <stdio.h>
2
3int main() {
4    FILE *file = fopen("output.txt", "w");
5    if (file == NULL) return 1;
6    
7    fputs("Hello, World!\n", file);
8    fputs("This is line 2\n", file);
9    
10    fclose(file);
11    return 0;
12}

Method 3: fprintf() - Formatted Writing (Most Used)

write_formatted.c

1#include <stdio.h>
2
3int main() {
4    FILE *file = fopen("report.txt", "w");
5    if (file == NULL) return 1;
6    
7    char name[] = "Alice";
8    int age = 25;
9    float score = 95.5;
10    
11    // Just like printf, but to file!
12    fprintf(file, "Name: %s\n", name);
13    fprintf(file, "Age: %d\n", age);
14    fprintf(file, "Score: %.1f\n", score);
15    
16    fclose(file);
17    printf("Report saved!\n");
18    return 0;
19}

report.txt

Name: Alice

Age: 25

Score: 95.5

07Binary Files

When to Use Binary?

Use binary for structs, arrays, images, or any raw data. Binary is faster and more compact than text, but not human-readable.

fwrite() and fread() Syntax

fwrite(ptr, size, count, file);

fread(ptr, size, count, file);

• ptr: Pointer to data
• size: Size of each element
• count: Number of elements
• file: FILE pointer

Write and Read a Struct

binary_struct.c

1#include <stdio.h>
2
3struct Student {
4    char name[50];
5    int age;
6    float grade;
7};
8
9int main() {
10    struct Student s1 = {"Alice", 20, 3.8};
11    
12    // Write struct to binary file
13    FILE *file = fopen("student.dat", "wb");
14    if (file == NULL) return 1;
15    
16    fwrite(&s1, sizeof(struct Student), 1, file);
17    fclose(file);
18    
19    // Read struct back
20    struct Student s2;
21    file = fopen("student.dat", "rb");
22    if (file == NULL) return 1;
23    
24    fread(&s2, sizeof(struct Student), 1, file);
25    fclose(file);
26    
27    printf("Name: %s, Age: %d, Grade: %.1f\n", 
28           s2.name, s2.age, s2.grade);
29    
30    return 0;
31}

Output

Name: Alice, Age: 20, Grade: 3.8

Write and Read Array

binary_array.c

1#include <stdio.h>
2
3int main() {
4    int arr[] = {10, 20, 30, 40, 50};
5    int n = 5;
6    
7    // Write array
8    FILE *file = fopen("numbers.bin", "wb");
9    fwrite(arr, sizeof(int), n, file);
10    fclose(file);
11    
12    // Read array
13    int loaded[5];
14    file = fopen("numbers.bin", "rb");
15    fread(loaded, sizeof(int), n, file);
16    fclose(file);
17    
18    for (int i = 0; i < n; i++) {
19        printf("%d ", loaded[i]);
20    }
21    
22    return 0;
23}

Output

10 20 30 40 50

08File Positioning

The Position Indicator (Cursor)

Every file has an invisible cursor that tracks where you are in the file.

data.txt contents:

HelloWorld

0123456789

↑ Position = 5 (ftell returns 5)

SEEK_SET

From beginning

fseek(f, 5, SEEK_SET)

→ Go to byte 5

SEEK_CUR

From current

fseek(f, 3, SEEK_CUR)

→ Move 3 bytes forward

SEEK_END

From end

fseek(f, -2, SEEK_END)

→ 2 bytes before end

Function	Purpose	Syntax
fseek()	Move to position	fseek(file, offset, origin)
ftell()	Get current position	long pos = ftell(file)
rewind()	Go to start	rewind(file)

fseek() Origin Constants

Constant	Value	Meaning
SEEK_SET	0	Beginning of file
SEEK_CUR	1	Current position
SEEK_END	2	End of file

file_position.c

1#include <stdio.h>
2
3int main() {
4    FILE *file = fopen("data.txt", "r");
5    if (file == NULL) return 1;
6    
7    // Get file size
8    fseek(file, 0, SEEK_END);  // Go to end
9    long size = ftell(file);   // Get position = size
10    printf("File size: %ld bytes\n", size);
11    
12    // Go back to start
13    rewind(file);  // Same as fseek(file, 0, SEEK_SET)
14    
15    // Read first character
16    int ch = fgetc(file);
17    printf("First char: %c\n", ch);
18    
19    // Skip 5 characters from current position
20    fseek(file, 5, SEEK_CUR);
21    ch = fgetc(file);
22    printf("6th char: %c\n", ch);
23    
24    fclose(file);
25    return 0;
26}

09Error Handling

Function	Purpose
feof(file)	Returns non-zero if end-of-file reached
ferror(file)	Returns non-zero if error occurred
clearerr(file)	Clear error and EOF flags
perror("msg")	Print error message

error_handling.c

1#include <stdio.h>
2
3int main() {
4    FILE *file = fopen("missing.txt", "r");
5    
6    if (file == NULL) {
7        perror("Error opening file");
8        return 1;
9    }
10    
11    char buffer[100];
12    while (fgets(buffer, sizeof(buffer), file) != NULL) {
13        printf("%s", buffer);
14    }
15    
16    // Check why loop ended
17    if (feof(file)) {
18        printf("\nEnd of file reached.\n");
19    }
20    if (ferror(file)) {
21        printf("\nError reading file!\n");
22    }
23    
24    fclose(file);
25    return 0;
26}

Output (if file missing)

Error opening file: No such file or directory

!Code Pitfalls: Common Mistakes & What to Watch For

These are the most common mistakes that trip up beginners. Study them carefully to avoid hours of debugging!

Not Checking fopen() Return

FILE *f = fopen("x.txt", "r");

fprintf(f, "Hi"); // Crash if NULL!

FILE *f = fopen("x.txt", "r");

if (f == NULL) return 1;

Forgetting to Close File

Always fclose(file) to flush buffer and release resources.

Using "w" Instead of "a"

"w" erases existing content! Use "a" to append without erasing.

Wrong Mode for Binary

Always use "rb"/"wb" for binary files. Text mode can corrupt binary data!

12Frequently Asked Questions

Q:What's the difference between text and binary mode?

Q:What happens if I don't close a file?

Q:How do I check if a file exists before opening?

Q:What's fseek and ftell used for?

A: fseek() moves the file position to read/write at specific locations. ftell() tells you the current position. Common pattern to get file size: seek to end, ftell, seek back to start.

Q:Should I use fread/fwrite or fprintf/fscanf?

12Summary

Complete File I/O Workflow

fopen()

Open

NULL check

Verify

read/write

Use

fclose()

Text File Functions

fgets(buf, n, file)read line

fputs(str, file)write string

fprintf(file, fmt, ...)formatted write

fscanf(file, fmt, ...)formatted read

Binary File Functions

fread(ptr, size, n, file)read bytes

fwrite(ptr, size, n, file)write bytes

fseek(file, offset, origin)move cursor

ftell(file)get position

Mode Cheat Sheet

"r" = read

"w" = write (erase!)

"a" = append

+ = add read+write

b = binary mode

✓Safe File Handling Template

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

if (file == NULL) {

perror("Error");

return 1;

}

// ... read or write ...

fclose(file);

Key Takeaways

✓FILE* = your bookmark/handle to a file
✓Always check if fopen() returns NULL
✓Always close files with fclose()

✓Text files: use fgets, fputs, fprintf
✓Binary files: use fread, fwrite with "rb"/"wb"
"w" mode erases existing content!

What You Will Learn

01What is File Handling?

?Why Do We Need Files?

Variables (RAM)

Files (Disk)

Understanding FILE* — The Bookmark Analogy

Your Bookmark Tracks:

The 3-Step Process:

Text Files

Binary Files

02The FILE Structure

What is FILE?

Why Buffering?

03Opening & Closing Files

fopen() Syntax

Always Close Files!

04File Modes

Read Only

Write Only

Append Only

Combined Modes (Read + Write)

Text vs Binary Mode

Text Mode (default)

Binary Mode

05Reading Text Files

Method 1: fgetc() - Read Character by Character

Method 2: fgets() - Read Line by Line (Recommended)

Method 3: fscanf() - Formatted Reading

06Writing Text Files

Method 1: fputc() - Write Character

Method 2: fputs() - Write String

Method 3: fprintf() - Formatted Writing (Most Used)

07Binary Files

When to Use Binary?

fwrite() and fread() Syntax

Write and Read a Struct

Write and Read Array

08File Positioning

The Position Indicator (Cursor)

fseek() Origin Constants

09Error Handling

!Code Pitfalls: Common Mistakes & What to Watch For

Not Checking fopen() Return

Forgetting to Close File

Using "w" Instead of "a"

Wrong Mode for Binary

12Frequently Asked Questions

Q:What's the difference between text and binary mode?

Q:What happens if I don't close a file?

Q:How do I check if a file exists before opening?

Q:What's fseek and ftell used for?

Q:Should I use fread/fwrite or fprintf/fscanf?

12Summary

Complete File I/O Workflow

Text File Functions

Binary File Functions

Mode Cheat Sheet

✓Safe File Handling Template

Key Takeaways

Test Your Knowledge

Related Tutorials

UNIX/Linux System Calls

Pointers in C

C errno.h Library Reference

Have Feedback?

What You Will Learn

01What is File Handling?

?Why Do We Need Files?

Variables (RAM)

Files (Disk)

Understanding FILE* — The Bookmark Analogy

Your Bookmark Tracks:

The 3-Step Process:

Text Files

Binary Files

02The FILE Structure

What is FILE?

Why Buffering?

03Opening & Closing Files

fopen() Syntax