C stdio.h Library Reference

?Why is stdio.h Important?

Every C program you'll ever write likely starts with #include <stdio.h>. It's the foundation of all input/output operations in C.

Console I/O

printf, scanf, getchar

›

File Operations

fopen, fread, fwrite

›

Formatted Output

%d, %f, %s specifiers

New to I/O? Start with the beginner-friendly Input/Output Tutorial first. This page is a quick reference for all stdio.h functions.

01Introduction to stdio.h

What is stdio.h?

<stdio.h> (Standard Input/Output) is the most commonly used header in C. It provides functions for console I/O, file handling, and formatted output.

Key Functions Overview

Category	Functions
Output	printf, fprintf, sprintf, snprintf
Input	scanf, fscanf, sscanf
Character	getchar, putchar, getc, putc, fgetc, fputc
String	gets, puts, fgets, fputs
File	fopen, fclose, fread, fwrite, fseek, ftell

02printf() Family - Formatted Output

printf() - Print to Console

int printf(const char *format, ...);

format: Format string with specifiers

...: Values to print

Returns: Number of characters printed

main.c

printf("Name: %s, Age: %d\n", "Alice", 25);
// Output: Name: Alice, Age: 25

fprintf() - Print to File/Stream

int fprintf(FILE *stream, const char *format, ...);

stream: File pointer (FILE *)

format: Format string

main.c

FILE *fp = fopen("log.txt", "w");
fprintf(fp, "Error code: %d\n", 404);
fprintf(stderr, "Error occurred!\n");  // Print to error stream
fclose(fp);

sprintf() - Print to String

int sprintf(char *str, const char *format, ...);

str: Destination buffer

format: Format string

main.c

char buffer[100];
sprintf(buffer, "Score: %d/%d", 85, 100);
printf("%s\n", buffer);  // Output: Score: 85/100

snprintf() - Safe Print to String

int snprintf(char *str, size_t size, const char *format, ...);

str: Destination buffer

size: Max bytes to write (including \\0)

main.c

char buffer[20];
snprintf(buffer, sizeof(buffer), "Long text: %s", "This is a very long string");
// Safe: Won't overflow buffer!

✓Recommended: Always use snprintf() over sprintf() to prevent buffer overflow!

03scanf() Family - Formatted Input

scanf() - Read from Console

int scanf(const char *format, ...);

format: Format string with specifiers

...: Pointers to variables

Returns: Number of items successfully read

main.c

int age;
char name[50];
printf("Enter name and age: ");
scanf("%s %d", name, &age);  // Note: & for non-arrays
printf("Hello %s, you are %d years old\n", name, age);

fscanf() - Read from File

int fscanf(FILE *stream, const char *format, ...);

main.c

FILE *fp = fopen("data.txt", "r");
int id;
float price;
fscanf(fp, "%d %f", &id, &price);
fclose(fp);

sscanf() - Read from String

int sscanf(const char *str, const char *format, ...);

main.c

char data[] = "John 25 85.5";
char name[20];
int age;
float score;
sscanf(data, "%s %d %f", name, &age, &score);
// name="John", age=25, score=85.5

04Character I/O Functions

Function	Prototype	Description
getchar()	int getchar(void)	Read one char from stdin
putchar()	int putchar(int c)	Write one char to stdout
fgetc()	int fgetc(FILE *stream)	Read char from file
fputc()	int fputc(int c, FILE *stream)	Write char to file
ungetc()	int ungetc(int c, FILE *stream)	Push char back to stream

char_io.c

1#include <stdio.h>
2
3int main() {
4    // Read and echo characters until Enter
5    printf("Type something: ");
6    int ch;
7    while ((ch = getchar()) != '\n') {
8        putchar(ch);  // Echo each character
9    }
10    putchar('\n');
11    
12    // File character I/O
13    FILE *fp = fopen("test.txt", "r");
14    if (fp) {
15        while ((ch = fgetc(fp)) != EOF) {
16            putchar(ch);  // Print file contents
17        }
18        fclose(fp);
19    }
20    return 0;
21}

05String I/O Functions

fgets() - Safe String Input

char *fgets(char *str, int n, FILE *stream);

str: Destination buffer

n: Max chars to read

stream: Input stream (stdin for console)

main.c

char name[50];
printf("Enter name: ");
fgets(name, sizeof(name), stdin);  // Safe: won't overflow!
// Note: fgets includes the newline character

fputs() - Write String to Stream

int fputs(const char *str, FILE *stream);

main.c

fputs("Hello World!\n", stdout);  // To console
FILE *fp = fopen("out.txt", "w");
fputs("Written to file", fp);
fclose(fp);

puts() - Simple String Output

int puts(const char *str);

main.c

puts("Hello World!");  // Automatically adds newline
// Same as: printf("Hello World!\n");

Never Use gets()!

gets() is deprecated and dangerous — no buffer size limit causes overflow. Always use fgets() instead!

06File Operations

Function	Prototype	Description
fopen()	FILE fopen(const char path, const char *mode)	Open file
fclose()	int fclose(FILE *stream)	Close file
fread()	size_t fread(void ptr, size_t size, size_t n, FILE s)	Read binary data
fwrite()	size_t fwrite(const void ptr, size_t size, size_t n, FILE s)	Write binary data
fseek()	int fseek(FILE *stream, long offset, int whence)	Move file position
ftell()	long ftell(FILE *stream)	Get current position
rewind()	void rewind(FILE *stream)	Go to file start
feof()	int feof(FILE *stream)	Check end of file
ferror()	int ferror(FILE *stream)	Check for error

file_ops.c

1#include <stdio.h>
2
3int main() {
4    // Write to file
5    FILE *fp = fopen("data.txt", "w");
6    if (fp == NULL) {
7        perror("Error opening file");
8        return 1;
9    }
10    fprintf(fp, "Hello, File!\n");
11    fclose(fp);
12    
13    // Read from file
14    fp = fopen("data.txt", "r");
15    char buffer[100];
16    while (fgets(buffer, sizeof(buffer), fp) != NULL) {
17        printf("%s", buffer);
18    }
19    fclose(fp);
20    
21    return 0;
22}

07File Opening Modes

Mode	Description	If File Exists	If Not Exists
"r"	Read	Open	Error (NULL)
"w"	Write	Truncate (empty)	Create
"a"	Append	Open at end	Create
"r+"	Read/Write	Open	Error
"w+"	Read/Write	Truncate	Create
"rb"	Read binary	Add 'b' for binary mode

08Standard Streams

stdin

Standard Input (keyboard)

stdout

Standard Output (console)

stderr

Standard Error (errors)

!Code Pitfalls: Common Mistakes & What to Watch For

Copied code often generates printf/scanf code with format string vulnerabilities or mismatched format specifiers.

If you search online to "print user input," it might generate printf(user_input) instead of printf("%s", user_input). This is a critical security vulnerability — attackers can crash your program or execute arbitrary code with format strings like %n.

The Trap: Online sources focus on functionality, not security. They might also mismatch format specifiers (using %d for size_t, %f for double in scanf), leading to undefined behavior, wrong output, or crashes. The bugs often work in testing but fail with different inputs.

The Reality: Never pass user input as the first argument to printf(). Always use literal format strings. Match specifiers to types exactly: %d for int, %zu for size_t, %f for float (printf) but %lf for double (scanf). Compile with -Wformat to catch mismatches.

10Frequently Asked Questions

Q:What does fflush(stdout) do and when should I use it?

A: fflush(stdout) forces buffered output to be written immediately. stdout is line-buffered on terminals, so output without a newline may not appear until later. Use it before asking for input with prompts like "Enter name: " to ensure the prompt displays.

Q:Why should I use fgets() instead of gets()?

A: gets() has no buffer size limit — it's a security vulnerability and was removed from C11. fgets(buf, size, stdin) is safe because you specify the maximum characters to read.

Q:What's the difference between printf and fprintf?

A: printf() writes to stdout (the screen). fprintf() writes to any FILE* — a file, stderr, or even stdout explicitly. Use fprintf(stderr, ...) for error messages.

Q:Why does fopen() return NULL?

A: The file doesn't exist (for "r" mode), you don't have permission, or the path is wrong. Always check for NULL and use perror() to see the actual error.

Q:When do I use "rb" vs "r" mode?

A: Use "rb" (binary) for non-text files (images, executables, data). Use "r" (text) for .txt, .c, etc. On Windows, text mode translates \\r\\n to \\n — binary mode reads bytes exactly as stored.

✓Additional Tips

Buffer Flushing

Output to stdout is line-buffered by default. If you need immediate output (e.g., progress indicators), call fflush(stdout) after printing. This forces buffered data to be written immediately.

Binary vs Text Mode

On Windows, text mode ("r", "w") converts \\n to \\r\\n on output and vice versa on input. Binary mode ("rb", "wb") reads/writes bytes exactly. Always use binary mode for non-text files (images, executables).

Error Checking

Always check return values! fopen() returns NULL on failure.printf() returns number of characters written. Use ferror() and feof()to diagnose stream problems.

10Summary

Key Functions

Output:

printf, fprintf, sprintf, snprintf

Input:

scanf, fscanf, sscanf, fgets

Character:

getchar, putchar, fgetc, fputc

File:

fopen, fclose, fread, fwrite

11Practical I/O Tips

Buffered vs Unbuffered I/O

stdio uses buffering by default for efficiency. Use fflush(stdout) to force output before expected, or setvbuf() to change buffering mode. stderr is typically unbuffered so error messages appear immediately.

Binary vs Text Mode

On Windows, text mode converts \n to \r\n. Use "rb" and "wb" for binary files to prevent corruption. On Unix, there's no difference, but using binary mode makes code portable.

Safer Alternatives

Always prefer fgets() over gets() (removed in C11), snprintf() over sprintf(), and check scanf return values. These prevent buffer overflows and detect input errors.

Handle Errors Gracefully

Always check return values: fopen() returns NULL on failure,scanf() returns the number of items read. Useferror() and feof() to distinguish between errors and end-of-file conditions.

Close What You Open

For every fopen(), there should be a correspondingfclose(). File handles are limited resources—leaving them open causes resource leaks and can prevent other processes from accessing files.

?Why is stdio.h Important?

Every C program you'll ever write likely starts with #include <stdio.h>. It's the foundation of all input/output operations in C.

Console I/O

printf, scanf, getchar

›

File Operations

fopen, fread, fwrite

›

Formatted Output

%d, %f, %s specifiers

New to I/O? Start with the beginner-friendly Input/Output Tutorial first. This page is a quick reference for all stdio.h functions.

01Introduction to stdio.h

What is stdio.h?

<stdio.h> (Standard Input/Output) is the most commonly used header in C. It provides functions for console I/O, file handling, and formatted output.

Key Functions Overview

Category	Functions
Output	printf, fprintf, sprintf, snprintf
Input	scanf, fscanf, sscanf
Character	getchar, putchar, getc, putc, fgetc, fputc
String	gets, puts, fgets, fputs
File	fopen, fclose, fread, fwrite, fseek, ftell

02printf() Family - Formatted Output

printf() - Print to Console

int printf(const char *format, ...);

format: Format string with specifiers

...: Values to print

Returns: Number of characters printed

main.c

printf("Name: %s, Age: %d\n", "Alice", 25);
// Output: Name: Alice, Age: 25

fprintf() - Print to File/Stream

int fprintf(FILE *stream, const char *format, ...);

stream: File pointer (FILE *)

format: Format string

main.c

FILE *fp = fopen("log.txt", "w");
fprintf(fp, "Error code: %d\n", 404);
fprintf(stderr, "Error occurred!\n");  // Print to error stream
fclose(fp);

sprintf() - Print to String

int sprintf(char *str, const char *format, ...);

str: Destination buffer

format: Format string

main.c

char buffer[100];
sprintf(buffer, "Score: %d/%d", 85, 100);
printf("%s\n", buffer);  // Output: Score: 85/100

snprintf() - Safe Print to String

int snprintf(char *str, size_t size, const char *format, ...);

str: Destination buffer

size: Max bytes to write (including \\0)

main.c

char buffer[20];
snprintf(buffer, sizeof(buffer), "Long text: %s", "This is a very long string");
// Safe: Won't overflow buffer!

✓Recommended: Always use snprintf() over sprintf() to prevent buffer overflow!

03scanf() Family - Formatted Input

scanf() - Read from Console

int scanf(const char *format, ...);

format: Format string with specifiers

...: Pointers to variables

Returns: Number of items successfully read

main.c

int age;
char name[50];
printf("Enter name and age: ");
scanf("%s %d", name, &age);  // Note: & for non-arrays
printf("Hello %s, you are %d years old\n", name, age);

fscanf() - Read from File

int fscanf(FILE *stream, const char *format, ...);

main.c

FILE *fp = fopen("data.txt", "r");
int id;
float price;
fscanf(fp, "%d %f", &id, &price);
fclose(fp);

sscanf() - Read from String

int sscanf(const char *str, const char *format, ...);

main.c

char data[] = "John 25 85.5";
char name[20];
int age;
float score;
sscanf(data, "%s %d %f", name, &age, &score);
// name="John", age=25, score=85.5

04Character I/O Functions

Function	Prototype	Description
getchar()	int getchar(void)	Read one char from stdin
putchar()	int putchar(int c)	Write one char to stdout
fgetc()	int fgetc(FILE *stream)	Read char from file
fputc()	int fputc(int c, FILE *stream)	Write char to file
ungetc()	int ungetc(int c, FILE *stream)	Push char back to stream

char_io.c

1#include <stdio.h>
2
3int main() {
4    // Read and echo characters until Enter
5    printf("Type something: ");
6    int ch;
7    while ((ch = getchar()) != '\n') {
8        putchar(ch);  // Echo each character
9    }
10    putchar('\n');
11    
12    // File character I/O
13    FILE *fp = fopen("test.txt", "r");
14    if (fp) {
15        while ((ch = fgetc(fp)) != EOF) {
16            putchar(ch);  // Print file contents
17        }
18        fclose(fp);
19    }
20    return 0;
21}

05String I/O Functions

fgets() - Safe String Input

char *fgets(char *str, int n, FILE *stream);

str: Destination buffer

n: Max chars to read

stream: Input stream (stdin for console)

main.c

char name[50];
printf("Enter name: ");
fgets(name, sizeof(name), stdin);  // Safe: won't overflow!
// Note: fgets includes the newline character

fputs() - Write String to Stream

int fputs(const char *str, FILE *stream);

main.c

fputs("Hello World!\n", stdout);  // To console
FILE *fp = fopen("out.txt", "w");
fputs("Written to file", fp);
fclose(fp);

puts() - Simple String Output

int puts(const char *str);

main.c

puts("Hello World!");  // Automatically adds newline
// Same as: printf("Hello World!\n");

Never Use gets()!

gets() is deprecated and dangerous — no buffer size limit causes overflow. Always use fgets() instead!

06File Operations

Function	Prototype	Description
fopen()	FILE fopen(const char path, const char *mode)	Open file
fclose()	int fclose(FILE *stream)	Close file
fread()	size_t fread(void ptr, size_t size, size_t n, FILE s)	Read binary data
fwrite()	size_t fwrite(const void ptr, size_t size, size_t n, FILE s)	Write binary data
fseek()	int fseek(FILE *stream, long offset, int whence)	Move file position
ftell()	long ftell(FILE *stream)	Get current position
rewind()	void rewind(FILE *stream)	Go to file start
feof()	int feof(FILE *stream)	Check end of file
ferror()	int ferror(FILE *stream)	Check for error

file_ops.c

1#include <stdio.h>
2
3int main() {
4    // Write to file
5    FILE *fp = fopen("data.txt", "w");
6    if (fp == NULL) {
7        perror("Error opening file");
8        return 1;
9    }
10    fprintf(fp, "Hello, File!\n");
11    fclose(fp);
12    
13    // Read from file
14    fp = fopen("data.txt", "r");
15    char buffer[100];
16    while (fgets(buffer, sizeof(buffer), fp) != NULL) {
17        printf("%s", buffer);
18    }
19    fclose(fp);
20    
21    return 0;
22}

07File Opening Modes

Mode	Description	If File Exists	If Not Exists
"r"	Read	Open	Error (NULL)
"w"	Write	Truncate (empty)	Create
"a"	Append	Open at end	Create
"r+"	Read/Write	Open	Error
"w+"	Read/Write	Truncate	Create
"rb"	Read binary	Add 'b' for binary mode

08Standard Streams

stdin

Standard Input (keyboard)

stdout

Standard Output (console)

stderr

Standard Error (errors)

!Code Pitfalls: Common Mistakes & What to Watch For

Copied code often generates printf/scanf code with format string vulnerabilities or mismatched format specifiers.

10Frequently Asked Questions

Q:What does fflush(stdout) do and when should I use it?

Q:Why should I use fgets() instead of gets()?

A: gets() has no buffer size limit — it's a security vulnerability and was removed from C11. fgets(buf, size, stdin) is safe because you specify the maximum characters to read.

Q:What's the difference between printf and fprintf?

A: printf() writes to stdout (the screen). fprintf() writes to any FILE* — a file, stderr, or even stdout explicitly. Use fprintf(stderr, ...) for error messages.

Q:Why does fopen() return NULL?

A: The file doesn't exist (for "r" mode), you don't have permission, or the path is wrong. Always check for NULL and use perror() to see the actual error.

Q:When do I use "rb" vs "r" mode?

A: Use "rb" (binary) for non-text files (images, executables, data). Use "r" (text) for .txt, .c, etc. On Windows, text mode translates \\r\\n to \\n — binary mode reads bytes exactly as stored.

✓Additional Tips

Buffer Flushing

Output to stdout is line-buffered by default. If you need immediate output (e.g., progress indicators), call fflush(stdout) after printing. This forces buffered data to be written immediately.

Binary vs Text Mode

Error Checking

Always check return values! fopen() returns NULL on failure.printf() returns number of characters written. Use ferror() and feof()to diagnose stream problems.

10Summary

Key Functions

Output:

printf, fprintf, sprintf, snprintf

Input:

scanf, fscanf, sscanf, fgets

Character:

getchar, putchar, fgetc, fputc

File:

fopen, fclose, fread, fwrite

11Practical I/O Tips

Buffered vs Unbuffered I/O

Binary vs Text Mode

On Windows, text mode converts \n to \r\n. Use "rb" and "wb" for binary files to prevent corruption. On Unix, there's no difference, but using binary mode makes code portable.

Safer Alternatives

Always prefer fgets() over gets() (removed in C11), snprintf() over sprintf(), and check scanf return values. These prevent buffer overflows and detect input errors.

Handle Errors Gracefully

Always check return values: fopen() returns NULL on failure,scanf() returns the number of items read. Useferror() and feof() to distinguish between errors and end-of-file conditions.

Close What You Open

For every fopen(), there should be a correspondingfclose(). File handles are limited resources—leaving them open causes resource leaks and can prevent other processes from accessing files.

What You Will Learn

?Why is stdio.h Important?

01Introduction to stdio.h

What is stdio.h?

Key Functions Overview

02printf() Family - Formatted Output

printf() - Print to Console

fprintf() - Print to File/Stream

sprintf() - Print to String

snprintf() - Safe Print to String

03scanf() Family - Formatted Input

scanf() - Read from Console

fscanf() - Read from File

sscanf() - Read from String

04Character I/O Functions

05String I/O Functions

fgets() - Safe String Input

fputs() - Write String to Stream

puts() - Simple String Output

Never Use gets()!

06File Operations

07File Opening Modes

08Standard Streams

!Code Pitfalls: Common Mistakes & What to Watch For

Copied code often generates printf/scanf code with format string vulnerabilities or mismatched format specifiers.

10Frequently Asked Questions

Q:What does fflush(stdout) do and when should I use it?

Q:Why should I use fgets() instead of gets()?

Q:What's the difference between printf and fprintf?

Q:Why does fopen() return NULL?

Q:When do I use "rb" vs "r" mode?

✓Additional Tips

Buffer Flushing

Binary vs Text Mode

Error Checking

10Summary

Key Functions

11Practical I/O Tips

Buffered vs Unbuffered I/O

Binary vs Text Mode

Safer Alternatives

Handle Errors Gracefully

Close What You Open

Test Your Knowledge

Related Tutorials

Input/Output in C

C Standard Library Overview

C stdlib.h Library Reference

Have Feedback?

What You Will Learn

?Why is stdio.h Important?

01Introduction to stdio.h

What is stdio.h?

Key Functions Overview

02printf() Family - Formatted Output

printf() - Print to Console

fprintf() - Print to File/Stream

sprintf() - Print to String

snprintf() - Safe Print to String

03scanf() Family - Formatted Input

scanf() - Read from Console

fscanf() - Read from File

sscanf() - Read from String

04Character I/O Functions

05String I/O Functions

fgets() - Safe String Input

fputs() - Write String to Stream

puts() - Simple String Output

Never Use gets()!

06File Operations

07File Opening Modes

08Standard Streams

!Code Pitfalls: Common Mistakes & What to Watch For

Copied code often generates printf/scanf code with format string vulnerabilities or mismatched format specifiers.

10Frequently Asked Questions

Q:What does fflush(stdout) do and when should I use it?

Q:Why should I use fgets() instead of gets()?

Q:What's the difference between printf and fprintf?

Q:Why does fopen() return NULL?

Q:When do I use "rb" vs "r" mode?