Input/Output in C

01What is Input/Output in C?

📺 Simple Definition

Input/Output (I/O) is how your program communicates with the outside world. Input = getting data IN (from keyboard, file). Output = sending data OUT (to screen, file).

📚 The stdio.h Header

All I/O functions come from the <stdio.h> header (Standard Input/Output).

#include <stdio.h>

📖 See the complete stdio.h Reference for all functions.

Function	Purpose	Type
printf()	Print formatted output to screen	Output
scanf()	Read formatted input from keyboard	Input
getchar()	Read single character	Input
putchar()	Print single character	Output
gets() / fgets()	Read string (line)	Input
puts()	Print string with newline	Output

02The printf() Function

📝 Syntax

printf("format string", value1, value2, ...);

The format string contains text and format specifiers (%d, %f, etc.)

📝 This program shows different ways to use printf for various data types.

printf_basic.c

1#include <stdio.h>
2
3int main() {
4    printf("Hello!\n");           // Text
5    printf("Age: %d\n", 25);      // Integer
6    printf("Price: %.2f\n", 9.99); // Float
7    printf("Grade: %c\n", 'A');   // Character
8    printf("Name: %s\n", "Bob");  // String
9    return 0;
10}

Output

Hello!

Age: 25

Price: 9.99

Grade: A

Name: Bob

03Format Specifiers

🎯 What are Format Specifiers?

Format specifiers are placeholders that tell printf/scanf what type of data to expect and how to format it.

Specifier	Data Type	Example	Output
%d or %i	int (signed)	printf("%d", 42)	42
%u	unsigned int	printf("%u", 42)	42
%ld	long int	printf("%ld", 123456789L)	123456789
%lld	long long int	printf("%lld", 9999999999LL)	9999999999
%f	float / double	printf("%f", 3.14)	3.140000
%.2f	float (2 decimals)	printf("%.2f", 3.14159)	3.14
%e	Scientific notation	printf("%e", 12345.67)	1.234567e+04
%c	char	printf("%c", 'A')	A
%s	string (char array)	printf("%s", "Hello")	Hello
%x / %X	Hexadecimal	printf("%x", 255)	ff
%o	Octal	printf("%o", 8)	10
%p	Pointer address	printf("%p", &x)	0x7fff...
%%	Literal %	printf("100%%")	100%

📏 Width and Precision Modifiers

format_modifiers.c

1#include <stdio.h>
2
3int main() {
4    printf("[%d]\n", 42);      // Normal: [42]
5    printf("[%5d]\n", 42);     // Width 5: [   42]
6    printf("[%05d]\n", 42);    // Zero-pad: [00042]
7    
8    printf("[%.2f]\n", 3.14159); // 2 decimals: [3.14]
9    printf("[%8.2f]\n", 3.14);   // Width 8: [    3.14]
10    return 0;
11}

Output

[42]

[ 42]

[00042]

[3.14]

[ 3.14]

04printf Variants

Function	Description	Use Case
printf()	Print to stdout (screen)	Normal console output
fprintf()	Print to file stream	Write to files, stderr
sprintf()	Print to string buffer	Format string in memory
snprintf()	Print to string (size limited)	Safe string formatting

printf_variants.c

1#include <stdio.h>
2
3int main() {
4    char buf[50];
5    
6    printf("Hello!\n");                    // To screen
7    fprintf(stderr, "Error!\n");           // To stderr
8    sprintf(buf, "Age: %d", 25);           // To string
9    printf("Buffer: %s\n", buf);
10    
11    return 0;
12}

Output

Hello!

Error!

Buffer: Age: 25

💡 Always Use snprintf!

sprintf() can cause buffer overflow! Always prefer snprintf() which limits the output to your buffer size.

05The scanf() Function

📥 Understanding scanf() - Reading Input

scanf() is a function that reads formatted input from the keyboard. The name stands for "scan formatted".

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

Defined in: <stdio.h>

format: Format string with specifiers like %d, %s, %f

...: Pointers to variables where input will be stored (use & before variables!)

Returns: Number of items successfully read, or EOF (-1) on failure

📝 Usage Syntax

scanf("format string", &variable1, &variable2, ...);

⚠️ Notice the & (address-of operator) before variables!

scanf_basic.c

1#include <stdio.h>
2
3int main() {
4    int age;
5    char name[20];
6    
7    printf("Enter age: ");
8    scanf("%d", &age);       // & required!
9    
10    printf("Enter name: ");
11    scanf("%s", name);       // No & for arrays
12    
13    printf("Hello %s, you are %d!\n", name, age);
14    return 0;
15}

Output

Enter age: 25

Enter name: Bob

Hello Bob, you are 25!

⚠️ scanf Gotchas

• Forgot &: Causes crash or undefined behavior
• %s stops at space: "John Doe" reads only "John"
• Buffer overflow: %s doesn't check array size
• Leftover newline: %c may read the newline from previous input

06scanf Variants

Function	Description	Use Case
scanf()	Read from stdin (keyboard)	Normal user input
fscanf()	Read from file stream	Parse file data
sscanf()	Read from string	Parse string data

sscanf_example.c

1#include <stdio.h>
2
3int main() {
4    char str[] = "Bob 25";
5    char name[20];
6    int age;
7    
8    // sscanf - parse from string
9    sscanf(str, "%s %d", name, &age);
10    printf("Name: %s, Age: %d\n", name, age);
11    
12    return 0;
13}

Output

Name: Bob, Age: 25

07Character and String I/O

Function	Prototype	Description
getchar	int getchar(void)	Returns next char from stdin (as int), or EOF on error
putchar	int putchar(int c)	Writes char c to stdout. Returns c or EOF on error
fgets	char fgets(char s, int n, FILE *stream)	Reads at most n-1 chars into s. Safe! Use stdin for keyboard
puts	int puts(const char *s)	Writes string s + newline to stdout. Returns non-negative or EOF

char_io.c

1#include <stdio.h>
2
3int main() {
4    char ch;
5    char name[50];
6    
7    // Single character
8    ch = getchar();     // Read one char
9    putchar(ch);        // Print one char
10    
11    // Full line (with spaces!)
12    fgets(name, 50, stdin);
13    puts(name);         // Print + newline
14    
15    return 0;
16}

Example

Input: A

Output: A

Input: John Doe

Output: John Doe

💡 Use fgets() Instead of scanf() for Strings!

scanf("%s", ...) stops at whitespace and can overflow buffers.
fgets() reads the entire line and is buffer-safe!

08main() with Command-Line Arguments

🎯 What are Command-Line Arguments?

Command-line arguments let you pass data to your program when you run it:

./program arg1 arg2 arg3

📝 main() Signatures

Without arguments:

int main(void)

int main()

With arguments:

int main(int argc, char *argv[])

int main(int argc, char **argv)

Parameter	Name	Description
argc	Argument Count	Number of arguments (including program name)
argv	Argument Vector	Array of strings (the actual arguments)
argv[0]	Program Name	Always the name of the program
argv[1..n]	User Arguments	Arguments passed by the user

main_args.c

1#include <stdio.h>
2
3int main(int argc, char *argv[]) {
4    printf("Arguments: %d\n", argc);
5    
6    for (int i = 0; i < argc; i++) {
7        printf("argv[%d] = %s\n", i, argv[i]);
8    }
9    
10    if (argc >= 2) {
11        printf("Hello, %s!\n", argv[1]);
12    }
13    return 0;
14}

$ ./program Bob

Arguments: 2

argv[0] = ./program

argv[1] = Bob

Hello, Bob!

🔧 String to Number Conversion

Function	Converts To	Example
atoi()	int	atoi("42") → 42
atof()	double	atof("3.14") → 3.14

atoi_example.c

1#include <stdio.h>
2#include <stdlib.h>
3
4int main(int argc, char *argv[]) {
5    if (argc >= 2) {
6        int num = atoi(argv[1]);  // String to int
7        printf("Number: %d\n", num);
8    }
9    return 0;
10}

$ ./program 42

Number: 42

09Common Mistakes

❌ Forgetting & in scanf

scanf("%d", num); // Wrong!

scanf("%d", &num); // Correct!

❌ Wrong format specifier

printf("%d", 3.14); // float with %d!

printf("%f", 3.14); // Correct!

❌ Buffer overflow with %s

char s[10];

scanf("%s", s); // Dangerous!

char s[10];

scanf("%9s", s); // Safe!

❌ Newline issue with %c

scanf("%c", &ch); // Reads leftover \\n

scanf(" %c", &ch); // Space skips whitespace

❌ Not checking return value

if (scanf("%d", &num) != 1) {

printf("Invalid input!\\n");

}

10Summary

🎯 Key Takeaways

•printf: Formatted output to screen (variants: fprintf, sprintf, snprintf)
•scanf: Formatted input from keyboard (variants: fscanf, sscanf)
•Format specifiers: %d (int), %f (float), %c (char), %s (string), %p (pointer)
•fgets(): Safer than scanf for strings (handles spaces, limits size)
•argc/argv: Access command-line arguments in main()
•Conversion: atoi(), atof(), strtol() for string to number

12Next Steps

Now that you understand input/output in C, learn about operators to perform calculations:

Next: Operators→Review: Data Types

What You Will Learn

01What is Input/Output in C?

📺 Simple Definition

📚 The stdio.h Header

02The printf() Function

📝 Syntax

03Format Specifiers

🎯 What are Format Specifiers?

📏 Width and Precision Modifiers

04printf Variants

💡 Always Use snprintf!

05The scanf() Function

📥 Understanding scanf() - Reading Input

📝 Usage Syntax

⚠️ scanf Gotchas

06scanf Variants

07Character and String I/O

💡 Use fgets() Instead of scanf() for Strings!

08main() with Command-Line Arguments

🎯 What are Command-Line Arguments?

📝 main() Signatures

🔧 String to Number Conversion

09Common Mistakes

❌ Forgetting & in scanf

❌ Wrong format specifier

❌ Buffer overflow with %s

❌ Newline issue with %c

❌ Not checking return value

10Summary

🎯 Key Takeaways

12Next Steps