C Input Output printf, scanf, Format Specifiers

Checked Standard I/O

C standard I/O uses streams and formatted conversion. Every input operation can reach end-of-file, encounter invalid text, or fail, and every output operation can fail after buffering. Correct code checks return values and keeps format specifiers consistent with argument types.

printf() - Formatted Output

printf() is the standard output function in C, defined in <stdio.h>. It prints formatted text to the console using format specifiers.

Format Specifier	Type	Example
%d or %i	int	printf("%d", 42);
%f	float	printf("%f", 3.14f);
%lf	double	printf("%lf", 3.14);
%c	char	printf("%c", 'A');
%s	string (char array)	printf("%s", "hello");
%u	unsigned int	printf("%u", 42u);
%ld	long int	printf("%ld", 1000000L);
%lld	long long int	printf("%lld", 9999999999LL);
%o	Octal	printf("%o", 8); -> 10
%x	Hexadecimal (lowercase)	printf("%x", 255); -> ff
%X	Hexadecimal (uppercase)	printf("%X", 255); -> FF
%e	Scientific notation	printf("%e", 12345.6);
%p	Pointer address	printf("%p", ptr);
%%	Literal % sign	printf("100%%");

Escape Sequences

Escape Sequence	Meaning
\n	Newline
\t	Horizontal tab
\	Backslash
\"	Double quote
\'	Single quote
\0	Null character (string terminator)
\r	Carriage return
\a	Alert (bell)

printf with Various Format Specifiers

#include <stdio.h>

int main() {
    int    age    = 25;
    float  price  = 9.99f;
    double pi     = 3.14159265;
    char   grade  = 'A';
    char   name[] = "Alice";

    printf("Name:   %s\n",  name);
    printf("Age:    %d\n",  age);
    printf("Grade:  %c\n",  grade);
    printf("Price:  %.2f\n", price);   // 2 decimal places
    printf("Pi:     %.4lf\n", pi);     // 4 decimal places

    // Width and alignment
    printf("\n%-10s %5d\n", "Alice",  25);  // left-align name, right-align age
    printf("%-10s %5d\n",  "Bob",    30);
    printf("%-10s %5d\n",  "Charlie", 22);

    // Hex and octal
    printf("\n255 in hex:   %x\n", 255);   // ff
    printf("255 in octal: %o\n",  255);   // 377
    printf("Percent sign: 100%%\n");

    return 0;
}

scanf() - Formatted Input

scanf() reads formatted input from the keyboard. You must pass the address of the variable using the & operator (except for strings/arrays).

getchar() / putchar() and gets() / puts()

getchar() reads a single character from stdin. putchar() writes a single character to stdout. gets() is unsafe (no bounds checking) - use fgets() instead.

scanf, getchar and putchar

#include <stdio.h>

int main() {
    int    age;
    float  salary;
    char   name[50];

    // scanf reads formatted input; & gives the address of the variable
    printf("Enter your name: ");
    fgets(name, sizeof(name), stdin);  // safer than gets()

    printf("Enter your age: ");
    scanf("%d", &age);

    printf("Enter your salary: ");
    scanf("%f", &salary);

    printf("\nName:   %s", name);
    printf("Age:    %d\n", age);
    printf("Salary: %.2f\n", salary);

    // getchar / putchar - single character I/O
    printf("\nEnter a character: ");
    getchar();  // consume leftover newline from scanf
    char ch = getchar();
    printf("You entered: ");
    putchar(ch);
    putchar('\n');

    // puts - prints string with automatic newline
    puts("Done!");

    return 0;
}

Input Boundaries

Prefer fgets into a bounded array for interactive text, then parse with strtol or another checked converter. scanf can be useful for controlled formats, but an unchecked %s can overflow and a failed conversion leaves unread text that may break the next attempt.

printf format mismatches cause undefined behavior because variadic arguments carry no automatic format correction. Enable format warnings. Check fflush or fclose for important output because a delayed disk error may appear after fprintf seemed to succeed.

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