printf formats values into text on stdout; scanf parses text from stdin into variables. Both are driven by format specifiers that must match the argument types exactly.

Format specifiers

Specifier	Type (printf)	Type (scanf)	Example output
`%d` / `%i`	`int`	`int *`	`-42`
`%u`	`unsigned int`	`unsigned *`	`42`
`%ld` / `%lld`	`long` / `long long`	pointer to same	`9000000000`
`%zu`	`size_t`	`size_t *`	`8`
`%f`	`double` (and `float`, promoted)	`float *`	`3.140000`
`%lf`	`double`	`double ` (required*)	`3.140000`
`%e` / `%g`	`double` scientific / shortest	same as `%f`	`3.14e+00` / `3.14`
`%c`	`int` (a char)	`char *`	`A`
`%s`	`char *` (NUL-terminated)	`char *` (array)	`hello`
`%x` / `%X` / `%o`	`unsigned` hex / HEX / octal	`unsigned *`	`ff` / `FF` / `377`
`%p`	`void *`	`void **`	`0x16d…`
`%%`	a literal `%`	—	`%`

Width, precision and flags

#include <stdio.h>

int main(void) {
    int n = 42;
    double pi = 3.14159265;
    printf("[%d] [%5d] [%-5d] [%05d] [%+d]\n", n, n, n, n, n);
    printf("[%f] [%.2f] [%8.3f] [%e] [%g]\n", pi, pi, pi, pi, pi);
    printf("[%x] [%X] [%#x] [%o] [%#o]\n", 255, 255, 255, 8, 8);
    printf("[%s] [%10s] [%-10s] [%.3s]\n", "hello", "hi", "hi", "truncate");
    printf("[%*d] [%.*f]\n", 6, n, 1, pi);    /* width / precision from args */
    printf("100%% done, char %c, code %d\n", 'A', 'A');
    return 0;
}

Output

[42] [   42] [42   ] [00042] [+42]
[3.141593] [3.14] [   3.142] [3.141593e+00] [3.14159]
[ff] [FF] [0xff] [10] [010]
[hello] [        hi] [hi        ] [tru]
[    42] [3.1]
100% done, char A, code 65

Reading input with scanf

#include <stdio.h>

int main(void) {
    int age;
    double height;
    char name[20];

    /* scanf needs ADDRESSES: &age, &height — but an array name already is one */
    int got = scanf("%19s %d %lf", name, &age, &height);
    printf("read %d items: %s is %d years, %.2f m\n", got, name, age, height);
    return 0;
}

Output

read 3 items: Asha is 21 years, 1.68 m

scanf returns the number of items successfully converted (or EOF). Always check it — on bad input the variables are left untouched:

#include <stdio.h>

int main(void) {
    int x = -1;
    if (scanf("%d", &x) != 1) {
        printf("not a number, x still %d\n", x);
    } else {
        printf("x = %d\n", x);
    }
    return 0;
}

Output

not a number, x still -1

Reading a whole line: fgets

#include <stdio.h>
#include <string.h>

int main(void) {
    char line[64];
    while (fgets(line, sizeof line, stdin) != NULL) {
        line[strcspn(line, "\n")] = '\0';   /* strip the trailing newline */
        printf("got \"%s\" (%zu chars)\n", line, strlen(line));
    }
    return 0;
}

Output

got "hello world" (11 chars)
got "second line" (11 chars)

Character I/O

#include <stdio.h>
#include <ctype.h>

int main(void) {
    int c;                         /* int, not char: must hold EOF (-1) too */
    while ((c = getchar()) != EOF) {
        putchar(toupper(c));
    }
    return 0;
}

Output

SHOUT THIS

Pitfalls

Mistake	Consequence	Fix
`scanf("%d", x)` — missing `&`	Writes to a garbage address → crash	`scanf("%d", &x)`
`scanf("%s", buf)` with no width	Buffer overflow on long input	`scanf("%19s", buf)` for `char buf[20]`
`%f` to read a `double`	Writes only 4 bytes → wrong value	Use `%lf` in `scanf`
`gets(buf)`	Unbounded — removed in C11	`fgets(buf, sizeof buf, stdin)`
`scanf("%d")` then `fgets`	`fgets` reads the leftover `\n` → empty line	Consume the rest of the line first
`printf(user_input)`	Format-string vulnerability	`printf("%s", user_input)`

Why is getchar's result stored in an int?

Why does printf use %f for both float and double, but scanf needs %lf for double?