Reference: Format Specifiers

Big idea

A format specifier is the %… code in a printf or scanf string that says how to print or read one value. The specifier must match the type of its argument — a mismatch is undefined behaviour, not just ugly output.

The main specifiers

The one asymmetry to remember: printf uses %f for double; scanf needs %lf for double.

Width, precision, and flags

The full shape is %[flags][width][.precision]specifier:

#include <stdio.h>

int main(void) {
    const char *names[] = {"Tea", "Coffee", "Hot chocolate"};
    double prices[] = {1.5, 2.25, 3};
    for (int i = 0; i < 3; i++) {
        printf("%-14s %6.2f\n", names[i], prices[i]);
    }
    printf("%05d %+d %x %X %o %%\n", 42, 42, 255, 255, 8);
    printf("%g %g\n", 1234.5, 0.0001);
    return 0;
}

output
Tea              1.50
Coffee           2.25
Hot chocolate    3.00
00042 +42 ff FF 10 %
1234.5 0.0001

scanf specifics

• %d, %f, %s and friends skip leading whitespace (spaces, newlines). %c does not — write " %c" with a space to skip it.
• %s reads one word and stops at whitespace. Always give a width one less than the array: char w[20]; scanf("%19s", w);.
• %[^\n] reads everything up to a newline (a whole line, spaces included): scanf(" %99[^\n]", line);.
• scanf returns how many items it filled — check it. See Taking Input with scanf.

Gotchas

• printf("%d", 3.5) — passing a double to %d prints garbage. Cast: printf("%d", (int) 3.5).
• printf("%f", 7) — passing an int to %f is just as wrong. Use 7.0 or (double) 7.
• printf("%d", sizeof x) — sizeof is size_t; use %zu.
• printf(user_text) — if the text contains %, printf goes looking for arguments that aren't there. Write printf("%s", user_text).
• Old Windows runtimes may not understand %zu or %lld; recent ones do.

Key terms

• Format string — The text with % codes that printf/scanf follow.
• Specifier — The letter(s) after % saying the type: d, f, s, …
• Width / precision — Minimum field width, and digits after the point (or max characters for %s).
• Flag — - (left-align), 0 (zero-pad), + (show sign), and others.