What You Will Learn

Prerequisites

Complete the earlier lectures in this chapter on printf and conversion specifiers. You need GCC or any C compiler. These printf practice questions are part of our free embedded systems course, and they make a good warm-up before the free Linux kernel development course, where printk uses nearly the same format rules.

How to Run Every printf Practice Question

Each question shows only the statements inside main. Wrap them in this template and compile with warnings enabled, because GCC will tell you when a format and its argument disagree:

#include <stdio.h>

int main(void)
{
    /* paste the statements here */
    return 0;
}

gcc -std=c17 -Wall -Wextra -o practice practice.c
./practice

Reading a printf Format Specification

Every conversion has the same anatomy: a percent sign, optional flags, optional width, optional precision, optional length modifier, and one conversion letter. Learn the shape once and every printf practice question becomes mechanical.

Escape Sequences You Must Know

The printf Practice Questions

For each question write your answer on paper first. Then open the box and compare. If your answer differs, run the program and find out which rule you missed.

Question 1: Tabs and new lines

    printf("Line1\nLine2\tTabbed\n");

Line1
Line2	Tabbed

Question 2: Carriage return

    printf("12345\rAB\n");

bytes sent : 1 2 3 4 5 \r A B \n

AB345

Question 3: Backspace

    printf("Cat\br\n");

bytes sent : C a t \b r \n

Car

Question 4: One number, four views

    int n = 255;
    printf("%d %o %x %X %c\n", n, n, n, n, 65);

255 377 ff FF A

Question 5: Signed versus unsigned

    int v = -1;
    printf("%d %u\n", v, (unsigned int)v);

-1 4294967295

Question 6: Characters are small integers

    char c = 'A';
    printf("%c %d %c\n", c, c, c + 2);

A 65 C

Question 7: Floating-point precision

    double pi = 3.14159265;
    printf("%f|%.2f|%8.3f|%-8.1f|\n", pi, pi, pi, pi);

3.141593|3.14|   3.142|3.1     |

Question 8: Width, left alignment, zero padding

    printf("[%5d][%-5d][%05d]\n", 42, 42, 42);

[   42][42   ][00042]

Question 9: Strings: width and precision

    printf("[%8s][%-8s][%.3s][%8.3s]\n", "Linux", "Linux", "Linux", "Linux");

[   Linux][Linux   ][Lin][     Lin]

Question 10: The return value of printf

    int n = printf("Hello\n");
    printf("%d%%\n", n);
    printf("%5.1f%%\n", 7.26);

Hello
6%
  7.3%

Question 11: Width taken from an argument

    printf("[%*d]\n", 6, 42);
    printf("[%-*d]\n", 6, 42);
    printf("[%.*f]\n", 3, 2.0 / 3);

[    42]
[42    ]
[0.667]

Question 12: Integer sizes and wrap-around

    short s = 50000;
    unsigned short u = 50000;
    int big = 50000;
    printf("%d %d %d\n", s, u, big);

-15536 50000 50000

Question 13: Bonus: alternate forms and exponent

    printf("%x %#x %#o\n", 255, 255, 8);
    printf("%e\n", 12345.678);

ff 0xff 010
1.234568e+04

Challenge: Align a Table with printf Format Specifiers

Print the numbers 1 to 5, their squares and half values in three tidy columns separated by bars. Try it before opening the solution.

#include <stdio.h>

int main(void)
{
    for (int i = 1; i <= 5; i++)
        printf("%2d | %3d | %5.1f\n", i, i * i, i / 2.0);
    return 0;
}

 1 |   1 |   0.5
 2 |   4 |   1.0
 3 |   9 |   1.5
 4 |  16 |   2.0
 5 |  25 |   2.5

Common printf Mistakes and Troubleshooting

Most wrong answers in these printf practice questions come from the mistakes below. Check this table whenever your prediction and the real output disagree.

char *text = get_line();   /* untrusted */
printf(text);              /* WRONG: input controls the format */
printf("%s", text);        /* RIGHT */

Best Practices for printf in Embedded Code

• Match every specifier to the exact argument type. Use %zu for size_t, and %lu for unsigned long.
• On small microcontrollers, a full printf with floating-point support can add many kilobytes of code. Many toolchains offer a reduced printf; check whether %f is enabled before relying on it.
• Print fixed-width integers from <stdint.h> with the macros in <inttypes.h>, for example PRIu32.
• Use width and zero flags for register dumps: %08x prints a 32-bit value with leading zeros.
• Leave warnings on. GCC’s format checking catches most of the mistakes in the table above.

Key Takeaways from These printf Practice Questions

If you could predict most outputs before opening the answers, your picture of how printf turns values into characters is solid. If not, repeat the questions you missed after a day. These printf practice questions are meant to be redone, not read once. Next in this free embedded C course we continue with formatted input and output using field widths with scanf.

Interview Questions

Frequently Asked Questions