Assigning unsigned to signed types and vice versa

UpdatedDecember 31, 2023

I am Jyotiprakash, a deeply driven computer systems engineer, software developer, teacher, and philosopher. With a decade of professional experience, I have contributed to various cutting-edge software products in network security, mobile apps, and healthcare software at renowned companies like Oracle, Yahoo, and Epic. My academic journey has taken me to prestigious institutions such as the University of Wisconsin-Madison and BITS Pilani in India, where I consistently ranked among the top of my class.

At my core, I am a computer enthusiast with a profound interest in understanding the intricacies of computer programming. My skills are not limited to application programming in Java; I have also delved deeply into computer hardware, learning about various architectures, low-level assembly programming, Linux kernel implementation, and writing device drivers. The contributions of Linus Torvalds, Ken Thompson, and Dennis Ritchie—who revolutionized the computer industry—inspire me. I believe that real contributions to computer science are made by mastering all levels of abstraction and understanding systems inside out.

In addition to my professional pursuits, I am passionate about teaching and sharing knowledge. I have spent two years as a teaching assistant at UW Madison, where I taught complex concepts in operating systems, computer graphics, and data structures to both graduate and undergraduate students. Currently, I am an assistant professor at KIIT, Bhubaneswar, where I continue to teach computer science to undergraduate and graduate students. I am also working on writing a few free books on systems programming, as I believe in freely sharing knowledge to empower others.

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Jyotiprakash's Blog

251 posts

I'm Jyotiprakash, a software dev and professor at KIIT, with expertise in system programming.

Let's explore the interactions between signed and unsigned variables in C with complete examples, including the binary representations in the outputs. These examples will illustrate the effects of sign and zero extension, especially in corner cases.

1. Unsigned to Signed Assignment

Example:

#include <stdio.h>

int main() {
    unsigned int u = 4294967295; // Maximum unsigned int value
    int s = u; // Assigning unsigned to signed

    printf("Unsigned: %u, Binary: %x\n", u, u);
    printf("Signed: %d, Binary: %x\n", s, s);

    return 0;
}

Expected Output:

Unsigned: 4294967295, Binary: ffffffff
Signed: -1, Binary: ffffffff

Explanation: The maximum unsigned int, represented in binary as all 1s, becomes -1 in a signed int due to two's complement representation.

2. Signed to Unsigned Assignment

Example:

#include <stdio.h>

int main() {
    int s = -1; // Negative signed int
    unsigned int u = s; // Assigning signed to unsigned

    printf("Signed: %d, Binary: %x\n", s, s);
    printf("Unsigned: %u, Binary: %x\n", u, u);

    return 0;
}

Expected Output:

Signed: -1, Binary: ffffffff
Unsigned: 4294967295, Binary: ffffffff

Explanation: -1 in signed int, with binary representation as all 1s, is interpreted as the maximum unsigned int value when assigned to an unsigned variable.

3. Small Unsigned to Larger Signed Type

Example:

#include <stdio.h>

int main() {
    unsigned char uc = 255; // Maximum unsigned char value
    int s = uc; // Assigning small unsigned to larger signed

    printf("Unsigned char: %u, Binary: %x\n", uc, uc);
    printf("Signed int: %d, Binary: %x\n", s, s);

    return 0;
}

Expected Output:

Unsigned char: 255, Binary: ff
Signed int: 255, Binary: 000000ff

Explanation: The unsigned char value 255 (0xff) is zero-extended to fit into a larger signed int, maintaining its value.

4. Small Signed to Larger Unsigned Type

Example:

#include <stdio.h>

int main() {
    signed char sc = -1; // Negative signed char
    unsigned int u = sc; // Assigning small signed to larger unsigned

    printf("Signed char: %d, Binary: %x\n", sc, sc);
    printf("Unsigned int: %u, Binary: %x\n", u, u);

    return 0;
}

Expected Output:

Signed char: -1, Binary: ffffffff
Unsigned int: 4294967295, Binary: ffffffff

Explanation: The signed char -1 (0xff when cast to unsigned for printing) is sign-extended to an unsigned int, becoming the maximum value of unsigned int.

5. Larger Unsigned to Smaller Signed Type

Example:

#include <stdio.h>

int main() {
    unsigned int u = 4294967295; // Large unsigned int
    signed char sc = u; // Assigning larger unsigned to smaller signed

    printf("Unsigned int: %u, Binary: %x\n", u, u);
    printf("Signed char: %d, Binary: %x\n", sc, sc);

    return 0;
}

Expected Output:

Unsigned int: 4294967295, Binary: ffffffff
Signed char: -1, Binary: ff

Explanation: When the large unsigned int (0xffffffff) is assigned to a smaller signed char, it's truncated to 0xff, which is -1 in signed representation.

These examples show how the binary representations of signed and unsigned integers interact during assignments in C, highlighting the effects of sign and zero extension, and potential issues like truncation.

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C Programming

More from this blog

1. Unsigned to Signed Assignment

Example:

Expected Output:

2. Signed to Unsigned Assignment

Example:

Expected Output:

3. Small Unsigned to Larger Signed Type

Example:

Expected Output:

4. Small Signed to Larger Unsigned Type

Example:

Expected Output:

5. Larger Unsigned to Smaller Signed Type

Example:

Expected Output: