C / STRUCTURES AND ENUMS
C unions
Share storage among alternative fields and track which member is active.
What you will learn
- Explain c unions in C using the correct mental model
- Trace a focused C example and predict its result before execution
- Recognize a boundary case involving the lesson topic and handle it deliberately
Understanding C unions
C unions is easier to use when you start from its execution model rather than memorizing punctuation. In this lesson, the central rule is that all union members overlap the same storage.
Read the example from the declarations toward the observable result. For unions, identify the objects involved, the operation performed on them, and the condition that must remain true for the operation to be valid.
C gives the programmer direct control, so many boundaries are not checked automatically. Compile with warnings, validate external data, and keep ownership, capacity, range, and lifetime visible whenever they matter.
After tracing the normal case, change one input and predict the result before running the program. That habit exposes misunderstandings earlier than copying a larger example.
C unions is a defining part of practical C work. Start by identifying the data or state involved, then trace the operation that changes or interprets it. Pay attention to the rules C applies at this boundary, because those rules explain both the useful behavior and the common failure modes. This lesson keeps the example deliberately small, then connects it to c typedef so the ideas form a coherent progression rather than a list of isolated syntax facts.
Worked examples
C unions example
A focused program demonstrating unions with a result that can be checked directly.
union Number { int integer; float decimal; };
#include <stdio.h>
int main(void) {
union Number number;
number.integer = 7;
printf("%d\n", number.integer);
return 0;
}Example explained
Line 1The declarations establish the values and types needed for unions.
Line 2The central statement demonstrates that all union members overlap the same storage.
Line 3The output makes the program state observable so the result can be compared with the prediction.
Line 4The return status reports successful completion after all required cleanup is finished.
Important notes
Keep the unions example small until every declaration and operation can be explained.
Compiler warnings are part of the feedback loop; do not silence one without understanding the condition it reports.
Common mistakes
Treating c unions as punctuation to memorize instead of a C behavior to reason about.
Ignoring an edge case until it appears in production data or a larger program.
Try it yourself
Change, predict, then run
Create a small C example that demonstrates c unions. Add a normal case and a boundary case, write down the expected result for each, then explain which C rule produces that result. Lesson 39 should remain small enough to trace without guessing.
Open C workspaceCheck your understanding
Which statement best describes the key rule for unions?
- all union members overlap the same storage
- C checks every boundary automatically
- Formatting changes runtime behavior
- Every value is stored as text
Show answer
The defining rule here is that all union members overlap the same storage.