Integer conversion occurs when a value of one integer type is converted to another integer type. The result depends on the destination type and whether it can represent the original value.
#include <stdio.h>
int main(void)
{
int number = 130;
signed char small = number;
unsigned int positive = -10;
printf("Original: %d\n", number);
printf("Converted: %d\n", small);
printf("Unsigned: %u\n", positive);
return 0;
}Example Output #
On a typical implementation with 8-bit signed char using two’s-complement representation and 32-bit unsigned int:
Original: 130
Converted: -126
Unsigned: 4294967286Explanation #
The assignment:
signed char small = number;converts the value 130 to signed char. If the destination type cannot represent the original value, the result is implementation-defined in C17 and earlier. C23 specifies the result differently. The example output reflects a common implementation.
The assignment:
unsigned int positive = -10;converts -10 to unsigned int. Unsigned integer conversion produces the value congruent to the original value modulo one more than the maximum value of the destination type.
For a 32-bit unsigned int, the result is:\[ 2^{32}-10 = 4294967286 \]
Integer Conversion Rules #
| Conversion | Behavior |
|---|---|
| Smaller value to a wider integer type that can represent it | Value is preserved |
| Signed integer to unsigned integer | Result is reduced modulo one more than the unsigned maximum |
| Unsigned integer to a signed integer type that can represent the value | Value is preserved |
| Value outside the destination signed type’s range | Result depends on the applicable C standard |
| Wider integer type to a narrower type | Value may change if it is not representable |
Integer conversions can change a value even when no arithmetic operation is performed. The destination type determines how the converted value is represented.
