Integer promotion in C

Small types promote to int, and mixing signed with unsigned converts the signed one.

#include <stdio.h>

void promotion(void) {
    char a = 100;
    char b = 100;
    int sum = a + b;

    signed int negative = -1;
    unsigned int positive = 1;

    printf("sum %d, fits in char: %d\n", sum, sum <= 127);
    printf("-1 < 1u is %d\n", negative < positive);
    printf("cast fixes it: %d\n", negative < (int) positive);

    size_t length = 3;
    for (int i = (int) length - 1; i >= 0; i--) {
        printf("%d ", i);
    }
    printf("\n");
}

How it works

1. char and short are promoted to int before arithmetic.
2. A signed value compared against unsigned becomes unsigned.
3. That is why -1 < 1u is false, which surprises everyone once.

Keywords and builtins used here

• char
• for
• int
• promotion
• signed
• size_t
• unsigned
• void

The run, in numbers

Lines

20

Characters to type

405

Tokens

125

Three-star pace

80 tpm

At the three-star pace of 80 tokens a minute, this run takes about 94 seconds.

Type this snippet

Step 7 of 7 in Variables & types, step 7 of 35 in Language basics.

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