Integer Promotions in C

Integer Promotions in C

Some data types like char, short int take less number of bytes than int, these data types are automatically promoted to int or unsigned int when an operation is performed on them. This is called integer promotion. For example, no arithmetic calculation happens on smaller types like char, short and enum. They are first converted to int or unsigned int, and then arithmetic is done on them. If an int can represent all values of the original type, the value is converted to an int. Otherwise, it is converted to an unsigned int.

For example see the following program:

#include <stdio.h>
int main()
{
    char a = 30, b = 40, c = 10;
    char d = (a * b) / c;
    printf ("%d ", d);
    return 0;
}

Output:

120
At first look, the expression (a*b)/c seems to cause arithmetic overflow because signed characters can have values only from -128 to 127 (in most of the C compilers), and the value of subexpression ‘(a*b)’ is 1200 which is greater than 128. But integer promotion happens here in arithmetic done on char types and we get the appropriate result without any overflow.