When the GCC extensions setting is on, you may skip the second expression in a conditional expression. The default value for this expression is the first expression. Using the shorter form of the conditional expression shows an example.
void f(int i, int j)
{
int a = i ? i : j;
int b = i ?: j; /* Equivalent to int b = i ? i : j; */
/* Variables a and b are both assigned the same value. */
}