Why would you use the ternary operator without assigning a value for the “true” condition (x = x ?: 1)

Question

In the Android open-source qemu code I ran across this line of code:

machine->max_cpus = machine->max_cpus ?: 1; /* Default to UP */

Answer 1

This is a GCC extension that means "if the condition is true, use it, else use this other value", so

machine->max_cpus = machine->max_cpus ?: 1;

is shorthand for

machine->max_cpus = machine->max_cpus ? machine->max_cpus : 1;

although if the conditional has side-effects, it will only be run once