the compiler can choose to call the function just once and cache the return value.
This attribute marks the function as considering only its numeric parameters. This is mainly intended for the compiler to optimize away repeated calls to a function that the compiler knows will return the same value repeatedly. It applies mostly to math functions that have no static state or side effects, and whose return is solely determined by the inputs.
In this highly-contrived example, the compiler normally must call the square() function in every loop even though we know that it's going to return the same value each time:
extern int square(int n) __attribute__((const));
...
for (i = 0; i < 100; i++ )
{
total += square(5) + i;
}
By adding __attribute__((const)), the compiler can choose to call the function just once and cache the return value.
In virtually every case, const can't be used on functions that take pointers, because the function is not considering just the function parameters but also the data the parameters point to, and it will almost certainly break the code very badly in ways that will be nearly impossible to track down.
Furthermore, the functions so tagged cannot have any side effects or static state, so things like getchar() or time() would behave very poorly under these circumstances.