void func(int c)
{
printf("%d\n", (getch() == 'a') ? c : 0);
}

halt(machine m,machine_code u,input x)

if m = u return Error illigal input

else return true if machine stops on x, false otherwise.

halt (Machine m,Input I)

if m = I return Error in input

else return true if m stops on i, false otherwise

halt(M,M1)

S (S + 1)
M = S
if Halt(S,S+1) loop
else halt.

S

S + 1

Halt(Machine M,Input I)
if M = I return Error
S = CallingAlgorithm(Halt)
S1 = CallintAlgorithmInput(S)
while S do
if S = M and S1 = I return Error
S = CallingAlgorithm(S)
end while
return true if M stops on I false otherwise

S = CallingAlgorithm(S)

while S do

Berger, R. (1966). "The undecidability of the domino problem", Memoirs Amer. Math. Soc. 66(1966).

Proving Program Termination, B. Cook et al., CACM v54 n5, May 2011

"In our new problem statement we will still require that a termination proving tool will always return answers that are correct, but we will not necessarily require an answer. If the termination prover cannot prove or disprove termination, it should return 'unknown."'

In contrast to popular belief, proving termination is not always impossible.