Is the output of printf (“%d %d”, c++, c); also undefined?

Question

I recently came across a post What is the correct answer for cout << c++ << c;? and was wondering whether the output of

int c = 0;  
printf (         


        

Answer 1

You are right: it is undefined. The reason is that, though it is guaranteed that the three arguments to printf() will be evaluated before printf() is called, the sequence in which the three arguments are evaluated is undefined.

It is technically incorrect that the incrementation occurs only after the semicolon, incidentally. What the standard guarantees is that the incrementation will occur no later than the semicolon. [Actually, in your case, I believe that the standard guarantees that it will occur before control is passed to the printf() function -- but now this answer is starting to spin off into realms of pedantic trivia, so let me let the matter rest there!]

Anyway, in short, you are right. The behavior is undefined.

Update: As @R.. rightly observes, the undefined behavior comes from the lack of a sequence point between arguments. The standard is quite careful regarding the participles unspecified and undefined, so the correction is accepted with thanks.