Default values for arguments in C

Question

Say I have a function in C defined as :

bool check ( int x, int y);

Now if I call it using check(4);

What will be the value of y that is taken?

It would read the second value of the stack and store it in y. Undefined behavior. — jn1kk
– jn1kk, Commented Jul 27, 2012 at 19:53
@jsn: Nothing in the language definition suggests that it would do anything with the stack; the standard doesn't even mention the word. — Keith Thompson
– Keith Thompson, Commented Jul 27, 2012 at 20:03
Why do you suppose that the 4 matches the first parameter? It could be that you "forgot" the first and not the second parameter. — Jens Gustedt
– Jens Gustedt, Commented Jul 27, 2012 at 20:51
@jsn: My point is that the behavior you described is not guaranteed. Even on systems with a contiguous stack, the behavior could vary depending on the order in which parameter are passed; I could easily imagine x being set to garbage and y to 4. (If you had written "could" rather than "should", I wouldn't have responded.) IMHO it's important to know what "undefined behavior" really means. — Keith Thompson
– Keith Thompson, Commented Jul 28, 2012 at 0:21

Community · Accepted Answer · 2020-06-20 09:12:55Z

7

Code like this can possibly compile only if the function is either undeclared (C89/90) or declared without a prototype (C89/90 and C99).

In any case the behavior will be undefined. If the number and/or type of promoted arguments used in the call do not match those used in the function definition, the behavior is undefined.

6.5.2.2 Function calls

6 [...] If the number of arguments does not equal the number of parameters, the behavior is undefined [...] If the function is defined with a type that does not include a prototype, and the types of the arguments after promotion are not compatible with those of the parameters after promotion, the behavior is undefined

edited Jun 20, 2020 at 9:12

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answered Jul 27, 2012 at 19:55

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2 Comments

Keith Thompson Over a year ago

A compiler is still permitted to reject the program; that's one possible consequence of undefined behavior.

Keith Thompson Over a year ago

@dasblinkenlight: Right, declarations are mandatory (as of C99), but prototypes are still optional, even as of C11. (Which is a pity, IMHO; non-prototype function declarations have been "obsolescent" since 1989.)

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