Is calling printf with excess arguments undefined behaviour?

Question

I wonder if this yields in undefined behaviour:

printf("Test %d %s", 123, "abc", "def", "ghi");

The first two arguments after the format string match the format string, so these are OK; but the 3rd and 4th arguments are in excess because there are no more corresponding format specifiers.

IMHO printf() should simply ignore these excess arguments and there should be no UB. Is this correct?

Simulant

I think this question is like asking "Can a function that accepts varargs take any number of arguments?". The mechanism that allows the handling of a variable number of arguments works whatever the particulars of how the function uses those arguments... – Renteria

Answer 1

Yes, this scenario is explicitly defined by the standard. It is not undefined behaviour.

To quote the C11 standard, chapter §7.21.6.1, The fprintf() function

[...] If the format is exhausted while arguments remain, the excess arguments are evaluated (as always) but are otherwise ignored [...]

Metapsychology

+1, I wasn't aware that it is explicitly defined. BTW "not UB" can be optimized, the "not" and the "un" cancel out, remains "defined behavior". – Maihem

As someone who isn't familiar with the implementation of C: why evaluate leftover arguments? Assuming the original format string (first argument) is evaluated before the replacements, this seems like it could be optimized. – Sulfonmethane

@JulesMazur Not evaluating arguments would mean that printf("%d", ++i); would increment i, while printf("foo", ++i); wouldn't. Which hardly makes any sense and is incredibly obscure. Hence, "as always", the arguments are evaluated before being passed in, then ignored by the function. – Metrical

@JulesMazur In addition to Quentin's point, arguments are always evaluated before being passed to functions. Making printf special in that regard would just make all our lives more complicated. – Archipenko

@JensGustedt - the term "undefined behavior" is defined by the standard; the term "defined behavior" is not. So the correct phrase is, indeed, "not undefined behavior". – Richart

@PeteBecker, how would it define the term "defined"? By saying something like ``the terms "definition" in this International standard is defined to ...'' Have a look a the beginning of Section 3. "Terms, definitions, and symbols" and you see that defining things is all what this International Standard is about. And Section 4 says Undefined behavior is otherwise indicated in this International Standard by the words ‘‘undefined behavior’’ or by the omission of any explicit definition of behavior. – Maihem

@JensGustedt - having spent eight years as project editor for C++ I'm quite familiar with the structure of standards. The term you suggested, "defined behavior," is not a technical term in the standard. – Richart

@PeteBecker, first I am talking of C and not C++, and then what do you think that the partial sentence explicit definition of behavior means (that I cited)? Definition of behavior, this is all what the standard is about. – Maihem

Answer 2

Basically, printf (or any formatting function) will look into only 'n' number of %d, %c, %f..., etc in the format string from the variable list argument. Others are simply ignored.

Hrvatska

How do you know that, and why should we trust your answer? – Foote