Under ISO C11 semantics, what is the correct classification of the following…
2025
Under ISO C11 semantics, what is the correct classification of the following program's behavior?
#include <stdio.h>
int main(void)
{
printf("%nxy");
printf("%bxy");
printf("%rxy");
return 0;
}Answer: C. Undefined behaviour — ConceptFor a formatted variadic function such as printf, the format string determines which additional arguments are consumed and what types they must have.…
- A.
Nothing gets printed
- B.
Error
- C.
Undefined behaviour
- D.
%nxy%bxy%rxy
Attempted by 40 students.
Show answer & explanation
Correct answer: C
Concept
For a formatted variadic function such as printf, the format string determines which additional arguments are consumed and what types they must have.
If a conversion specification requires an argument but no matching argument is supplied, ISO C defines the behavior as undefined. Undefined behavior gives no guarantee about output, diagnostics, or termination.
Application
In printf("%nxy"), the %n conversion consumes an int * argument and stores through that pointer the number of characters written so far.
This call supplies only the format string; it does not supply the int * required by %n. The format therefore attempts to consume a missing argument, so a portable output sequence cannot be predicted.
Even if the first mismatch were ignored, %r is not an ISO C11 conversion specification; an invalid conversion specification independently leaves the behavior undefined.
Cross-check and contrast
“Nothing gets printed” claims a fixed output of zero characters, but this execution has no guaranteed output.
“Error” claims a required diagnostic or termination, but undefined behavior does not require either one.
“%nxy%bxy%rxy” claims that every percent sequence is emitted literally, but the execution is not guaranteed to produce that string.
Result
Therefore, the standards-correct classification is undefined behaviour.