Question

Or in other words: Could a wrong printf / fprintf decimal integer (%d, %u, %ld, %lld) format string cause a program to crash or lead to undefined behavior?

Cosinder following lines of code:

#include <iostream>
#include <cstdio>

int main() {
    std::cout << sizeof(int) << std::endl
              << sizeof(long) << std::endl;

    long a = 10;
    long b = 20;
    std::printf("%d, %d\n", a, b);

    return 0;
}

Result on 32 bit architecture:

4
4
10, 20

Result on 64 bit architecture:

4
8
10, 20

In any case the program prints the expected result. I know, if the long value exceeds the int range, the program prints wrong numbers – which is ugly, but doesn’t effect the main purpose of the program –, but beside this, could anything unexpected happen?

Answer 1

What can happen if printf is called with a wrong format string?

Anything can happen. It is Undefined behavior!
Undefined behavior means that anything can happen. It may show you results which you expect or it may not or it may crash. Anything can happen and you can blame no one but yourself about it.

Reference:

c99 Standard: 7.19.6.1:
para 9:

If a conversion specification is invalid, the behavior is undefined.²²⁵⁾ If any argument is
not the correct type for the corresponding coversion specification, the behavior is
undefined.