Adding newline character to printf() changes code behaviour

Question

For some reason, adding \\n to printf() changes the behaviour of below code. The code without \\n prints (null) whereas t

Answer 1

Both are undefined behavior, so an answer could stop right here.

But there's at least an explanation for the output of (null). This is an extension in glibc (the GNU C library). Passing 0 for %s in printf() is considered undefined in the C standard and therefore could very well result in a crash. The developers of glibc decided to do something meaningful instead.

The reason the second crashes nevertheless is that with the newline, the compiler decides to optimize: Instead of printf("%s\n", argv[1]), it executes puts(argv[1]), which is semantically equivalent according to the C standard, therefore an allowed optimization. But glibcs "(null)-trick" is only implemented in printf().

There's another undefined behavior in your program: You potentially access argv out of bounds. There's no guarantee what value you will find at argv[i] when i > argc. There's a slight chance argc could be 0, so you could experience anything else as well.