string literals and strcat

Question

I am not sure why strcat works in this case for me:

char* foo="foo";

printf(strcat(foo,"bar"));

It successfully prints "foobar" for me.

However, as per an earlier topic discussed on stackoverflow here: I just can't figure out strcat

It says, that the above should not work because foo is declared as a string literal. Instead, it needs to be declared as a buffer (an array of a predetermined size so that it can accommodate another string which we are trying to concatenate).

In that case, why does the above program work for me successfully?

Answer 1

This code invokes Undefined Behavior (UB), meaning that you have no guarantee of what will happen (failure here).

The reason is that string literals are immutable. That means that they are not mutable, and any attempt of doing so, will invoke UB.

Note what a difficult logical error(s) can arise with UB, since it might work (today and in your system), but it's still wrong, which makes it very likely that you might miss the error, and get along as everything was fine.

---

PS: In this Live Demo, I am lucky enough to get a Segmentation fault. I say lucky, because this seg fault will make me investigate and debug the code.

It's worth noting that GCC issues no warning, and the warning from Clang are also irrelevant:

p

rog.c:7:8: warning: format string is not a string literal (potentially insecure) [-Wformat-security]
printf(strcat(foo,"bar"));
       ^~~~~~~~~~~~~~~~~
prog.c:7:8: note: treat the string as an argument to avoid this
printf(strcat(foo,"bar"));
       ^
       "%s", 
1 warning generated.