Can it cause problems to pass the address to an array instead of the array?

  • A+

I ran into this code:

char str[600]; scanf("%s", &str); 

Of course, this emits this warning:

a.c:6:17: warning: format specifies type 'char *' but the argument has type       'char (*)[600]' [-Wformat]     scanf("%s", &str);            ~~   ^~~~~~~ 

I know that the correct way is to remove the & and type scanf("%s", str) instead. But it does work, so my question is if this could cause any problems. Is it UB? When I switched str to a pointer instead of an array it (obviously) did not work. But can this cause any problem when using an array?

(I had a discussion about an answer here, where the answerer did not want to change his answer to the correct way)


Yes, the code is undefined behaviour. The argument corresponding to %s must have the type char *. This is described in C17 under the s specifier:

[...] the corresponding argument shall be a pointer to the initial element of a character array large enough to accept the sequence and a terminating null character, which will be added automatically.

which says fairly clearly that the pointer should have pointer-to-character type, and not point to the whole array.

Undefined behaviour means that anything can happen. It might behave as if you omitted the &, or it might format your hard drive.

Given that it is extremely easy to avoid undefined behaviour in this case, I don't really see any reason to engage in arguments about whether it is OK to rely on the behaviour of undefined behaviour in this situation.


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