cstringvalgrindnull-terminated

What happened when we do not include '\0' at the end of string in C?

In C, when I initialize my array this way:

char full_name[] = {
    't', 'o', 'a', 'n'
};

and print it with printf("%s", full_name);

and run it with valgrind I got error

Uninitialised value was create by stack allocation

Why does that happen?

Solution

  • Since %s format specifier expects a null-terminated string, the resulting behavior of your code is undefined. Your program is considered ill-formed, and can produce any output at all, produce no output, crash, and so on. To put this shortly, don't do that.

    This is not to say that all arrays of characters must be null-terminated: the rule applies only to arrays of characters intended to use as C strings, e.g. to be passed to printf on %s format specifier, or to be passed to strlen or other string functions of the Standard C library.

    If you are intended to use your char array for something else, it does not need to be null terminated. For example, this use is fully defined:

    char full_name[] = {
    't', 'o', 'a', 'n'
};
for (size_t i = 0 ; i != sizeof(full_name) ; i++) {
    printf("%c", full_name[i]);
}