Where is the scanf_s() buffer defined?

Question

If I didn't have access to the internet, but knew that I wanted to use the scanf_s() function to take input from the keyboard (stdin), how would I know where to declare the buffer?

At the moment, when I step into the scanf_s() function in Visual Studio, I see this:

#if __STDC_WANT_SECURE_LIB__

    _Check_return_opt_
    _CRT_STDIO_INLINE int __CRTDECL scanf_s(
        _In_z_ _Scanf_s_format_string_ char const* const _Format,
        ...)
#if defined _NO_CRT_STDIO_INLINE // SCANF
    ;

So, I can see that scanf_s() takes 1 defined argument (char const * _Format) which, in this case, could be some placeholder such as be "%s", but nothing for the second argument - the memory address of the buffer to store the result of the scanning.

Answer 1

scanf_s takes a variable list of arguments. The first argument is mandatory, a string with the conversion specifications, the following arguments depend on what conversion specifications are present in the format string.

For each conversion specification %s, %c and %[, scanf_s expects 2 arguments for the conversion: a pointer to char (char *) and a number passed with type rsize_t that specifies the size of the array pointed to by the char * argument.

Here is an example:

#define __STDC_WANT_LIB_EXT1__ 1 
#include <stdio.h>

int main() {
    char buf[32];
    int res;

    printf("Type a word: ");
    res = scanf_s("%s", buf, sizeof(buf));
    switch (res) {
    case EOF:
        printf("Input failure\n");
        break;
    case 0:
        printf("Matching failure\n");
        break;
    case 1:
        printf("Success: you typed '%s'\n", buf);
        break;
    default:
        printf("Unexpected return value %d\n", res);
        break;
    }
    return 0;
}

Note however that support for scanf_s is far from universal. Microsoft platforms usually support it, but with varying semantics, while linux and Mac C libraries usually don't. Read this question to understand Why didn't gcc implement _s functions?