C - Scanf with only read/write

Question

I'm trying to write my own scanf function in C using only read/write (no libary functions, no va_args etc.). I came up with this code (only for decimal integers for now):

void myscanf(char* text, ... ) {
  char* ap = (char *) &text + sizeof text;
  char* p;
  int tempk;

  for(p=text; *p != '\0'; p++){
    if(*p=='%'){
        p++;
        switch (*p) {
            case 'd':
                char data[128] = { 0 };
                read(0, data, 128);
                tempk = StringToInt(data, 10);
                int* k = (int*)ap;
                *k = tempk;
                break;
        }
    }
  }

}

For some reason the address of *k is different from pointer I pass as an argument. I tried different ways, but I sill don't know how to get the right address.

what do you want to achieve with int* k = (int*)ap and line after this *k=tempk, one of these has no reason to be there — Angen, Dec 17 '17 at 12:16
Assuming you already was told, that what you are doing is very dirty, sick, implementation dependent etc... What exact system/compiller are you using? — Fat-Zer, Dec 17 '17 at 12:16
For actually to do `scanf` you *need* to implement buffering of at least one character. Minimally you will read the stream character by character, then implement the `ungetc` function. Your immediate problem, however, is that you need to learn how to use the variable arguments (varargs). They do not work at all like you expect them to be here. — Antti Haapala -- Слава Україні, Dec 17 '17 at 12:23
In main I have variable int k; then I call myscanf("%d", &k); but the address myscanf gets is different from the actual k's address. The difference is very small. c0 or c8 (hex) — iamjustasking, Dec 17 '17 at 12:26
@iamjustasking that's your problem. You get a value that "almost makes sense". If you got total garbage, you'd not think you're right at all (your approach is very wrong). — Antti Haapala -- Слава Україні, Dec 17 '17 at 12:29
Instead follow the advice of the duplicate, and to get the `int *`, use `int * ptr = va_arg(list, int *);` — Antti Haapala -- Слава Україні, Dec 17 '17 at 12:29
@iamjustasking what! there is no way of doing this portably in C then. (i.e. it wouldn't work in any 64-bit computer one could find) Whose course is this?! — Antti Haapala -- Слава Україні, Dec 17 '17 at 12:38
it doesn't have to be done portably and i'm compiling it for 32-bit machine — iamjustasking, Dec 17 '17 at 12:44
@iamjustasking You have already use va_arg with your function, "`...`" already use `va_arg`, tell this to your school for me. — Stargateur, Dec 17 '17 at 12:44
Ok, I've found the solution should be int* k = (int **)ap; and -fno-stack-protector when compiling — iamjustasking, Dec 17 '17 at 13:28

C - Scanf with only read/write

0 Answers0