"fmemopen(3)" that creates FILE* that writes to pre-allocate dbuffer

"open_memstream(3)" that creates FILE* that writes to auto-allocated buffer;

and if that's not sufficient, there is "fopencookie(3)" which takes general callbacks and creates FILE* that redirects all operations to those callbacks.

If that does not work for some reason, then having custom callback with user-passed 3 parameters is too much. Why add dedicated FILE* or "size" parameters which are only ever used in one specific case? Do a generic "void * context" argument ("int (write)(char data, void * context)" + "void * context") and let user figure out how to use it.

- For some conversions, you can establish an upper bound on how many characters they will produce. E.g. a positive decimal integer not more than 9999 does not consume more than four characters.

- It's possible to specify truncation. e.g. "%.64s" prints at most 64 characters from the string argument.

- There are enirely static cases that can be worked out at compile time, e.g.

  char big_enuf_buf[BIG_ENUF_BUF_SIZE];
  sprintf(big_enuf_buf, "%x-%04x-%04x", MAJOR, MINOR, BUILD); // preprocessor constants

The reasons for analyzing to it wouldn't necessarily just be for diagnostics, but possibly for compiling it down to a literal:

  char big_enuf_buf[BIG_ENUF_BUF_SIZE] = "A1-0013-000A";

https://github.com/eyalroz/printf

which is independent of a C standard library (it doesn't actually do any I/O itself). Originally by Marco Paland, now maintained, or 'curated' by myself (so, this is a bit of a self-plug, even though I can barely claim authorship). It offers this generalization :

  int fctprintf(void (*out)(char c, void* extra_arg), void* extra_arg, const char* format, ...);
  int vfctprintf(void (*out)(char c, void* extra_arg), void* extra_arg, const char* format, va_list arg);

  typedef struct {
    void (*function)(char c, void* extra_arg);
    void* extra_function_arg;
    char* buffer;
    printf_size_t pos;
    printf_size_t max_chars;
  } output_gadget_t;