Generalizing Printf in C

Post by nwellnhof » Fri, Nov 28, 2025, 10:34 PM UTC

theamk wrote: post #2

On GNU systems, if you want to generalize printf, all you need is vfprintf - because there is: "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…

Post by theamk » Sat, Nov 29, 2025, 8:20 PM UTC

lelanthran wrote: post #12

kazinator wrote: post #4

sprintf can be safely used. - 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…

If you really hit the pathological edge case of needed sprintf at runtime with runtime-only buffer sizes, you can still work out the size safely, albeit slowly. I've done it to make a "safe" sprintf function that allocates the destination buffer, so that the caller cannot overrun it: https://github.com/lelanthran/libds/blob/56d6e18c8970b84c9fa...