This functionality is already implemented for asprintf(): It does this the print using a "null stream" to get the size, allocates memory of that size, then does the real print into the allocated memory.
This logic could be easily extracted: https://github.com/apache/incubator-nuttx/blob/master/libs/libc/stdio/lib_vasprintf.c#L82 It is just a couple of lines of code and could be shared or duplicated in snprintf. Adding the functionality is then trivial. On Thu, Jan 20, 2022 at 6:50 AM Xiang Xiao <xiaoxiang781...@gmail.com> wrote: > On Thu, Jan 20, 2022 at 8:43 PM Arie de Muijnck <nu...@ademu.com> wrote: > > > > > > > On 2022-01-20 13:11, Xiang Xiao wrote: > > > On Thu, Jan 20, 2022 at 7:33 PM Jukka Laitinen <jukka.laiti...@iki.fi> > > > wrote: > > > > > >> Hi, > > >> > > >> Sorry if this question comes several times, it seems that for some > > >> reason my emails are not always coming through to the mailing list... > So > > >> re-sending. > > >> > > >> I started getting this build error from the latest NuttX: > > >> > > >> misc/lib_execinfo.c:45:17: error: null argument where non-null > required > > >> (argument 1) [-Werror=nonnull] > > >> 45 | int ret = sprintf(NULL, "%pS", *buffer++); > > >> | ^~~~~~~ > > >> In function 'backtrace_malloc', > > >> inlined from 'backtrace_symbols' at misc/lib_execinfo.c:67:10: > > >> misc/lib_execinfo.c:45:17: error: null destination pointer > > >> [-Werror=format-overflow=] > > >> 45 | int ret = sprintf(NULL, "%pS", *buffer++); > > >> | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > >> cc1: all warnings being treated as errors > > >> > > >> What is the deal with sprintf to NULL ptr? > > >> > > >> > > > The code wants to get the length of the formatted string. But, the > right > > > API is snprintf, not sprintf. Could you try this: > > > https://github.com/apache/incubator-nuttx/pull/5290 > > > > > > > > > > Uhm, no, the first parameter is the target buffer location. That cannot > > be a NULL pointer. > > > > No, snprintf support this special case by spec explicitly: > https://en.cppreference.com/w/c/io/fprintf > > Calling snprintf with zero bufsz and null pointer for buffer is useful to > determine the necessary buffer size to contain the output: > > const char *fmt = "sqrt(2) = %f";int sz = snprintf(NULL > <http://en.cppreference.com/w/c/types/NULL>, 0, fmt, sqrt > <http://en.cppreference.com/w/c/numeric/math/sqrt>(2));char buf[sz + > 1]; // note +1 for terminating null byte > snprintf(buf, sizeof buf, fmt, sqrt > <http://en.cppreference.com/w/c/numeric/math/sqrt>(2)); > > > > > > I fully agree on the snprintf recommendation to prevent overflows. > > > > Arie > > >
