I need to perform simple arithmetic on struct tm from time.h. I need to add or subtract seconds or minutes, and be able to normalize the structure. Normally, I'd use mktime(3) which performs this normalization as a side effect:
struct tm t = {.tm_hour=0, .tm_min=59, .tm_sec=40};
t.tm_sec += 30;
mktime(&t);
// t.tm_hour is now 1
// t.tm_min is now 0
// t.tm_sec is now 10
I'm doing this on an STM32 with 32 kB of flash, and binary gets very big. mktime(3) and the other stuff it pulls in take up 16 kB of flash--half the available space.
Is there a function in newlib that is specifically responsible for struct tm normalization? I realize that linking to a private function like that would make the code less portable.
There is a validate_structure() function in newlib/libc/time/mktime.c which does a part of the job, normalizes month, day-of-month, hour, min, sec, but leaves day-of-week and day-of-year alone.
It's declared static, so you can't simply call it, but you can copy the function from the sources. (There might be licensing issues though). Or you can just reimplement it, it's quite straightforward.
The tm_wday and tm_yday is calculated later in mktime(), so you'd need the whole mess including the timezone stuff in order to have these two normalized.
The bulk of that 16kB code is related to a call to siscanf(), a variant of sscanf() without floating point support, which is (I believe) used to parse timezone and DST information in environment variables.
You can cut lots of unnecessary code by using --specs=nano.specs when linking, which would switch to simplified printf/scanf code, saving about 10kB of code in your case.