[c] warning: implicit declaration of function

I think the question is not 100% answered. I was searching for issue with missing typeof(), which is compile time directive.

Following links will shine light on the situation:

https://gcc.gnu.org/onlinedocs/gcc-5.3.0/gcc/Typeof.html

https://gcc.gnu.org/onlinedocs/gcc-5.3.0/gcc/Alternate-Keywords.html#Alternate-Keywords

as of conculsion try to use __typeof__() instead. Also gcc ... -Dtypeof=__typeof__ ... can help.