Problem:
You have C code like
int v = 123; intptr_t vptr = (intptr_t)&v;
but when you try to compile it you see an error message like
main.c: In function ‘main’: main.c:5:1: error: unknown type name ‘intptr_t’; did you mean ‘__int128_t’? intptr_t vptr = (intptr_t)&v; ^~~~~~~~ __int128_t main.c:5:18: error: ‘intptr_t’ undeclared (first use in this function) intptr_t vptr = (intptr_t)&v; ^~~~~~~~ main.c:5:18: note: each undeclared identifier is reported only once for each function it appears in
Solution:
Add
#include <stdint.h>
at the top of the source file where the error occured.