How to fix C error ‘mode_t undeclared’

Problem:

You have C code like

mode_t mode;

but when you try to compile it you see an error message like

main.c: In function ‘main’:
main.c:4:2: error: unknown type name ‘mode_t’
  mode_t mode;
  ^~~~~~

Solution:

Add

#include <sys/stat.h>

at the top of the source file where the error occured. In case you want further information about what mode_t represents, I recommend reading this blogpost.