* Jonny Grant <jg@jguk.org>, 2020-06-16, 12:36:
>SYNOPSIS         top
>   #define _GNU_SOURCE
>   #include <unistd.h>
>   #include <sys/types.h>
>
>   pid_t gettid(void);
>
>I can compile on Ubuntu without #define _GNU_SOURCE and call gettid()
>
>Maybe that line can be removed?

I don't think so. Here's what I get (with GCC 9):

   $ gcc test.c
   test.c: In function ‘main’:
   test.c:6:9: warning: implicit declaration of function ‘gettid’; did you mean ‘getgid’? [-Wimplicit-function-declaration]
       6 |  return gettid();
         |         ^~~~~~
         |         getgid

Prepending "#define _GNU_SOURCE" fixes the above.

-- 
Jakub Wilk