cc -O2 -pipe  -pthread  -Wall -Wextra -Werror -Wno-error=deprecated -Wno-error=deprecated-declarations signal/pthread_kill.c -o signal/pthread_kill -D_POSIX_C_SOURCE=202405L  -lm -lpthread 
signal/pthread_kill.c:5:23: error: expected identifier
int (*foo)(pthread_t, int) = pthread_kill;
                      ^
signal/pthread_kill.c:5:30: error: use of undeclared identifier 'pthread_kill'
int (*foo)(pthread_t, int) = pthread_kill;
                             ^
2 errors generated.