cc -O2 -pipe -pthread -Wall -Wextra -Werror=implicit-function-declaration stdlib/quick_exit.c -o stdlib/quick_exit -D_GNU_SOURCE -D_BSD_SOURCE -D_ALL_SOURCE -D_DEFAULT_SOURCE -lm -lpthread stdlib/quick_exit.c:9:2: error: call to undeclared function 'quick_exit'; ISO C99 and later do not support implicit function declarations [-Werror,-Wimplicit-function-declaration] quick_exit(0); ^ 1 error generated.