cc -O2 -pipe  -pthread  -Wall -Wextra at_quick_exit.c -o at_quick_exit -D_GNU_SOURCE -D_BSD_SOURCE -D_ALL_SOURCE -D_DEFAULT_SOURCE  -lm -lpthread 
at_quick_exit.c:14:7: warning: call to undeclared function 'at_quick_exit'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
        if ( at_quick_exit(handler) )
             ^
at_quick_exit.c:16:2: warning: call to undeclared function 'quick_exit'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
        quick_exit(1);
        ^
2 warnings generated.
ld: error: undefined symbol: at_quick_exit
>>> referenced by at_quick_exit.c
>>>               ../out/openbsd/basic/at_quick_exit.o:(main)

ld: error: undefined symbol: quick_exit
>>> referenced by at_quick_exit.c
>>>               ../out/openbsd/basic/at_quick_exit.o:(main)
cc: error: linker command failed with exit code 1 (use -v to see invocation)