cc -O2 -pipe  -pthread  -Wall -Wextra -Werror=implicit-function-declaration strings/ffsl.c -o strings/ffsl -D_GNU_SOURCE -D_BSD_SOURCE -D_ALL_SOURCE -D_DEFAULT_SOURCE  -lm -lpthread 
strings/ffsl.c:11:15: error: call to undeclared function 'ffsl'; ISO C99 and later do not support implicit function declarations [-Werror,-Wimplicit-function-declaration]
   11 |         int output = ffsl(input);
      |                      ^
strings/ffsl.c:11:15: note: did you mean 'ffs'?
/usr/include/strings.h:73:6: note: 'ffs' declared here
   73 | int      ffs(int);
      |          ^
1 error generated.