gcc -pthread -Wall -Wextra -Werror=implicit-function-declaration sys_mman/posix_mem_offset.c -o sys_mman/posix_mem_offset -D_GNU_SOURCE -D_BSD_SOURCE -D_ALL_SOURCE -D_DEFAULT_SOURCE -lm -lpthread -lrt -lxnet -lsocket -latomic sys_mman/posix_mem_offset.c: In function 'main': sys_mman/posix_mem_offset.c:21:9: error: implicit declaration of function 'posix_mem_offset' [-Wimplicit-function-declaration] 21 | posix_mem_offset(NULL, 0, NULL, NULL, NULL); | ^~~~~~~~~~~~~~~~