Linker does not link compiled file

[Anonymous]

in the C file:

#include "sntp.h" 

    sntp_start_auto_time_sync( 1 * DAYS );

in the makefile:

$(NAME)_COMPONENTS := protocols/sntp

Compiles fine, but the linker cannot find the function I called: "undefined reference to `sntp_start_auto_time_sync'"

I looked in the build directory (Modules/Library/protocols/sntp) and I can see the snap.o file, compiled at the right timestamp.

The .o contains the string "sntp_start_auto_time_sync"

What's missing? What other info does the linker need?

[Update: If I run make clean, and then try again, the compilation is successful. Why?]

[Anonymous]

I changed this from a question to "tip": Linker issues can be resolved by cleaning the build environment.