Multiple definition error but "load once"-#ifndef should catch it...

[Tobe]

How is this possible with the #ifndef catch?

[TDK · Accepted solution]

If you have multiple compilations units (i.e. multiple *.c files), this gets included from each of them (only once). When the linker tries to resolve everything at the end, it sees multiple definitions of ledState.

Solution: don't put variables definitions in header files. Put them in source files. If you need them in multiple source files, additional put an "extern" declaration in the header.

In a header file:

extern bool ledState;

In one source file:

bool ledState = false;