I meet the problem : 5623.H(4): error C129: missing ';' before 'data'

so either 5623.h must include usb.h before the declaration or main.c must include usb.h before including 5623.h.

And here's one strong piece of advice: by all means, whenever remotely possibly, avoid that second choice.

Header files should always be self-contained and idempotent, i.e. their effect should not depend on anything being present or absent in the file they're getting #included in, before their #include line. The way to achieve that is this

*) if a header file references any elements defined in some other header file, it has to #include that other header

*) except for rare exceptions, every header file shall have multiple-inclusion guards, so it can be included in a single translation unit arbitrarily many times.

As a rough test, each header file you write should be compilable without errors, all by itself, and #including it 20 times in a row may not cause errors.

so either 5623.h must include usb.h before the declaration or main.c must include usb.h before including 5623.h.

And here's one strong piece of advice: by all means, whenever remotely possibly, avoid that second choice.

Header files should always be self-contained and idempotent, i.e. their effect should not depend on anything being present or absent in the file they're getting #included in, before their #include line. The way to achieve that is this

*) if a header file references any elements defined in some other header file, it has to #include that other header

*) except for rare exceptions, every header file shall have multiple-inclusion guards, so it can be included in a single translation unit arbitrarily many times.

As a rough test, each header file you write should be compilable without errors, all by itself, and #including it 20 times in a row may not cause errors.