search

nasa / CF

The Core Flight System (cFS) CFDP application.
Apache License 2.0
79 stars 45 forks source link

Instantiating globals in header files (FSW version) #94

Closed jphickey closed 2 years ago

jphickey commented 2 years ago

The FSW has a macro called "DECLARE_FIELD" which creates a constant at global scope:

https://github.com/nasa/CF/blob/7b99b91cd50a347f8553fc68ea3b074ff0672251/fsw/src/cf_field.h#L54-L55

The constant is scoped as "static" so it doesn't create a linker error, but it still creates a separate instance of this global variable for each time the header is included.

Confirmed by checking cf.so and observing that each of these 8 fields occur in the binary file 8 times:

https://github.com/nasa/CF/blob/7b99b91cd50a347f8553fc68ea3b074ff0672251/fsw/src/cf_cfdp_pdu.h#L73-L80

jphickey commented 2 years ago

Calling this a "bug" because space efficiency is one of the core requirements/expectations of this CF implementation, and this is not meeting that expectation. Plus, instantiating values from a header file is against most/all standards - it is not expected that simple inclusion of a header will cause extra memory to be allocated in the binary.

jphickey commented 2 years ago

This was partially addressed by #137, in that the DECLARE_FIELDS are only used in a single source file now, but it is not an ideal way to do it.

Better fix for this can be combined with #65, moving the field definitions into the codec code should achieve both.