Dr. Dobb's Journal - February 2009 - (Page 36)

d02klatch_p3db 12/12/08 9:42 AM Page 36 State of the Art by R. Samuel Klatchko Decoupling C Header Files Avoiding confusion One issue with the common C compilation model is that it can easily lead to unnecessary coupling between files. This coupling can require users of an API to either directly or indirectly Samuel is a software architect for Symantec. He can be contacted at samuel_klatchko@symantec.com. include unnecessary header files. These additional header files may cause frustration as users attempt to figure out what additional header files or include paths are needed to use the API. Even if your header file includes the necessary header files or your build environment centralizes all header files in a single location, these additional header files can still be problematic as they can lead to longer compile times and add additional symbols to the global namespace of the translation unit, which increases the chance of conflicts. Consider a hypothetical SMTP library in which one important input to the library is the contents of the message that the caller wishes to send. To make the library easy to use by different developers, the library is designed to accept the data in a variety of formats. In addition to built-in data sources such as strings (NULL-terminated character arrays, for instance), the library may also accept locally developed data sources such as Location Transparent Data Source (LTDS), a common data structure that keeps data in memory or on disk, depending on how much data is present in a specific instance. One common way to handle this would require the SMTP header to include the LTDS header; see Listing One. For users of the API who only want to use smtp_send_str(), the fact that smtp_send_ltds() is in the same header file is the first problem you face. Before you can include the SMTP header file, you must first include the LTDS header file. Even if the SMTP header includes the LTDS header file, the fact that the LTDS header is required can cause problems. If the build environment keeps header files in different locations, the client of the SMTP header needs to add the header path in the local build environment. The additional symbols introduced by the LTDS header file could cause conflicts as well as longer build times and larger object files. Decoupling Header Files The solution lies in C’s distinction between declarations and definitions, and how C allows duplicate declarations to appear in the same translation unit (which in the general case must all agree; in the case of structures that is guaranteed as they have their own namespace and have no modifiers that can conflict). One issue that can be confusing is the difference between structure names and typedef names. For this 36 Dr. Dobb’s Journal l www.ddj.com l February 2009 http://www.ddj.com

Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - February 2009

Dr. Dobb's Journal - February 2009 Contents Friday Night Fish Fry Alia Vox Developer Diaries Conversations Computing in the Clouds Software Development in the Cloud Videos and Oracle Forms 10g Parallel LINQ Decoupling C Header Files Effective Concurrency Disciplined Agility Swaine’s Flames

Dr. Dobb's Journal - February 2009

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.