Do I need to compile the header files in a C program

Question

Sometimes I see someone compile a C program like this   gcc -o hello hello c hello h  As I know  we just need to put the header files into the C program like    include  somefile   and compile the C program  gcc -o hello hello c   When do we need to compile the header files or why

User · Answer

In some systems  attempts to speed up the assembly of fully resolved   c  files call the pre-assembly of include files  compiling header files    However  it is an optimization technique that is not necessary for actual C development   Such a technique basically computed the include statements and kept a cache of the flattened includes   Normally the C toolchain will cut-and-paste in the included files recursively  and then pass the entire item off to the compiler   With a pre-compiled header cache  the tool chain will check to see if any of the inputs  defines  headers  etc  have changed   If not  then it will provide the already flattened text file snippets to the compiler   Such systems were intended to speed up development  however  many such systems were quite brittle   As computers sped up  and source code management techniques changed  fewer of the header pre-compilers are actually used in the common project   Until you actually need compilation optimization  I highly recommend you avoid pre-compiling headers

User · Answer

You don t need to compile header files  It doesn t actually do anything  so there s no point in trying to run it  However  it is a great way to check for typos and mistakes and bugs  so it ll be easier later

User · Answer

Firstly  in general   If these  h files are indeed typical C-style header files  as opposed to being something completely different that just happens to be named with  h extension   then no  there s no reason to  compile  these header files independently  Header files are intended to be included into implementation files  not fed to the compiler as independent translation units   Since a typical header file usually contains only declarations that can be safely repeated in each translation unit  it is perfectly expected that  compiling  a header file will have no harmful consequences  But at the same time it will not achieve anything useful   Basically  compiling hello h as a standalone translation unit equivalent to creating a degenerate dummy c file consisting only of  include  hello h  directive  and feeding that dummy c file to the compiler  It will compile  but it will serve no meaningful purpose     Secondly  specifically for GCC   Many compilers will treat files differently depending on the file name extension  GCC has special treatment for files with  h extension when they are supplied to the compiler as command-line arguments  Instead of treating it as a regular translation unit  GCC creates a precompiled header file for that  h file   You can read about it here  http   gcc gnu org onlinedocs gcc Precompiled-Headers html  So  this is the reason you might see  h files being fed directly to GCC

User · Answer

I think we do need preprocess maybe NOT call the compile  the head file  Because from my understanding  during the compile stage  the head file should be included in c file  For example  in test h we have  typedef enum      a      b      c  test t   and in test c we have  void foo         test t test              during the compile  i think the compiler will put the code in head file and c file together and code in head file will be pre-processed and substitute the code in c file  Meanwhile  we d better to define the include path in makefile

User · Answer

Okay  let s understand the difference between active and passive code   The active code is the implementation of functions  procedures  methods  i e  the pieces of code that should be compiled to executable machine code  We store it in  c files and sure we need to compile it   The passive code is not being execute itself  but it needed to explain the different modules how to communicate with each other  Usually   h files contains only prototypes  function headers   structures   An exception are macros  that formally can contain an active pieces  but you should understand that they are using at the very early stage of building  preprocessing  with simple substitution  At the compile time macros already are substituted to your  c file   Another exception are C   templates  that should be implemented in  h files  But here is the story similar to macros  they are substituted on the early stage  instantiation  and formally  each other instantiation is another type   In conclusion  I think  if the modules formed properly  we should never compile the header files

[c] Do I need to compile the header files in a C program?

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