What s an object file in C

Question

I am reading about libraries in C but I have not yet found an explanation on what an object file is  What s the real difference between any other compiled file and an object file  I would be glad if someone could explain in human language

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An object file is just what you get when you compile one (or several) source file(s).

It can be either a fully completed executable or library, or intermediate files.

The object files typically contain native code, linker information, debugging symbols and so forth.

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There are 3 kind of object files  Relocatable object files Contain machine code in a form that can be combined with other relocatable object files at link time  in order to form an executable object file  If you have an a c source file  to create its object file with GCC you should run  gcc a c -c The full process would be  preprocessor  cpp  would run over a c  Its output  still source  will feed into the compiler  cc1   Its output  assembly  will feed into the assembler  as   which will produce the relocatable object file  That file contains object code and linking  and debugging if -g was used  metadata  and is not directly executable  Shared object files Special type of relocatable object file that can be loaded dynamically  either at load time  or at run time  Shared libraries are an example of these kinds of objects  Executable object files Contain machine code that can be directly loaded into memory  by the loader  e g execve  and subsequently executed  The result of running the linker over multiple relocatable object files is an executable object file  The linker merges all the input object files from the command line  from left-to-right  by merging all the same-type input sections  e g   data  to the same-type output section  It uses symbol resolution and relocation  Bonus read  When linking against a static library the functions that are referenced in the input objects are copied to the final executable  With dynamic libraries a symbol table is created instead that will enable a dynamic linking with the library s functions globals  Thus  the result is a partially executable object file  as it depends on the library  If the library doesn t exist  the file can no longer execute   The linking process can be done as follows  ld a o -o myexecutable The command  gcc a c -o myexecutable will invoke all the commands mentioned at point 1 and at point 3  cpp - gt  cc1 - gt  as - gt  ld1  1  actually is collect2  which is a wrapper over ld

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An Object file is the compiled file itself  There is no difference between the two  An executable file is formed by linking the Object files  Object file contains low level instructions which can be understood by the CPU  That is why it is also called machine code  This low level machine code is the binary representation of the instructions which you can also write directly using assembly language and then process the assembly language code  represented in English   into machine language  represented in Hex  using an assembler    Here s a typical high level flow for this process for code in High Level Language such as C  --  goes through pre-processor  --  to give optimized code  still in C   --  goes through compiler  --  to give assembly code  --  goes through an assembler  --  to give code in machine language which is stored in OBJECT FILES  --  goes through Linker  --  to get an executable file   This flow can have some variations for example most compilers can directly generate the machine language code  without going through an assembler  Similarly  they can do the  pre-processing for you  Still  it is nice to break up the constituents for a better understanding

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An object file is the real output from the compilation phase   It s mostly machine code  but has info that allows a linker to see what symbols are in it as well as symbols it requires in order to work    For reference   symbols  are basically names of global objects  functions  etc    A linker takes all these object files and combines them to form one executable  assuming that it can  ie  that there aren t any duplicate or undefined symbols    A lot of compilers will do this for you  read  they run the linker on their own  if you don t tell them to  just compile  using command-line options    -c is a common  just compile  don t link  option

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Object files are codes that are dependent on functions  symbols  and text to run the program  Just like old telex machines  which required teletyping to send signals to other telex machine   In the same way processor s require binary code to run  object files are like binary code but not linked  Linking creates additional files so that the user does not have to have compile the C language themselves  Users can directly open the exe file once the object file is linked with some compiler like c language   or vb etc

[c] What's an object file in C?

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