How do I link object files in C Fails with Undefined symbols for architecture x86 64

Question

So I m trying trying to use a function defined in another C  file1 c  file in my file  file2 c   I m including the header of file1  file1 h  in order to do this   However  I keep getting the following error whenever I try to compile my file using gcc   Undefined symbols for architecture x86 64      init filenames   referenced from         run worker in cc8hoqCM o     read list   referenced from         run worker in cc8hoqCM o ld  symbol s  not found for architecture x86 64   I ve been told I need to  link the object files together  in order to use the functions from file1 in file2  but I have no clue what that means

User · Answer

The existing answers already cover the  how   but I just wanted to elaborate on the  what  and  why  for others who might be wondering   What a compiler  gcc  does  The term  compile  is a bit of an overloaded term because it is used at a high-level to mean  convert source code to a program   but more technically means to  convert source code to object code    A compiler like gcc actually performs two related  but arguably distinct functions to turn your source code into a program  compiling  as in the latter definition of turning source to object code  and linking  the process of combining the necessary object code files together into one complete executable    The original error that you saw is technically a  linking error   and is thrown by  ld   the linker   Unlike  strict  compile-time errors  there is no reference to source code lines  as the linker is already in object space   By default  when gcc is given source code as input  it attempts to compile each and then link them all together   As noted in the other responses  it s possible to use flags to instruct gcc to just compile first  then use the object files later to link in a separate step   This two-step process may seem unnecessary  and probably is for very small programs  but it is very important when managing a very large program  where compiling the entire project each time you make a small change would waste a considerable amount of time

User · Answer

I assume you are using gcc  to simply link object files do     gcc -o output file1 o file2 o   To get the object-files simply compile using    gcc -c file1 c   this yields file1 o and so on   If you want to link your files to an executable do    gcc -o output file1 c file2 c

User · Answer

Add foo1 c   foo2 c   foo3 c and makefile in one folder the type make in bash   if you do not want to use the makefile  you can run the command  gcc -c foo1 c foo2 c foo3 c   then  gcc -o output foo1 o foo2 o foo3 o   foo1 c   include  lt stdio h gt   include  lt string h gt   void funk1     void funk1         printf    nfunk1 n        int main void         char  arg2      size t nbytes   100       while   1              printf    nargv2    s n    arg2           printf    n  gt              getline   amp arg2    amp nbytes   stdin           if  strcmp  arg2    1 n      0                 funk1               else if  strcmp  arg2    2 n      0                 funk2               else if  strcmp  arg2    3 n      0                 funk3               else if  strcmp  arg2    4 n      0                 funk4               else               funk5                         foo2 c   include  lt stdio h gt  void funk2        printf   nfunk2 n      void funk3        printf   nfunk3 n        foo3 c   include  lt stdio h gt   void funk4        printf   nfunk4 n      void funk5        printf   nfunk5 n        makefile  outputTest  foo1 o foo2 o foo3 o     gcc -o output foo1 o foo2 o foo3 o     make removeO  outputTest o  foo1 c foo2 c foo3 c     gcc -c foo1 c foo2 c foo3 c  clean      rm -f   o output  removeO      rm -f   o

User · Answer

Since there s no mention of how to compile a  c file together with a bunch of  o files  and this comment asks for it      where s the main c in this answer     if file1 c is the main  how do   you link it with other already compiled  o files       Tom Brito Oct 12    14 at 19 45     gcc main c lib obj1 o lib obj2 o lib objN o -o x0rbin  Here  main c is the C file with the main   function and the object files    o  are precompiled  GCC knows how to handle these together  and invokes the linker accordingly and results in a final executable  which in our case is x0rbin    You will be able to use functions not defined in the main c but using an extern reference to functions defined in the object files    o    You can also link with  obj or other extensions if the object files have the correct format  such as COFF

User · Answer

You could compile and link in one command   gcc file1 c file2 c -o myprogram   And run with     myprogram   But to answer the question as asked  simply pass the object files to gcc   gcc file1 o file2 o -o myprogram

[c] How do I link object files in C? Fails with "Undefined symbols for architecture x86_64"

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