C error undefined reference to function but it IS defined

Question

Just a simple program  but I keep getting this compiler error  I m using MinGW for the compiler   Here s the header file  point h     type for a Cartesian point typedef struct     double x    double y    Point   Point create double x  double y   Point midpoint Point p  Point q     And here s point c     This is the implementation of the point type  include  point h   int main       return 0    Point create double x  double y      Point p    p x   x    p y   y    return p     Point midpoint Point p  Point q      Point mid    mid x    p x   q x    2    mid y    p y   q y    2    return mid      And here s where the compiler issue comes in  I keep getting      testpoint c  undefined reference to  create double x  double y     While it is defined in point c   This is a separate file called testpoint c    include  point h   include  lt assert h gt   include  lt stdio h gt  int main       double x   1    double y   1    Point p   create x  y      assert p x    1     return 0      I m at a loss as to what the issue could be

User · Answer

I had this issue recently  In my case  I had my IDE set to choose which compiler  C or C    to use on each file according to its extension  and I was trying to call a C function  i e  from a  c file  from C   code   The  h file for the C function wasn t wrapped in this sort of guard    ifdef   cplusplus extern  C     endif     all of your legacy C code here   ifdef   cplusplus    endif   I could ve added that  but I didn t want to modify it  so I just included it in my C   file like so   extern  C     include  legacy C header h       Hat tip to UncaAlby for his clear explanation of the effect of extern  C

User · Answer

How are you doing the compiling and linking  You ll need to specify both files  something like   gcc testpoint c point c      so that it knows to link the functions from both together  With the code as it s written right now  however  you ll then run into the opposite problem  multiple definitions of main  You ll need want to eliminate one  undoubtedly the one in point c    In a larger program  you typically compile and link separately to avoid re-compiling anything that hasn t changed  You normally specify what needs to be done via a makefile  and use make to do the work  In this case you d have something like this   OBJS testpoint o point o  testpoint exe    OBJS      gcc   OJBS    The first is just a macro for the names of the object files  You get it expanded with   OBJS   The second is a rule to tell make 1  that the executable depends on the object files  and 2  telling it how to create the executable when if it s out of date compared to an object file   Most versions of make  including the one in MinGW I m pretty sure  have a built-in  implicit rule  to tell them how to create an object file from a C source file  It normally looks roughly like this    c o        CC  -c   CFLAGS    lt    This assumes the name of the C compiler is in a macro named CC  implicitly defined like CC gcc  and allows you to specify any flags you care about in a macro named CFLAGS  e g   CFLAGS -O3 to turn on optimization  and   lt  is a special macro that expands to the name of the source file   You typically store this in a file named Makefile  and to build your program  you just type make at the command line  It implicitly looks for a file named Makefile  and runs whatever rules it contains   The good point of this is that make automatically looks at the timestamps on the files  so it will only re-compile the files that have changed since the last time you compiled them  i e   files where the   c  file has a more recent time-stamp than the matching   o  file    Also note that 1  there are lots of variations in how to use make when it comes to large projects  and 2  there are also lots of alternatives to make  I ve only hit on the bare minimum of high points here

User · Answer

Add the  extern  keyword to the function definitions in point h

User · Answer

I think the problem is that when you re trying to compile testpoint c  it includes point h but it doesn t know about point c  Since point c has the definition for create  not having point c will cause the compilation to fail   I m not familiar with MinGW  but you need to tell the compiler to look for point c  For example with gcc you might do this   gcc point c testpoint c   As others have pointed out  you also need to remove one of your main functions  since you can only have one

[c] C error: undefined reference to function, but it IS defined

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