Trying to include a library but keep getting undefined reference to messages

Question

I am attempting to use the libtommath library  I m using the NetBeans IDE for my project on Ubuntu linux  I have downloaded and built the library  I have done a  make install  to put the resulting  a file into  usr lib  and the  h files into  usr include  It appears to be finding the files appropriately  since I no longer get those errors  which I did before installing into the  usr directories    However  when I create a simple main making a call to mp init  which is in the library   I get the following error when I attempt to make my project   mkdir -p build Debug GNU-Linux-x86 rm -f build Debug GNU-Linux-x86 main o d gcc -c -g -MMD -MP -MF build Debug GNU-Linux-x86 main o d -o build Debug GNU-Linux-x86 main o main c mkdir -p dist Debug GNU-Linux-x86 gcc -o dist Debug GNU-Linux-x86 cproj1 build Debug GNU-Linux-x86 main o build Debug GNU-Linux-x86 main o  In function  main    home   myusername   NetBeansProjects CProj1 main c 18  undefined reference to  mp init  collect2  ld returned 1 exit status make 2        dist Debug GNU-Linux-x86 cproj1  Error 1   So  it looks like the linker can t find the function within the library  however it IS there  so I just don t know what could be causing this   I get the same error if I type the gcc command directly and skip the makefile  I also made sure the static library got compiled with gcc as well   Edited to Add   I get these same errors if I do the compile directly and add the library with -l or -L     gcc -l  usr lib libtommath a main c   usr bin ld  cannot find -l usr lib libtommath a collect2  ld returned 1 exit status    gcc -llibtommath a main c   usr bin ld  cannot find -llibtommath a collect2  ld returned 1 exit status    gcc -Llibtommath a main c   tmp ccOxzclw o  In function  main   main c   text 0x18   undefined reference to  mp init  collect2  ld returned 1 exit status    gcc -Llibtommath a main c   tmp ccOxzclw o  In function  main   main c   text 0x18   undefined reference to  mp init  collect2  ld returned 1 exit status   I am very rusty on this stuff  so I m not sure I m using the right command here  in the -L examples are the libraries being found  If the library isn t being found how on earth do I get it to find the library  It s in  usr lib  I ve tried it with the  a file in the current directory  etc   Is there an environment variable I need to set   If so  how  etc   I ve tried a completely different library  GMP  and had the EXACT same problem  This has got to be some kind of Ubuntu environment issue  Anyone have any idea how to fix this

User · Answer

Yes  It is required to add libraries after the source files objects files   This command will solve the problem   gcc -static -L usr lib -I usr lib main c -ltommath

User · Answer

If the  c source files are converted  cpp  like as in parsec   then the extern needs to be followed by  C  as in   extern  C  void foo

User · Answer

The trick here is to put the library AFTER the module you are compiling  The problem is a reference thing  The linker resolves references in order  so when the library is BEFORE the module being compiled  the linker gets confused and does not think that any of the functions in the library are needed  By putting the library AFTER the module  the references to the library in the module are resolved by the linker

[ubuntu] Trying to include a library, but keep getting 'undefined reference to' messages

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