eclipse - build error related to g++ -c flag -

i dont understand g++ -c flag . based on definition:compile or assemble source files, not link. linking stage not done. ultimate output in form of object file each source file. need understand cause error following build process. thanks

i try compile sample helloworld program in eclipse.

#include <iostream> using namespace std;  int main () {   cout << "hello world!";   return 0; } 

without -c. eclipse gives me error:

make  building file: ../app.cpp invoking: gcc c++ compiler g++ -o0 -g3 -wall -fmessage-length=0 -fpic -mmd -mp -mf"app.d" -mt"app.d" -o "app.o" "../app.cpp" finished building: ../app.cpp  building target: app.so invoking: gcc c++ linker g++ -shared -o "app.so"  ./app.o    ./app.o: in function `_fini': (.fini+0x0): multiple definition of `_fini' /usr/lib/gcc/x86_64-redhat-linux/4.6.3/../../../../lib64/crti.o:(.fini+0x0): first defined here ./app.o: in function `_init': (.init+0x0): multiple definition of `_init' /usr/lib/gcc/x86_64-redhat-linux/4.6.3/../../../../lib64/crti.o:(.init+0x0): first defined here /usr/lib/gcc/x86_64-redhat-linux/4.6.3/crtends.o:(.dtors+0x0): multiple definition of `__dtor_end__' ./app.o:(.dtors+0x8): first defined here /usr/bin/ld: warning: cannot create .eh_frame_hdr section, --eh-frame-hdr ignored. /usr/bin/ld: error in ./app.o(.eh_frame); no .eh_frame_hdr table created. collect2: ld returned 1 exit status make: *** [app.so] error 1  11:25:49 build finished (took 463ms) 

with -c , build fine:

11:33:16 **** incremental build of configuration debug project app **** make  building file: ../app.cpp invoking: gcc c++ compiler g++ -o0 -g3 -wall -c -fmessage-length=0 -fpic -mmd -mp -mf"app.d" -mt"app.d" -o "app.o" "../app.cpp" finished building: ../app.cpp  building target: app.so invoking: gcc c++ linker g++ -shared -o "app.so"  ./app.o    finished building target: app.so 

11:33:16 build finished (took 311ms)

new fix generate executable file

13:32:44 **** incremental build of configuration debug project app **** make  building file: ../app.cpp invoking: gcc c++ compiler g++ -o0 -g3 -wall -c -fmessage-length=0 -fpic -mmd -mp -mf"app.d" -mt"app.d" -o "app.o" "../app.cpp" finished building: ../app.cpp  building target: app invoking: gcc c++ linker g++ -shared -o "app"  ./app.o    finished building target: app 

in first case, without -c, first g++ invocation generates linked executable misleadingly named "app.o". (try typing "file ./app.o" describe file in both cases. might interesting.) able run it. (type ./app.o)

with -c flag, g++ invocation generates object code , is suitable further link stage (as observe).