linux - Undefine reference for libraries, so How could I find the right path? -

i trying compile v4l2 example in ubuntu getting following error:

guilherme@notedev01:~/downloads/v4l2_samples-0.4.1$ make gcc -o2  -l/usr/include -lx11 -lxext -o viewer viewer.c /tmp/ccujnjwq.o: in function `image_destroy': viewer.c:(.text+0x234): undefined reference `xdestroyimage' viewer.c:(.text+0x256): undefined reference `xfreegc' viewer.c:(.text+0x277): undefined reference `xshmdetach' viewer.c:(.text+0x2ac): undefined reference `xfreepixmap' /tmp/ccujnjwq.o: in function `image_create': viewer.c:(.text+0x305): undefined reference `xcreategc' viewer.c:(.text+0x31d): undefined reference `xgetwindowattributes' viewer.c:(.text+0x39e): undefined reference `xshmcreateimage' viewer.c:(.text+0x3f5): undefined reference `xshmattach' viewer.c:(.text+0x44e): undefined reference `xcreateimage' viewer.c:(.text+0x494): undefined reference `xshmqueryextension' viewer.c:(.text+0x4b4): undefined reference `xshmpixmapformat' viewer.c:(.text+0x4dc): undefined reference `xshmcreatepixmap' /tmp/ccujnjwq.o: in function `image_put': viewer.c:(.text+0x54c): undefined reference `xputimage' viewer.c:(.text+0x586): undefined reference `xshmputimage' /tmp/ccujnjwq.o: in function `main': viewer.c:(.text.startup+0x18b): undefined reference `xopendisplay' viewer.c:(.text.startup+0x1b1): undefined reference `xscreenofdisplay' viewer.c:(.text.startup+0x1ee): undefined reference `xcreatesimplewindow' viewer.c:(.text.startup+0x249): undefined reference `xmapraised' viewer.c:(.text.startup+0x263): undefined reference `xstorename' viewer.c:(.text.startup+0x280): undefined reference `xgetwindowattributes' viewer.c:(.text.startup+0x92f): undefined reference `xpending' viewer.c:(.text.startup+0x94c): undefined reference `xnextevent' viewer.c:(.text.startup+0xaee): undefined reference `xpending' viewer.c:(.text.startup+0xb0b): undefined reference `xnextevent' viewer.c:(.text.startup+0xf39): undefined reference `xpending' viewer.c:(.text.startup+0xf56): undefined reference `xnextevent' collect2: error: ld returned 1 exit status make: *** [viewer] error 1 

what can see path -lx11 , -lxext isn't -l/usr/include. how can find right path libraries?

thanks.

as chris has pointed out, order wrong, need put -lx11 -lxext after source-code/object-files.

this because modern compilers try optimize final result , not link against unused libraries. maintaining list of unresolved symbols within object , use binary files come aferwards in linker arguments resolve symbols.

example

your program test uses function do_foo() libfoo , function do_bar_do() libbar.

you link using:

$ gcc -o test test.o -lfoo -lbar 

the linker first searches test.o , notices symbols (do_foo , do_bar_do) not defined anywhere. proceeds libfoo (specified right after test.o) , finds provides do_foo, creates code use program. do_bar_do still unresolved, until linker checks upon libbar.

consider doing wrong way:

$ gcc -o test -lfoo test.o -lbar 

the linker first check libfoo , see doesn't contain unresolved symbols. cool. proceed test.o , notice do_bar_do , do_foo. do_bar_do resolved right-hand libbar do_foo not resolved @ all, , error:

undefined reference `do_foo'

"but code meant tutorial..."

so why not working?

older compilers bit lax order of dependencies (they check binaries/libraries/objects whether given symbol resolved); that's why can still find code out there puts libraries link against before object files.