CODEWITHSUNDEEP
ubuntuopengllinkerglew
Reinderien
Reinderien

Reputation: 15308

Cannot link to libglew or libglfw

I'm following a (modified) version of this OpenGL tutorial.

With this trivial source file:

#include <GL/glew.h>

int main() {
    glewInit();
}

And this makefile:

cflags=-ggdb -Wall -std=c++17

all: 01

01: 01.o makefile
    g++ $(cflags) $(shell pkg-config --libs glew glfw3 glm) -o $@ $<
01.o: 01.cpp makefile
    g++ $(cflags) $(shell pkg-config --cflags glew glfw3 glm) -o $@ $< -c

# The following have pkg-config entries:
# x11 xext glu glfw3 glew glm

It can compile but not link. 

$ make
g++ -ggdb -Wall -std=c++17 -I/usr/include/libdrm -o 01.o 01.cpp -c
g++ -ggdb -Wall -std=c++17 -lGLEW -lGLU -lGL -lglfw -o 01 01.o
/usr/bin/ld: 01.o: in function `main':
/home/greg/OpenGL_GEOM_2D/01/01.cpp:6: undefined reference to `glewInit'

Checking what pkg-config is doing in the background:

$ pkg-config --libs glew
-lGLEW -lGLU -lGL

The library it's using is here:

$ dpkg-query -L libglew-dev
/usr/lib/x86_64-linux-gnu/libGLEW.so

That file definitely does have the symbols that are failing to link:

$ readelf -Ws /usr/lib/x86_64-linux-gnu/libGLEW.so | grep glewInit
1146: 000000000005f2f0 38819 FUNC    GLOBAL DEFAULT   10 glewInit

So what's going on? Why is the linker failing to find that symbol?

Upvotes: 0

Views: 985

Answers (1)

datenwolf
datenwolf

Reputation: 162317

For the old GNU linker the order of inputs specified on the command line does matter. It makes a difference if the libraries are specified before or after the input object files.

Try this for your Makefile

01: 01.o makefile
    g++ -o $@ $< $(shell pkg-config --libs glew glfw3 glm)

BTW, you should learn about implicit rules, your whole Makefile can be replaced with:

.PHONY: 01
all: 01

01: LDFLAGS += $(shell pkg-config --libs glew glfw3 glm)
01: 01.o

01.o: CFLAGS += $(shell pkg-config --cflags glew glfw3 glm)
01.o: 01.cpp Makefile

Thanks to the implcit rules you don't have to write the actual commands at all, just set the desired variables.

Upvotes: 1

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