CODEWITHSUNDEEP
clinuxshared-libraries
Gauthier
Gauthier

Reputation: 41945

How do I link to a shared library?

I installed a new lib from source and I am trying to link to it. The linker complains that it cannot find the symbols that should be in the lib.

Here are the errors I get:

$ make all
gcc -std=c99 -g -Wall -O3 `pkg-config --cflags --libs libmodbus` -c client.c -o client.o
gcc -std=c99 -g -Wall -O3 `pkg-config --cflags --libs libmodbus` client.o -o client
client.o: In function `main':
/home/gauthier/code/modbus/client.c:29: undefined reference to `modbus_new_tcp'
/home/gauthier/code/modbus/client.c:30: undefined reference to `modbus_connect'
/home/gauthier/code/modbus/client.c:33: undefined reference to `modbus_read_registers'
/home/gauthier/code/modbus/client.c:35: undefined reference to `modbus_close'
/home/gauthier/code/modbus/client.c:36: undefined reference to `modbus_free'
collect2: error: ld returned 1 exit status
make: *** [client] Error 1

pkg-config says this:

$ pkg-config --cflags --libs libmodbus
-I/usr/local/include/modbus  -L/usr/local/lib -lmodbus

and the files in /usr/local/lib are these:

$ ls /usr/local/lib/ | grep mod
libmodbus.la
libmodbus.so
libmodbus.so.5
libmodbus.so.5.0.5

Also, the so file contains the undefined references:

$ grep modbus_connect /usr/local/lib/*
Binary file /usr/local/lib/libmodbus.so matches
Binary file /usr/local/lib/libmodbus.so.5 matches
Binary file /usr/local/lib/libmodbus.so.5.0.5 matches

I tried adding /usr/local/lib to LD_LIBRARY_PATH, to no av. Not that I though it would help since I have -L in my compiler command anyway.

I also tried to manually write the -I, -L, and -l (not using pkg-config), that didn't help.

How do I make the linker understand that the references to the modbus_* functions are right there?

Upvotes: 1

Views: 1724

Answers (1)

user184968
user184968

Reputation:

Let me guess.

It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, ‘foo.o -lz bar.o’ searches library ‘z’ after file foo.o but before bar.o. If bar.o refers to functions in ‘z’, those functions may not be loaded.

http://gcc.gnu.org/onlinedocs/gcc/Link-Options.html

So try put pkg-config --cflags --libs libmodbus at the end

Upvotes: 5

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