CODEWITHSUNDEEP
c++linuxg++
flammi88
flammi88

Reputation: 401

Gcc - Undefined reference but library contains matching symbol

i have tried to link a programm of mine with the libmodbus opensource lib. After some trouble with automake, i finally managed to compile it.

But now I get a undefined reference error from gcc, when I use one of the functions the library provides in my own programm:

main.cpp:(.text+0x21): undefined reference to `modbus_udp_init(char*, int, __modbus_udp_handle*)'
collect2: Fehler: ld gab 1 als Ende-Status zurück

From my point of view, the arguments of gcc to compile my project are correct:

g++ -v -I ../libmodbus/modbus -o main main.cpp -L../libmodbus/modbus/.libs/ -lmodbus

I checked with nm if the function is really in that library, but everything looks fine for me (I have deleted some parts from the output, to prevent the output from getting too big): 

$ nm ../libmodbus/modbus/.libs/libmodbus.a 

modbus.o:
0000000000000590 T crc16
                 U free
                 U malloc
                 U memcpy
0000000000000b40 T modbus_diagnostics
0000000000000d90 T modbus_error_get_exception_code
0000000000000100 C modbus_error_str
0000000000000cf0 T modbus_exception
[...]

modbus-udp.o:
                 U bcopy
                 U close
                 U __errno_location
                 U gethostbyname
                 U modbus_error_str
                 U modbus_tcp_frame_pack
                 U modbus_tcp_frame_parse
0000000000000000 T modbus_udp_close
0000000000000030 T modbus_udp_init
0000000000000220 T modbus_udp_recv
0000000000000190 T modbus_udp_send
0000000000000010 r __PRETTY_FUNCTION__.4582
0000000000000000 r __PRETTY_FUNCTION__.4592
                 U recvfrom
                 U sendto
                 U setsockopt
                 U snprintf
                 U socket
                 U strerror

modbus-tcp.o:
[...]


modbus-serial.o:
[...]

Does someone have an idea why gcc can't resolve the symbol?

Upvotes: 4

Views: 1726

Answers (1)

flammi88
flammi88

Reputation: 401

As noloader suggested, i missed adding extern "C" as libmodbus is a C-library. I edited my source:

extern "C" {
    #include <modbus.h>
    #include <modbus-udp.h>
}

Now it compiles fine. Thank you!

Upvotes: 3

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