CODEWITHSUNDEEP
compilationshared-librarieslinkerg++
stdcerr
stdcerr

Reputation: 15658

unable to compile with library

I would like to experiment with the OpenALPR SDK and have written a little test program. The problem is, I can't get it to compile properly and I'm not sure why. Here's the SDK documentation.

My source file looks like: 

$ cat test.cpp 
#include <alpr.h>
#include <iostream>

std::string key =
"MyKey";

int main (void) 
{
    alpr::Alpr openalpr("us", "/etc/openalpr/openalpr.conf", "/usr/share/openalpr/runtime_data/", key);

    // Make sure the library loaded before continuing.  
    // For example, it could fail if the config/runtime_data is not found
    if (openalpr.isLoaded() == false)
    {
            std::cerr << "Error loading OpenALPR" << std::endl;
                return 1;
    }

    std::cout << "Hello World!" << std::endl;

    return 0;
}

I use the following command to compile and get the output:

$ g++ -Wall -lopenalpr test.cpp -o test
/tmp/ccGjLkrk.o: In function `main':
test.cpp:(.text+0xc5): undefined reference to `alpr::Alpr::Alpr(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)'
test.cpp:(.text+0x134): undefined reference to `alpr::Alpr::isLoaded()'
test.cpp:(.text+0x18e): undefined reference to `alpr::Alpr::~Alpr()'
test.cpp:(.text+0x1ce): undefined reference to `alpr::Alpr::~Alpr()'
test.cpp:(.text+0x202): undefined reference to `alpr::Alpr::~Alpr()'
test.cpp:(.text+0x236): undefined reference to `alpr::Alpr::~Alpr()'
test.cpp:(.text+0x273): undefined reference to `alpr::Alpr::~Alpr()'
/tmp/ccGjLkrk.o:test.cpp:(.text+0x290): more undefined references to `alpr::Alpr::~Alpr()' follow
collect2: error: ld returned 1 exit status

Just confirming my library is where it should be: libopenalpr.so is a symlink to libopenalpr.so.2.

$ locate libopenalpr.so
/usr/lib/libopenalpr.so
/usr/lib/libopenalpr.so.2

Can anyone point out what I'm doing wrong here?

Upvotes: 0

Views: 275

Answers (1)

Ignacio Vazquez-Abrams
Ignacio Vazquez-Abrams

Reputation: 799230

From the gcc(1) man page:

... the placement of the -l option is significant.

[...]

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.

$ g++ -Wall test.cpp -lopenalpr -o test

Upvotes: 4

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