Headers not being recognised

The Background

I have the following source code

#include <libubuntuone-1.0/u1-music-store.h>
#include <libsyncdaemon-1.0/libsyncdaemon/libsyncdaemon.h>

static void
get_credentials (U1MusicStore *music_store,
                                 gchar **oauth_consumer_token,
                                 gchar **oauth_consumer_secret,
                                 gchar **oauth_token,
                                 gchar **oauth_token_secret)
{
    SyncdaemonCredentials *credentials;
    *oauth_consumer_token = *oauth_consumer_secret = *oauth_token = *oauth_token_secret = NULL;

    *oauth_consumer_token = g_strdup (syncdaemon_credentials_get_consumer_key (credentials));
    *oauth_consumer_secret = g_strdup (syncdaemon_credentials_get_consumer_secret (credentials));
    *oauth_token = g_strdup (syncdaemon_credentials_get_token (credentials));
    *oauth_consumer_secret = g_strdup (syncdaemon_credentials_get_token_secret (credentials));
}

int main() 
{
    return 0;
}

and I am compiling it with the following makefile

main: main.o
    g++ main.o -o main

main.o: main.cpp
    g++ -c main.cpp `pkg-config --cflags --libs gtk+-2.0`

I need to include the pkg-config option since the u1-music-store.h header tried to included gtk/gtk.h, but the compiler isn't able to find it on it's own.

libsyncdaemon.h is a meta-header whose only purpose is to include a larger list of headers, which can be seen below

#include <libsyncdaemon-1.0/libsyncdaemon/syncdaemon-authentication.h>
#include <libsyncdaemon-1.0/libsyncdaemon/syncdaemon-config-interface.h>
#include <libsyncdaemon-1.0/libsyncdaemon/syncdaemon-credentials.h>
#include <libsyncdaemon-1.0/libsyncdaemon/syncdaemon-daemon.h>
#include <libsyncdaemon-1.0/libsyncdaemon/syncdaemon-events-interface.h>
#include <libsyncdaemon-1.0/libsyncdaemon/syncdaemon-file-info.h>
#include <libsyncdaemon-1.0/libsyncdaemon/syncdaemon-filesystem-interface.h>
#include <libsyncdaemon-1.0/libsyncdaemon/syncdaemon-folder-info.h>
#include <libsyncdaemon-1.0/libsyncdaemon/syncdaemon-folders-interface.h>
#include <libsyncdaemon-1.0/libsyncdaemon/syncdaemon-interface.h>
#include <libsyncdaemon-1.0/libsyncdaemon/syncdaemon-publicfiles-interface.h>
#include <libsyncdaemon-1.0/libsyncdaemon/syncdaemon-share-info.h>
#include <libsyncdaemon-1.0/libsyncdaemon/syncdaemon-status-info.h>
#include <libsyncdaemon-1.0/libsyncdaemon/syncdaemon-status-interface.h>
#include <libsyncdaemon-1.0/libsyncdaemon/syncdaemon-transfer-info.h>

My problem

Whenever I try to compile my code, I get the following error:

main.o: In function `get_credentials(_U1MusicStore*, char**, char**, char**, char**)':
main.cpp:(.text+0x34): undefined reference to `syncdaemon_credentials_get_consumer_key'
main.cpp:(.text+0x3c): undefined reference to `g_strdup'
main.cpp:(.text+0x4e): undefined reference to `syncdaemon_credentials_get_consumer_secret'
main.cpp:(.text+0x56): undefined reference to `g_strdup'
main.cpp:(.text+0x68): undefined reference to `syncdaemon_credentials_get_token'
main.cpp:(.text+0x70): undefined reference to `g_strdup'
main.cpp:(.text+0x82): undefined reference to `syncdaemon_credentials_get_token_secret'
main.cpp:(.text+0x8a): undefined reference to `g_strdup'
collect2: ld returned 1 exit s开发者_如何学JAVAtatus
make: *** [main] Error

Using grep, I've tracked down the four syncdaemon_credentials_get_* functions to syncdaemon-credentials.h, which I would expect the compiler to be able to find since it's listed in libsyncdaemon.h, but for some reason that's not happening. I'm assuming it's for the reason that u1-music-store.h was unable to find gtk/gtk.h that compelled me to use the pkg-config option in my makefile, but I'm having trouble understanding why this is even the case to begin with. If the file's #included, I would expect it to be included by the compiler.

Using grep again I was able to track down g_strdup to a number of headers, but I also discovered that when I replaced by makefile with the single command

g++ main.cpp -o main `pkg-config --cflags --libs gtk+-2.0`

I can eliminate the g_strdup warning, and I'm just left with the function errors.

My question

There are two things I'm looking to know here:

1. What should my makefile look like in order to solve my specific problem
2. What is the general solution to my problem? I'm guessing it's something to do with daisy-chaining #include directives together, and having to use pkg-config to fix that, but I'm not sure.

1. You need to link against libsyncdaemon.so. Get the appropriate arguments from pkg-config ... libsyncdaemon-1.0.

2. You need to link against libraries that export the required symbols. If there is a .pc file then you can use it to get the appropriate arguments.

"Unresolved external symbol" (MSVC) and "undefined reference to" (GCC) mean that the compiler found the declarations but the linker couldn't find the definitions.
This either means that you forgot to compile and/or link against a .cpp or that you forgot to link against an external library (.lib (Windows) / .a (Unix/Linux)) or object file (.o) which contains said definitions.