Why does the order in which libraries are linked sometimes cause errors in GCC

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Here s an example to make it clear how things work with GCC when static libraries are involved  So let s assume we have the following scenario    myprog o - containing main   function  dependent on libmysqlclient libmysqlclient - static  for the sake of the example  you d prefer the shared library  of course  as the libmysqlclient is huge   in  usr local lib  and dependent on stuff from libz libz  dynamic    How do we link this   Note  examples from compiling on Cygwin using gcc 4 3 4   gcc -L usr local lib -lmysqlclient myprog o   undefined reference to   mysql init    myprog depends on libmysqlclient   so myprog has to come earlier on the command line  gcc myprog o -L usr local lib -lmysqlclient   undefined reference to   uncompress    we have to link with libz  too  gcc myprog o -lz -L usr local lib -lmysqlclient   undefined reference to   uncompress    libz is needed by libmysqlclient   so it has to appear  after  it on the command line  gcc myprog o -L usr local lib -lmysqlclient -lz   this works

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If you add -Wl --start-group to the linker flags it does not care which order they re in or if there are circular dependencies   On Qt this means adding    QMAKE LFLAGS    -Wl --start-group   Saves loads of time messing about and it doesn t seem to slow down linking much  which takes far less time than compilation anyway

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The GNU ld linker is a so-called smart linker   It will keep track of the functions used by preceding static libraries  permanently tossing out those functions that are not used from its lookup tables   The result is that if you link a static library too early  then the functions in that library are no longer available to static libraries later on the link line     The typical UNIX linker works from left to right  so put all your dependent libraries on the left  and the ones that satisfy those dependencies on the right of the link line   You may find that some libraries depend on others while at the same time other libraries depend on them   This is where it gets complicated   When it comes to circular references  fix your code

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Link order certainly does matter  at least on some platforms  I have seen crashes for applications linked with libraries in wrong order  where wrong means A linked before B but B depends on A

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See the history on this answer to get the more elaborate text  but I now think it s easier for the reader to see real command lines    Common files shared by all below commands   cat a cpp extern int a  int main       return a       cat b cpp extern int b  int a   b     cat d cpp int b   Linking to static libraries   g   -c b cpp -o b o   ar cr libb a b o   g   -c d cpp -o d o   ar cr libd a d o    g   -L  -ld -lb a cpp   wrong order   g   -L  -lb -ld a cpp   wrong order   g   a cpp -L  -ld -lb   wrong order   g   a cpp -L  -lb -ld   right order  The linker searches from left to right  and notes unresolved symbols as it goes  If a library resolves the symbol  it takes the object files of that library to resolve the symbol  b o out of libb a in this case   Dependencies of static libraries against each other work the same - the library that needs symbols must be first  then the library that resolves the symbol  If a static library depends on another library  but the other library again depends on the former library  there is a cycle  You can resolve this by enclosing the cyclically dependent libraries by -  and -   such as -  -la -lb -   you may need to escape the parens  such as -   and -     The linker then searches those enclosed lib multiple times to ensure cycling dependencies are resolved  Alternatively  you can specify the libraries multiple times  so each is before one another  -la -lb -la  Linking to dynamic libraries   export LD LIBRARY PATH     not needed if libs go to  usr lib etc   g   -fpic -shared d cpp -o libd so   g   -fpic -shared b cpp -L  -ld -o libb so   specifies its dependency     g   -L  -lb a cpp   wrong order  works on some distributions    g   -Wl --as-needed -L  -lb a cpp   wrong order   g   -Wl --as-needed a cpp -L  -lb   right order  It s the same here - the libraries must follow the object files of the program  The difference here compared with static libraries is that you need not care about the dependencies of the libraries against each other  because dynamic libraries sort out their dependencies themselves  Some recent distributions apparently default to using the --as-needed linker flag  which enforces that the program s object files come before the dynamic libraries  If that flag is passed  the linker will not link to libraries that are not actually needed by the executable  and it detects this from left to right   My recent archlinux distribution doesn t use this flag by default  so it didn t give an error for not following the correct order  It is not correct to omit the dependency of b so against d so when creating the former  You will be required to specify the library when linking a then  but a doesn t really need the integer b itself  so it should not be made to care about b s own dependencies  Here is an example of the implications if you miss specifying the dependencies for libb so   export LD LIBRARY PATH     not needed if libs go to  usr lib etc   g   -fpic -shared d cpp -o libd so   g   -fpic -shared b cpp -o libb so   wrong  but links     g   -L  -lb a cpp   wrong  as above   g   -Wl --as-needed -L  -lb a cpp   wrong  as above   g   a cpp -L  -lb   wrong  missing libd so   g   a cpp -L  -ld -lb   wrong order  works on some distributions    g   -Wl --as-needed a cpp -L  -ld -lb   wrong order  like static libs    g   -Wl --as-needed a cpp -L  -lb -ld    quot right quot   If you now look into what dependencies the binary has  you note the binary itself depends also on libd  not just libb as it should  The binary will need to be relinked if libb later depends on another library  if you do it this way  And if someone else loads libb using dlopen at runtime  think of loading plugins dynamically   the call will fail as well  So the  quot right quot  really should be a wrong as well

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A quick tip that tripped me up  if you re invoking the linker as  gcc  or  g     then using  --start-group  and  --end-group  won t pass those options through to the linker -- nor will it flag an error  It will just fail the link with undefined symbols if you had the library order wrong   You need to write them as  -Wl --start-group  etc  to tell GCC to pass the argument through to the linker

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You may can use -Xlinker option   g   -o foobar  -Xlinker -start-group  -Xlinker libA a -Xlinker libB a -Xlinker libC a  -Xlinker -end-group    is ALMOST equal to  g   -o foobar  -Xlinker -start-group  -Xlinker libC a -Xlinker libB a -Xlinker libA a  -Xlinker -end-group    Careful     The order within a group is important   Here s an example  a debug library has a debug routine  but the non-debug library has a weak version of the same  You must put the debug library FIRST in the group or you will resolve to the non-debug version  You need to precede each library in the group list with -Xlinker

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I have seen this a lot  some of our modules link in excess of a 100 libraries of our code plus system  amp  3rd party libs    Depending on different linkers HP Intel GCC SUN SGI IBM etc you can get unresolved functions variables etc  on some platforms you have to list libraries twice    For the most part we use structured hierarchy of libraries  core  platform  different layers of abstraction  but for some systems you still have to play with the order in the link command   Once you hit upon a solution document it so the next developer does not have to work it out again   My old lecturer used to say   high cohesion  amp  low coupling   it   s still true today

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Another alternative would be to specify the list of libraries twice   gcc prog o libA a libB a libA a libB a -o prog x   Doing this  you don t have to bother with the right sequence since the reference will be resolved in the second block

[gcc] Why does the order in which libraries are linked sometimes cause errors in GCC?

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