How to specify preference of library path

Question

I m compiling a c   program using g   and ld  I have a  so library I want to be used during linking  However  a library of the same name exists in  usr local lib  and ld is choosing that library over the one I m directly specifying  How can I fix this   For the examples below  my library file is  my dir libfoo so 0  Things I ve tried that don t work    my g   command is g   -g -Wall -o my binary -L my dir -lfoo bar cpp adding  my dir to the beginning or end of my  PATH en  variable adding  my dir libfoo so 0 as an argument to g

User · Answer

As an alternative  you can use the environment variables LIBRARY PATH and CPLUS INCLUDE PATH  which respectively indicate where to look for libraries and where to look for headers  CPATH will also do the job   without specifying the -L and -I options   Edit  CPATH includes header with -I and CPLUS INCLUDE PATH with -isystem

User · Answer

Add the path to where your new library is to LD LIBRARY PATH  it has slightly different name on Mac       Your solution should work with using the -L my dir -lfoo options  at runtime use LD LIBRARY PATH to point to the location of your library   Careful with using LD LIBRARY PATH - in short  from link         implications          Security  Remember that the directories specified in LD LIBRARY PATH get searched before    the standard locations  In that   way  a nasty person could get your application to load a version of a   shared library that contains malicious code  That   s one reason why   setuid setgid executables do neglect that variable        Performance  The link loader has to search all the directories specified  until it finds the directory where the shared library   resides     for ALL shared libraries the application is linked against    This means a lot of system calls to open    that will fail with      ENOENT  No such file or directory      If the path contains many   directories  the number of failed calls will increase linearly  and   you can tell that from the start-up time of the application  If some    or all  of the directories are in an NFS environment  the start-up   time of your applications can really get long     and it can slow down   the whole system        Inconsistency  This is the most common problem  LD LIBRARY PATH forces an application to load a shared library it wasn   t linked   against  and that is quite likely not compatible with the original   version  This can either be very obvious  i e  the application   crashes  or it can lead to wrong results  if the picked up library not   quite does what the original version would have done  Especially the   latter is sometimes hard to debug    OR  Use the rpath option via gcc to linker - runtime library search path  will be used instead of looking in standard dir  gcc option    -Wl -rpath   DEFAULT LIB INSTALL PATH    This is good for a temporary solution  Linker first searches the LD LIBRARY PATH for libraries before looking into standard directories   If you don t want to permanently update LD LIBRARY PATH you can do it on the fly on command line   LD LIBRARY PATH  some custom dir   fooo   You can check what libraries linker knows about using  example     sbin ldconfig -p   grep libpthread         libpthread so 0  libc6  OS ABI  Linux 2 6 4    gt   lib libpthread so 0   And you can check which library your application is using   ldd foo         linux-gate so 1   gt    0xffffe000          libpthread so 0   gt   lib libpthread so 0  0xb7f9e000          libxml2 so 2   gt   usr lib libxml2 so 2  0xb7e6e000          librt so 1   gt   lib librt so 1  0xb7e65000          libm so 6   gt   lib libm so 6  0xb7d5b000          libc so 6   gt   lib libc so 6  0xb7c2e000           lib ld-linux so 2  0xb7fc7000          libdl so 2   gt   lib libdl so 2  0xb7c2a000          libz so 1   gt   lib libz so 1  0xb7c18000

User · Answer

Specifying the absolute path to the library should work fine   g    my dir libfoo so 0        Did you remember to remove the -lfoo once you added the absolute path

User · Answer

If one is used to work with DLL in Windows and would like to skip  so version numbers in linux QT  adding CONFIG    plugin will take version numbers out  To use absolute path to  so  giving it to linker works fine  as Mr  Klatchko mentioned

User · Answer

This is an old question  but no one seems to have mentioned this   You were getting lucky that the thing was linking at all   You needed to change  g   -g -Wall -o my binary -L my dir -lfoo bar cpp   to this   g   -g -Wall -o my binary -L my dir bar cpp -lfoo   Your linker keeps track of symbols it needs to resolve  If it reads the library first  it doesn t have any needed symbols  so it ignores the symbols in it  Specify the libraries after the things that need to link to them so that your linker has symbols to find in them   Also  -lfoo makes it search specifically for a file named libfoo a or libfoo so as needed  Not libfoo so 0  So either ln the name or rename the library as appopriate   To quote the gcc man page   -l library           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    Adding the file directly to g   s command line should have worked  unless of course  you put it prior to bar cpp  causing the linker to ignore it for lacking any needed symbols  because no symbols were needed yet

[c++] How to specify preference of library path?

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