Where is LD_LIBRARY_PATH? how to set the LD_LIBRARY_PATH env variable

environment-variables

I am trying to build a c++ program using Unix.

I got the error

Linking CXX executable ../../bin/ME
/usr/bin/ld: cannot find -lboost_regex-mt

I heard that I just need to set the location of libboost* in my LD_LIBRARY_PATH env variable and then invoke make as I originally did, by typing

-L /usr/lib64 -l boost_regex-mt

export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/usr/lib64

But where is LD_LIBRARY_PATH? how do I set the LD_LIBRARY_PATH env variable?

Best Answer

how do I set the LD_LIBRARY_PATH env variable?

You already set it when you did this:

export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/usr/lib64

But that will not solve your problem. $LD_LIBRARY_PATH is consulted at time of execution, to provide a list of additional directories in which to search for dynamically linkable libraries. It is not consulted at link time (except maybe for locating libraries required by the built tools themselves!).

In order to tell the linker where to find libraries at build time, you need to use the -L linker option. You already did that too:

-L /usr/lib64

If you are still getting the error, then you need to make sure that the library is actually there. Do you have a file libboost_regex-mt.so or libboost_regex-mt.a in that (or any) directory? Note that a file like libboost_regex-mt.so.othersuffix doesn't count for this purpose. If you don't have that, then you probably need to install your distribution's development package for this library.

Related Solutions

LD_LIBRARY_PATH environment variable

Use ld -soname:

$ mkdir dir1 dir2
$ gcc -shared -fPIC -o dir1/func.so func1.c -Wl,-soname,func.so
$ gcc -shared -fPIC -o dir2/func.so func2.c -Wl,-soname,func.so
$ gcc test.c dir1/func.so
$ ldd a.out
    linux-vdso.so.1 =>  (0x00007ffda80d7000)
    func.so => not found
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f639079e000)
    /lib64/ld-linux-x86-64.so.2 (0x00007f6390b68000)
$ LD_LIBRARY_PATH='$ORIGIN/dir1:$ORIGIN/dir2' ./a.out
hello world
$ LD_LIBRARY_PATH='$ORIGIN/dir2:$ORIGIN/dir1' ./a.out
No print

-Wl,-soname,func.so (this means -soname func.so is passed to ld) embeds SONAME attribute of func.so in the output. You can examine it by readelf -d:

$ readelf -d dir1/func.so 

Dynamic section at offset 0xe08 contains 25 entries:
  Tag        Type                         Name/Value
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
 0x000000000000000e (SONAME)             Library soname: [func.so]
...

Linked with this func.so with SONAME, a.out has that in its NEEDED attribute:

$ readelf -d a.out

Dynamic section at offset 0xe18 contains 25 entries:
  Tag        Type                         Name/Value
 0x0000000000000001 (NEEDED)             Shared library: [func.so]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
...

Without -Wl,-soname,func.so, you'll get the following by readelf -d:

$ readelf -d dir1/func.so 

Dynamic section at offset 0xe18 contains 24 entries:
  Tag        Type                         Name/Value
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
...

$ readelf -d a.out

Dynamic section at offset 0xe18 contains 25 entries:
  Tag        Type                         Name/Value
 0x0000000000000001 (NEEDED)             Shared library: [dir1/func.so]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
...

Set LD_LIBRARY_PATH correctly

You have a couple of options:

Prefix the path to your shared libraries in the export statement above (more than likely $LD_LIBRARY_PATH is not set on your system and so the only paths that are getting set are the ones explicitly set in your export statement.)
```
 export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/usr/local/lib/:/usr/lib/:/usr/lib64/:/usr/local/MATLAB/MATLAB_Runtime/v94/runtime/glnxa64:/usr/local/MATLAB/MATLAB_Runtime/v94/bin/glnxa64:/usr/local/MATLAB/MATLAB_Runtime/v94/sys/os/glnxa64:/usr/local/MATLAB/MATLAB_Runtime/v94/extern/bin/glnxa64
```
This export statement will not be a permanent part of your shell sessions unless you add it to .bashrc fyi.
Add the MATLAB library path to /etc/ld.so.conf.d/ and run ldconfig to identify the new paths for the linker on your system.

(BTW I am guessing at the MATLAB library name. You might need to make the name of the file something other than MATLAB like all lower case or something else. Basically we need the name of the library as it is referenced during the linking process. You could inspect a make file to see how this is referenced.)
```
 # Add the file '/etc/ld.so.conf.d/MATLAB' with the following lines 
 /usr/local/MATLAB/MATLAB_Runtime/v94/runtime/glnxa64
 /usr/local/MATLAB/MATLAB_Runtime/v94/bin/glnxa64
 /usr/local/MATLAB/MATLAB_Runtime/v94/sys/os/glnxa64
 /usr/local/MATLAB/MATLAB_Runtime/v94/extern/bin/glnxa64
```
Then run ldconfig.
```
 sudo ldconfig
```
You may need to unset the $LD_LIBRARY_PATH variable to make the linker use the normal system search path and pickup the new search path for MATLAB.
```
 unset LD_LIBRARY_PATH
```

Best Answer

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LD_LIBRARY_PATH environment variable

Set LD_LIBRARY_PATH correctly

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