[linux] ld cannot find an existing library


As just formulated by grepsedawk, the answer lies in the -l option of g++, calling ld. If you look at the man page of this command, you can either do:

  • g++ -l:libmagic.so.1 [...]
  • or: g++ -lmagic [...] , if you have a symlink named libmagic.so in your libs path

I am attempting to link an application with g++ on this Debian lenny system. ld is complaining it cannot find specified libraries. The specific example here is ImageMagick, but I am having similar problems with a few other libraries too.

I am calling the linker with:

g++ -w (..lots of .o files/include directories/etc..) \
-L/usr/lib -lmagic

ld complains:

/usr/bin/ld: cannot find -lmagic

However, libmagic exists:

$ locate libmagic.so
$ ls -all /usr/lib/libmagic.so.1*
lrwxrwxrwx 1 root root    17 2008-12-01 03:52 /usr/lib/libmagic.so.1 -> libmagic.so.1.0.0
-rwxrwxrwx 1 root root 84664 2008-09-09 00:05 /usr/lib/libmagic.so.1.0.0
$ ldd /usr/lib/libmagic.so.1.0.0 
    linux-gate.so.1 =>  (0xb7f85000)
    libz.so.1 => /usr/lib/libz.so.1 (0xb7f51000)
    libc.so.6 => /lib/i686/cmov/libc.so.6 (0xb7df6000)
    /lib/ld-linux.so.2 (0xb7f86000)
$ sudo ldconfig -v | grep "libmagic"
    libmagic.so.1 -> libmagic.so.1.0.0

How do I diagnose this problem further, and what could be wrong? Am I doing something completely stupid?

As mentioned above the linker is looking for libmagic.so, but you only have libmagic.so.1.

To solve this problem just perform an update cache.

ldconfig -v 

To verify you can run:

$ ldconfig -p | grep libmagic

In Ubuntu, you can install libtool which resolves the libraries automatically.

$ sudo apt-get install libtool

This resolved a problem with ltdl for me, which had been installed as libltdl.so.7 and wasn't found as simply -lltdl in the make.