ld cannot find an existing library

Question

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
/usr/lib/libmagic.so.1
/usr/lib/libmagic.so.1.0.0
$ 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?

Answer 1

The problem is the linker is looking for libmagic.so but you only have libmagic.so.1

A quick hack is to symlink libmagic.so.1 to libmagic.so

Answer 2

Unless I'm badly mistaken libmagic or -lmagic is not the same library as Image Magick. You state that you want image magick.

Image Magick comes with a utility to supply all appropriate options to the compiler.

Ex:

g++ program.cpp `Magick++-config --cppflags --cxxflags --ldflags --libs` -o "prog"

Answer 3

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

Answer 4

I do have a file in /lib named "libgdk_pixbuf-2.0.so" However, I still get a message " ld: cannot find -lgdk_pixbuf-2.0"! Any suggestions?

Answer 5

It is Debian convention to separate shared libraries into their runtime components (libmagic1: /usr/lib/libmagic.so.1 → libmagic.so.1.0.0) and their development components (libmagic-dev: /usr/lib/libmagic.so → …).

Because the library's soname is libmagic.so.1, that's the string that gets embedded into the executable so that's the file that is loaded when the executable is run.

However, because the library is specified as -lmagic to the linker, it looks for libmagic.so, which is why it is needed for development.

See Diego E. Pettenò: Linkers and names for details on how this all works on Linux.

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In short, you should apt-get install libmagic-dev. This will not only give you libmagic.so but also other files necessary for compiling like /usr/include/magic.h.