Can't find .so in the same directory as the executable?

While you can set LD_LIBRARY_PATH to let the dynamic linker know where to look, there are better options. You can put your shared library in one of the standard places, see /etc/ld.so.conf (on Linux) and /usr/bin/crle (on Solaris) for the list of these places

You can pass -R <path> to the linker when building your binary, which will add <path> to the list of directories scanned for your shared library. Here's an example. First, showing the problem:

libtest.h:

void hello_world(void);

libtest.c:

#include <stdio.h>
void hello_world(void) {
  printf("Hello world, I'm a library!\n");
}

hello.c:

#include "libtest.h"
int main(int argc, char **argv) {
  hello_world();
}

Makefile (tabs must be used):

all: hello
hello: libtest.so.0
%.o: %.c
        $(CC) $(CFLAGS) -fPIC -c -o $@ $<
libtest.so.0.0.1: libtest.o
        $(CC) -shared -Wl,-soname,libtest.so.0 -o libtest.so.0.0.1 libtest.o
libtest.so.0: libtest.so.0.0.1
        ln -s $< $@
clean:
        rm -f hello libtest.o hello.o libtest.so.0.0.1 libtest.so.0

Let's run it:

$ make
cc  -fPIC -c -o libtest.o libtest.c
cc -shared -Wl,-soname,libtest.so.0 -o libtest.so.0.0.1 libtest.o
ln -s libtest.so.0.0.1 libtest.so.0
cc     hello.c libtest.so.0   -o hello
$ ./hello 
./hello: error while loading shared libraries: libtest.so.0: cannot open shared object file: No such file or directory

How to fix it? Add -R <path> to the linker flags (here, by setting LDFLAGS).

$ make clean
(...)
$ make LDFLAGS="-Wl,-R -Wl,/home/maciej/src/tmp"
(...)
cc   -Wl,-R -Wl,/home/maciej/src/tmp  hello.c libtest.so.0   -o hello
$ ./hello 
Hello world, I'm a library!

Looking at the binary, you can see that it needs libtest.so.0:

$ objdump -p hello | grep NEEDED
  NEEDED               libtest.so.0
  NEEDED               libc.so.6

The binary will look for its libraries, apart from the standard places, in the specified directory:

$ objdump -p hello | grep RPATH
  RPATH                /home/maciej/src/tmp

If you want the binary to look in the current directory, you can set the RPATH to $ORIGIN. This is a bit tricky, because you need to make sure that the dollar sign is not interpreted by make. Here's one way to do it:

$ make CFLAGS="-fPIC" LDFLAGS="-Wl,-rpath '-Wl,\$\$ORIGIN'"
$ objdump -p hello | grep RPATH
  RPATH                $ORIGIN
$ ./hello 
Hello world, I'm a library!

The loader never checks the current directory for shared objects unless it is explicitly directed to via $LD_LIBRARY_PATH. See the ld.so(8) man page for more details.

To load the shared objects from the same directory as your executable, simply execute:

$ LD_LIBRARY_PATH=. ./binary

Note: It will not modify the LD_LIBRARY_PATH variable of your system. The change only affects to this, and only this, execution of your program.

For anyone using CMake for their build, you can set the CMAKE_EXE_LINKER_FLAGS to the following:

set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -Wl,-rpath='$ORIGIN'")

This will properly propagate the linker flags for all build types (e.g., Debug, Release, etc...) to look for .so files in the current working directory first.

For anyone that still struggles without an answer I found one myself with the following suggestion:

You could try updating the ld.so.cache using: sudo ldconfig -v

Worked for me.

There can be subtle situations showing the same symptoms. I use to build a CMake project, both locally at /path/to/myproj and remotely on a server, at /path/to/myproj-deploy. Project contains an .so build artifact and multiple ELF files depending on that.

When I pulled back binaries from the build server, I found out that (locally) calling ELFs results in the same problems. Thanks to authomatthias, calling

$ objdump -p bin/myelf  | grep my
bin/myelf:     file format elf64-x86-64
  NEEDED               libmyso.so
  RUNPATH              /path/to/myproj-deploy/bin

ends up in a non-surprising consequence: CMake system hardcoded remote path into the RUNPATH, demonstrating some non-portability (without proper additional actions, I suppose)

The dynamic linker will decide where to look for libraries. In case of Linux, the dynamic linker usually is GNU ld.so (or an alternative that will usually behave identical for compatibility reasons).

To quotes from the Wikipedia:

The dynamic linker of the GNU C Library searches for shared libraries in the following locations:

1. The (colon-separated) paths in the DT_RPATH dynamic section attribute of the binary if present and the DT_RUNPATH attribute does not exist.
2. The (colon-separated) paths in the environment variable LD_LIBRARY_PATH, unless the executable is a setuid/setgid binary, in which case it is ignored. LD_LIBRARY_PATH can be overridden by calling the dynamic linker with the option --library-path (e.g. /lib/ld-linux.so.2 --library-path $HOME/mylibs myprogram).
3. The (colon-separated) paths in the DT_RUNPATH dynamic section attribute of the binary if present.
4. Lookup based on the ldconfig cache file (often located at /etc/ld.so.cache) which contains a compiled list of candidate libraries previously found in the augmented library path (set by /etc/ld.so.conf). If, however, the binary was linked with the -z nodefaultlib linker option, libraries in the default library paths are skipped.
5. In the trusted default path /lib, and then /usr/lib. If the binary was linked with the -z nodefaultlib linker option, this step is skipped.

Source: https://en.wikipedia.org/wiki/Rpath

I got it working with this :

$ gcc file.c -L. -lmylib

and to run :

$ LD_LIBRARY_PATH=. ./a.out