Calling a shared library using ./ - c

Question

I am curious about using dlopen in Linux to call shared libraries.
Suppose I want to use a shared library in C whose name is fileName.so. I am working in a 64bit Ubuntu Linux and I include dlfcn.h and use dlopen function to access the shared library.
When I use dlopen(fileName.so, RTLD_LAZY), a NULL handle is returned and shared library is not opened. However, when I use dlopen("./fileName.so", RTLD_LAZY) the dlopen does its job and opens the shared library. It seems that the main point is in using ./ before file name.
It is appreciated if help me figure out why I should use ./ in my code. Thanks

Answer 1

POSIX says that dlopen() has to know where to look for the file and leaves the behaviour when the file name does not include a / implementation defined. On Linux, if you don't supply a pathname (a name with a / in it somewhere), then dlopen() only looks in 'standard places', specified by environment variables such as LD_LIBRARY_PATH or via /etc/ld.so.conf (or /etc/ld.so.cache; see also ldconfig(8)) or in standard places such as /lib and /usr/lib.
When you specify the relative name ./fileName.so, it knows to look in the current directory, which is not normally a place it looks.
Note that you can run into some interesting issues on systems that support both 32-bit and 64-bit executables, with various conventions being used for the locations of the different classes of library. Other variants of Unix use vaguely related systems — mostly using dlopen() et al these days (historically, it was not always thus), and using a wide variety of environment variables (DYLD_LIBRARY_PATH, LIBPATH, SHLIB_PATH, LD_RUN_PATH, LD_LIBRARY_PATH_32, LD_LIBRARY_PATH_64, ...).

Answer 2

./ is a relative path to the .so file. It means that the file is in the current directory.
In *nix, by default, when given a file name without an absolute or relative path, dlopen will search for the library in a set list of default locations.

Answer 3

The "main point" is in using double-quotes in your second example:
dlopen("./fileName.so", RTLD_LAZY)
If you want to include your own library/filename enclose it in double-quotes. You don't even need the ./ for that, provided that the file is in the current directory, as ./ suggests.
As per the dlopen manpage's example:
handle = dlopen("libm.so", RTLD_LAZY);
if (!handle) {
fprintf(stderr, "%s\n", dlerror());
exit(EXIT_FAILURE);
}
the filename is enclosed in quotes.
Though, as specified by a previous answer, dlopen will look in the "standard" places for "includes". Another way of including a library that's inside your working directory (though, obviously, not a shared system library) is to use the preprocessor directive with the filename enclosed within double quotes:
#include <stdio.h>
#include <stdlib.h>
#include "myCustomLibrary.h"