Conventions

For shared libraries to support all of these desired properties, a number of conventions and guidelines must be followed. You need to understand the difference between a library's names, in particular its ``soname'' and ``real name'' (and how they interact). You also need to understand where they should be placed in the filesystem.

How Libraries are Used

On GNU glibc-based systems, including all Linux systems, starting up an ELF binary executable automatically causes the program loader to be loaded and run. On Linux systems, this loader is named /lib/ld-linux.so.X (where X is a version number). This loader, in turn, finds and loads all other shared libraries used by the program.

The list of directories to be searched is stored in the file /etc/ld.so.conf. Many Red Hat-derived distributions don't normally include /usr/local/lib in the file /etc/ld.so.conf. I consider this a bug, and adding /usr/local/lib to /etc/ld.so.conf is a common ``fix'' required to run many programs on Red Hat-derived systems.

If you want to just override a few functions in a library, but keep the rest of the library, you can enter the names of overriding libraries (.o files) in /etc/ld.so.preload; these ``preloading'' libraries will take precedence over the standard set. This preloading file is typically used for emergency patches; a distribution usually won't include such a file when delivered.

Searching all of these directories at program start-up would be grossly inefficient, so a caching arrangement is actually used. The program ldconfig(8) by default reads in the file /etc/ld.so.conf, sets up the appropriate symbolic links in the dynamic link directories (so they'll follow the standard conventions), and then writes a cache to /etc/ld.so.cache that's then used by other programs. This greatly speeds up access to libraries. The implication is that ldconfig must be run whenever a DLL is added, when a DLL is removed, or when the set of DLL directories changes; running ldconfig is often one of the steps performed by package managers when installing a library. On start-up, then, the dynamic loader actually uses the file /etc/ld.so.cache and then loads the libraries it needs.

Environment Variables

Various environment variables can control this process, and in fact there are environment variables that permit you to override this process. For example, you can temporarily substitute a different library for this particular execution. In Linux, the environment variable LD_LIBRARY_PATH is a colon-separated set of directories where libraries should be searched for first, before the standard set of directories; this is useful when debugging a new library or using a nonstandard library for special purposes. The environment variable LD_PRELOAD lists object files with functions that override the standard set, just as /etc/ld.so.preload does. These are implemented by the loader /lib/ld-linux.so.

There are actually a number of other environment variables that control the loading process; their names begin with LD_ or RTLD_. Most of the others are for low-level debugging of the loader process or for implementing specialized capabilities. Most of them aren't well-documented; if you need to know about them, the best way to learn about them is to read the source code.

Permitting user control over dynamically linked libraries would be disastrous for setuid/setgid programs if special measures weren't taken. Therefore, in GNU loader, if the program is setuid or setgid these variables (and other similar variables) are ignored or greatly limited in what they can do. The loader determines if a program is setuid or setgid by checking the program's credentials; if the uid and euid differ, or the gid and the egid differ, the loader presumes the program is setuid/setgid (or descended from one) and therefore greatly limits its abilities to control linking. If you read the GNU glibc library source code, you can see this; see especially the files elf/rtld.c and sysdeps/generic/dl-sysdep.c. This means that if you cause the uid and gid to equal the euid and egid, and then call a program, these variables will have full effect. Other Unix-like systems handle the situation differently but for the same reason: a setuid/setgid program should not be unduly affected by the environment variables set.

Creating a Shared Library

Creating a shared library is easy. First, create the object files that will go into the shared library using the gcc -fPIC flag (this enables the ``position indendependent code'' generation, a requirement for shared libraries). Then create the shared library using this format:

gcc -shared -Wl,-soname,your_soname \
    -o library_name file_list library_list

Here's an example, which creates two object files (a.o and b.o) and then creates a shared library that contains both of them. Note that this compilation includes debugging information (-g) and will generate warnings (-Wall), which aren't required for shared libraries but are recommended. The compilation generates object files (using -c), and obviously includes the required -fPIC option:

gcc -fPIC -g -c -Wall a.c
gcc -fPIC -g -c -Wall b.c
gcc -shared -Wl,-soname,libmystuff.so.1 \
    -o libmystuff.so.1.0.1 a.o b.o -lc

Here are a few points worth noting:

• Don't strip the resulting library, and don't use the compiler option -fomit-frame-pointer unless you really have to. The resulting library will work, but these actions make debuggers mostly useless.

• Use -fPIC to generate code, not -fpic (the latter may not work, because if branches need large displacements -fpic may not generate fully position-independent code).

Installing and Using a Shared Library

Once you've created a shared library, you'll want to install it. The simple approach is simply to copy the library into one of the standard directories (e.g., /usr/lib) and run ldconfig(8).

If you can't do that (e.g., you don't have the right to modify /usr/lib), then you can use environment variables to control things. First, you'll need to create the shared libraries somewhere. Then, you'll need to set up the necesary symbolic links, in particular a link from a soname to the real name (as well as from a versionless soname, that is, a soname that ends in ``.so'' for users who don't specify a version at all). The simplest approach is to run:

 ldconfig -n directory_with_shared_libraries

Then you can set LD_LIBRARY_PATH, which is a colon-separated list of directories in which to search for shared libraries before the usual places. If you're using bash, you could invoke my_program this way using:

LD_LIBRARY_PATH=.:$LD_LIBRARY_PATH  my_program

If you want to override just a few selected functions, you can do this by creating an overriding object file and setting LD_PRELOAD; the functions in this object file will override just those functions.

Usually you can update libraries without concern; if there was an API change, the library creator will change the soname. However, if a program breaks on an update to a library that kept the same soname, you can force it to use the older library version by copying the old library back somewhere, renaming the program (say to the old name plus ``.orig''), and then create a small ``wrapper'' script that resets the library to use and calls the real (renamed) program. You could place the old library in its own special area, if you like, though the numbering conventions do permit multiple versions to live in the same directory. The wrapper script could look something like this:

#!/bin/sh export LD_LIBRARY_PATH=/usr/local/my_lib:$LD_LIBRARY_PATH exec /usr/bin/my_program.orig $*

You can see the list of the shared libraries used by a program using ldd(1). So, for example, you can see the shared libraries used by ls by typing:

ldd /bin/ls

Generally you'll see a list of the sonames being depended on, along with the directory that those names resolve to. In practically all cases you'll have at least two dependencies:

• /lib/ld-linux.so.N (where N is 1 or more, usually at least 2). This is the library that loads all other libraries.

• libc.so.N (where N is 6 or more). This is the C library. Even other languages tend to use the C library (at least to implement their own libraries), so most programs at least include this one.

Incompatible Libraries

When a new version of a library is binary-incompatible with the old one the soname needs to change. There are four basic reasons that a library would cease to be binary compatible:

1. The behavior of a function changes so that it no longer meets its original specification,

2. Exported data items change (exception: adding optional items to the ends of structures is okay, as long as those structures are only allocated within the library).

3. An exported function is removed.

4. The interface of an exported function changes.

If you can avoid these reasons, you can keep your libraries binary-compatible. Said another way, you can keep your Application Binary Interface (ABI) compatible if you avoid such changes. For example, you might want to add new functions but not delete the old ones. You can add items to structures but only if you can make sure that old programs won't be sensitive to such changes by adding items only to the end of the structure, only allowing the library (and not the application) to allocate the structure, making the extra items optional (or having the library fill them in), and so on. Watch out - you probably can't expand structures if users are using them in arrays.

dlopen()

The dlopen(3) function opens a library and prepares it for use. In C its prototype is:

void * dlopen(const char *filename, int flag);

If filename begins with ``/'' (i.e., it's an absolute path), dlopen() won't search for a library. Otherwise, dlopen() will search for the library in the following order:

1. A colon-separated list of directories in the user's LD_LIBRARY path environment variable.

2. The list of libraries specified in /etc/ld.so.cache.

3. /usr/lib, followed by /lib.

If the libraries depend on each other (e.g., X depends on Y), then you need to load the dependees first (in this example, load Y first, and then X).

The return value of dlopen() is a ``handle'' that should be considered an opaque value to be used by the other DL library routines. dlopen() will return NULL if the attempt to load does not succeed, and you need to check for this. If the same library is loaded more than once with dlopen(), the same file handle is returned.

If the library exports a routine named _init, then that code is executed before dlopen() returns. You can use this fact in your own libraries to implement initialization routines.

dlerror()

Errors can be reported by calling dlerror(), which returns a string describing the error from the last call to dlopen(), dlsym(), or dlclose(). One oddity is that after calling dlerror(), future calls to dlerror() will return NULL until another error has been encountered.

dlsym()

There's no point in loading a DL library if you can't use it. The main routine for using a DL library is dlsym(3), which looks up the value of a symbol in a given (opened) library. This function is defined as:

void * dlsym(void *handle, char *symbol);

the handle is the value returned from dlopen, and symbol is a NIL-terminated string. If you can avoid it, don't store the result of dlsym() into a void* pointer, because then you'll have to cast it each time you use it (and you'll give less information to other people trying to maintain the program).

dlsym() will return a NULL result if the symbol wasn't found. If you know that the symbol could never have the value of NULL or zero, that may be fine, but there's a potential ambiguity otherwise: if you got a NULL, does that mean there is no such symbol, or that NULL is the value of the symbol? The standard solution is to call dlerror() first (to clear any error condition that may have existed), then call dlsym() to request a symbol, then call dlerror() again to see if an error occurred. A code snippet would look like this:

dlerror(); /* clear error code */ s = (actual_type) dlsym(handle, symbol_being_searched_for); if ((err = dlerror()) != NULL) { /* handle error, the symbol wasn't found */ } else { /* symbol found, its value is in s */ }

dlclose()

The converse of dlopen() is dlclose(), which closes a DL library. The dl library maintains link counts for dynamic file handles, so a dynamic library is not actually deallocated until dlclose has been called on it as many times as dlopen has succeeded on it. Thus, it's not a problem for the same program to load the same library multiple times.

DL Library Example

Here's an example from the man page of dlopen(3). This example loads the math library and prints the cosine of 2.0, and it checks for errors at every step (recommended):

#include <stdio.h> #include <dlfcn.h> int main(int argc, char **argv) { void *handle; double (*cosine)(double); char *error; handle = dlopen ("/lib/libm.so", RTLD_LAZY); if (!handle) { fputs (dlerror(), stderr); exit(1); } cosine = dlsym(handle, "cos"); if ((error = dlerror()) != NULL) { fputs(error, stderr); exit(1); } printf ("%f\n", (*cosine)(2.0)); dlclose(handle); }

If this program were in a file named "foo.c", you would build the program with the following command:

gcc -Wl,export-dynamic -o foo foo.c -ldl

The ``-Wl,export-dynamic'' option isn't actually required, but you may sometimes find it useful. It is defined in ld(1): ``When creating an ELF file, [this option adds] all symbols to the dynamic symbol table. Normally, the dynamic symbol table contains only symbols which are used by a dynamic object. This option is needed for some uses of dlopen.'' Note that you could say ``-rdynamic'' instead of ``-Wl,export-dynamic'' if you only work with Linux systems, but according to the ELF documentation the ``-rdynamic'' flag doesn't always work for gcc on non-Linux systems.

nm command

The nm(1) command can report the list of symbols in a given library. It works on both static and shared libraries. For a given library nm(1) can list the symbol names defined, each symbol's value, and the symbol's type. It can also identify where the symbol was defined in the source code (by filename and line number), if that information is available in the library (see the -l option).

The symbol type requires a little more explanation. The type is displayed as a letter; lowercase means that the symbol is local, while uppercase means that the symbol is global (external). Typical symbol types include T (a normal definition in the code section), D (initialized data section), B (uninitialized data section), U (undefined; the symbol is used by the library but not defined by the library), and W (weak; if another library also defines this symbol, that definition overrides this one).

If you know the name of a function, but you truly can't remember what library it was defined in, you can use nm's ``-o'' option (which prefixes the filename in each line) along with grep to find the library name. From a Bourne shell, you can search all the libraries in /lib, /usr/lib, direct subdirectories of /usr/lib, and /usr/local/lib for ``cos'' as follows:

nm -o /lib/* /usr/lib/* /usr/lib/*/* \ /usr/local/lib/* 2> /dev/null | grep 'cos$'

Much more information about nm can be found in the nm ``info'' documentation locally installed at info:binutils#nm.

Shared Libraries Can Be Scripts

It's worth noting that the GNU loader permits shared libraries to be text files using a specialized scripting language instead of the usual library format. This is useful for indirectly combining other libraries. For example, here's the listing of /usr/lib/libc.so on one of my systems:

/* GNU ld script Use the shared library, but some functions are only in the static library, so try that secondarily. */ GROUP ( /lib/libc.so.6 /usr/lib/libc_nonshared.a )

For more information about this, see the texinfo documentation on ld linker scripts (ld command language). General information is at info:ld#Options and info:ld#Commands, with likely commands discussed in info:ld#Option Commands.

GNU libtool

If you're building an application that should port to many systems, you might consider using GNU libtool to build and install libraries. GNU libtool is a generic library support script. Libtool hides the complexity of using shared libraries behind a consistent, portable interface. Libtool provides portable interfaces to create object files, link libraries (static and shared), link executables, debug executables, install libraries, install executables. It also includes libltdl, a portability wrapper for dynamically loading programs. For more information, see its documentation at http://www.gnu.org/software/libtool/manual.html

File libhello.c

/* libhello.c - demonstrate library use. */ #include <stdio.h> void hello(void) { printf("Hello, library world.\n"); }

File libhello.h

/* libhello.h - demonstrate library use. */ void hello(void);

File demo_use.c

/* demo_use.c -- demonstrate direct use of the "hello" routine */ #include "libhello.h" int main(void) { hello(); return 0; }

File script_static

#!/bin/sh # Static library demo # Create static library's object file, libhello-static.o. # I'm using the name libhello-static to clearly # differentiate the static library from the # dynamic library examples, but you don't need to use # "-static" in the names of your # object files or static libraries. gcc -Wall -g -c -o libhello-static.o libhello.c # Create static library. ar rcs libhello-static.a libhello-static.o # At this point we could just copy libhello-static.a # somewhere else to use it. # For demo purposes, we'll just keep the library # in the current directory. # Compile demo_use program file. gcc -Wall -g -c demo_use.c -o demo_use.o # Create demo_use program; -L. causes "." to be searched during # creation of the program. Note that this command causes # the relevant object file in libhello-static.a to be # incorporated into file demo_use_static. gcc -g -o demo_use_static demo_use.o -L. -lhello-static # Execute the program. ./demo_use_static

File script_shared

#!/bin/sh # Shared library demo # Create shared library's object file, libhello.o. gcc -fPIC -Wall -g -c libhello.c # Create shared library. # Use -lc to link it against C library, since libhello # depends on the C library. gcc -g -shared -Wl,-soname,libhello.so.0 \ -o libhello.so.0.0 libhello.o -lc # At this point we could just copy libhello.so.0.0 into # some directory, say /usr/local/lib. # Now we need to call ldconfig to fix up the symbolic links. # Set up the soname. We could just execute: # ln -sf libhello.so.0.0 libhello.so.0 # but let's let ldconfig figure it out. /sbin/ldconfig -n . # Set up the linker name. # In a more sophisticated setting, we'd need to make # sure that if there was an existing linker name, # and if so, check if it should stay or not. ln -sf libhello.so.0 libhello.so # Compile demo_use program file. gcc -Wall -g -c demo_use.c -o demo_use.o # Create program demo_use. # The -L. causes "." to be searched during creation # of the program; note that this does NOT mean that "." # will be searched when the program is executed. gcc -g -o demo_use demo_use.o -L. -lhello # Execute the program. Note that we need to tell the program # where the shared library is, using LD_LIBRARY_PATH. LD_LIBRARY_PATH="." ./demo_use

File demo_dynamic.c

/* demo_dynamic.c -- demonstrate dynamic loading and use of the "hello" routine */ /* Need dlfcn.h for the routines to dynamically load libraries */ #include <dlfcn.h> #include <stdio.h> /* Note that we don't have to include "libhello.h". However, we do need to specify something related; we need to specify a type that will hold the value we're going to get from dlsym(). */ /* The type "simple_demo_function" describes a function that takes no arguments, and returns no value: */ typedef void (*simple_demo_function)(void); int main(void) { const char *error; void *module; simple_demo_function demo_function; /* Load dynamically loaded library */ module = dlopen("libhello.so", RTLD_LAZY); if (!module) { fprintf(stderr, "Couldn't open libhello.so: %s\n", dlerror()); exit(1); } /* Get symbol */ dlerror(); demo_function = dlsym(module, "hello"); if ((error = dlerror())) { fprintf(stderr, "Couldn't find hello: %s\n", error); exit(1); } /* Now call the function in the DL library */ (*demo_function)(); /* All done, close things cleanly */ dlclose(module); return 0; }

File script_dynamic

#!/bin/sh # Dynamically loaded library demo # Presume that libhello.so and friends have # been created (see dynamic example). # Compile demo_dynamic program file into an object file. gcc -Wall -g -c demo_dynamic.c # Create program demo_use. # Note that we don't have to tell it where to search for DL libraries, # since the only special library this program uses won't be # loaded until after the program starts up. # However, we DO need the option -ldl to include the library # that loads the DL libraries. gcc -g -o demo_dynamic demo_dynamic.o -ldl # Execute the program. Note that we need to tell the # program where get the dynamically loaded library, # using LD_LIBRARY_PATH. LD_LIBRARY_PATH="." ./demo_dynamic