Linux find linked libs

Where do executables look for shared objects at runtime?

I understand how to define include shared objects at linking/compile time. However, I still wonder how do executables look for the shared object ( *.so libraries) at execution time.

For instance, my app a.out calls functions defined in the lib.so library. After compiling, I move lib.so to a new directory in my $HOME .

How can I tell a.out to go look for it there?

4 Answers 4

The shared library HOWTO explains most of the mechanisms involved, and the dynamic loader manual goes into more detail. Each unix variant has its own way, but most use the same executable format (ELF) and have similar dynamic linkers¹ (derived from Solaris). Below I’ll summarize the common behavior with a focus on Linux; check your system’s manuals for the complete story.

(Terminology note: the part of the system that loads shared libraries is often called “dynamic linker”, but sometimes “dynamic loader” to be more precise. “Dynamic linker” can also mean the tool that generates instructions for the dynamic loader when compiling a program, or the combination of the compile-time tool and the run-time loader. In this answer, “linker” refers to the run-time part.)

In a nutshell, when it’s looking for a dynamic library ( .so file) the linker tries:

• directories listed in the LD_LIBRARY_PATH environment variable ( DYLD_LIBRARY_PATH on OSX);
• directories listed in the executable’s rpath;
• directories on the system search path, which (on Linux at least) consists of the entries in /etc/ld.so.conf plus /lib and /usr/lib .

The rpath is stored in the executable (it’s the DT_RPATH or DT_RUNPATH dynamic attribute). It can contain absolute paths or paths starting with $ORIGIN to indicate a path relative to the location of the executable (e.g. if the executable is in /opt/myapp/bin and its rpath is $ORIGIN/../lib:$ORIGIN/../plugins then the dynamic linker will look in /opt/myapp/lib and /opt/myapp/plugins ). The rpath is normally determined when the executable is compiled, with the -rpath option to ld , but you can change it afterwards with chrpath .

In the scenario you describe, if you’re the developer or packager of the application and intend for it to be installed in a …/bin , …/lib structure, then link with -rpath=’$ORIGIN/../lib’ . If you’re installing a pre-built binary on your system, either put the library in a directory on the search path ( /usr/local/lib if you’re the system administrator, otherwise a directory that you add to $LD_LIBRARY_PATH ), or try chrpath .

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How to find out the dynamic libraries executables loads when run?

I want to find out the list of dynamic libraries a binary loads when run (With their full paths). I am using CentOS 6.0. How to do this?

8 Answers 8

You can do this with ldd command:

readelf -d $executable | grep ‘NEEDED’

Can be used if you can’t run the executable, e.g. if it was cross compiled, or if you don’t trust it:

In the usual case, ldd invokes the standard dynamic linker (see ld.so(8)) with the LD_TRACE_LOADED_OBJECTS environment variable set to 1, which causes the linker to display the library dependencies. Be aware, however, that in some circumstances, some versions of ldd may attempt to obtain the dependency information by directly executing the program. Thus, you should never employ ldd on an untrusted executable, since this may result in the execution of arbitrary code.

Note that libraries can depend on other libraries, so now you need to find the dependencies.

A naive approach that often works is:

but the more precise method is to understand the ldd search path / cache. I think ldconfig is the way to go.

Choose one, and repeat:

/maps for running processes

Mentioned by Basile, this is useful to find all the libraries currently being used by running executables. E.g.:

shows all currently loaded dynamic dependencies of init (PID 1 ):

This method also shows libraries opened with dlopen , tested with this minimal setup hacked up with a sleep(1000) on Ubuntu 18.04.

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Static, Shared Dynamic and Loadable Linux Libraries

This tutorial discusses the philosophy behind libraries and the creation and use of C/C++ library «shared components» and «plug-ins». The various technologies and methodologies used and insight to their appropriate application, is also discussed. In this tutorial, all libraries are created using the GNU Linux compiler.

Related YoLinux Tutorials:

Libraries employ a software design also known as «shared components» or «archive libraries», which groups together multiple compiled object code files into a single file known as a library. Typically C functions/C++ classes and methods which can be shared by more than one application are broken out of the application’s source code, compiled and bundled into a library. The C standard libraries and C++ STL are examples of shared components which can be linked with your code. The benefit is that each and every object file need not be stated when linking because the developer can reference the library collective. This simplifies the multiple use and sharing of software components between applications. It also allows application vendors a way to simply release an API to interface with an application. Components which are large can be created for dynamic use, thus the library can remain separate from the executable reducing it’s size and thus less disk space is used for the application. The library components are then called by various applications for use when needed.

Benefits include:

• Component reuse: update one library, shared resource takes up less disk space.
• Version management: Linux libraries can cohabitate old and new versions on a single system.
• Component Specialization: niche and specialized developers can focus on their core competency on a single library. Simplifies testing and verification.

There are two Linux C/C++ library types which can be created:

1. Static libraries (.a): Library of object code which is linked with, and becomes part of the application.
2. Dynamically linked shared object libraries (.so): There is only one form of this library but it can be used in two ways.
   1. Dynamically linked at run time. The libraries must be available during compile/link phase. The shared objects are not included into the executable component but are tied to the execution.
   2. Dynamically loaded/unloaded and linked during execution (i.e. browser plug-in) using the dynamic linking loader system functions.

Library naming conventions:

Consider the following compile and link command: gcc src-file.c -lm -lpthread
The libraries referenced in this example for inclusion during linking are the math library («m») and the thread library («pthread»). They are found in /usr/lib/libm.a and /usr/lib/libpthread.a.

Note: The GNU compiler now has the command line option «-pthread» while older versions of the compiler specify the pthread library explicitly with «-lpthread». Thus now you are more likely to see gcc src-file.c -lm -pthread

How to generate a static library (object code archive file):

• Compile: cc -Wall -c ctest1.c ctest2.c
  Compiler options:
  • -Wall: include warnings. See man page for warnings specified.
• Create library «libctest.a»: ar -cvq libctest.a ctest1.o ctest2.o
• List files in library: ar -t libctest.a
• Linking with the library:
  • cc -o executable-name prog.c libctest.a
  • cc -o executable-name prog.c -L/path/to/library-directory -lctest
• Example files:
  • ctest1.c
  • ctest2.c
  • prog.c

Historical note: After creating the library it was once necessary to run the command: ranlib ctest.a. This created a symbol table within the archive. Ranlib is now embedded into the «ar» command.

Note for MS/Windows developers: The Linux/Unix «.a» library is conceptually the same as the Visual C++ static «.lib» libraries.

How to generate a shared object: (Dynamically linked object library file.) Note that this is a two step process.

1. Create object code
2. Create library
3. Optional: create default version using a symbolic link.

Library creation example: This creates the library libctest.so.1.0 and symbolic links to it.

It is also valid to cascade the linkage: If you look at the libraries in /lib/ and /usr/lib/ you will find both methodologies present. Linux developers are not consistent. What is important is that the symbolic links eventually point to an actual library.

• -Wall: include warnings. See man page for warnings specified.
• -fPIC: Compiler directive to output position independent code, a characteristic required by shared libraries. Also see «-fpic».
• -shared: Produce a shared object which can then be linked with other objects to form an executable.
• -Wl,options: Pass options to linker.
  In this example the options to be passed on to the linker are: «-soname libctest.so.1«. The name passed with the «-o» option is passed to gcc.
• Option -o: Output of operation. In this case the name of the shared object to be output will be «libctest.so.1.0«

• The link to /opt/lib/libctest.so allows the naming convention for the compile flag -lctest to work.
• The link to /opt/lib/libctest.so.1 allows the run time binding to work. See dependency below.

Compile main program and link with shared object library:

Compiling for run-time linking with a dynamically linked libctest.so.1.0: Use: Where the name of the library is libctest.so. (This is why you must create the symbolic links or you will get the error «/usr/bin/ld: cannot find -lctest».)
The libraries will NOT be included in the executable but will be dynamically linked during run-time execution.

The shared library dependencies of the executable can be listed with the command: ldd name-of-executable

Example: ldd prog [Potential Pitfall] : Unresolved errors within a shared library may cause an error when the library is loaded. Example:

Error message at run-time:

The first three libraries show that there is a path resolution. The last two are problematic.

The fix is to resolve dependencies of the last two libraries when linking the library libname-of-lib.so:

• Add the unresolved library path in /etc/ld.so.conf.d/name-of-lib-x86_64.conf and/or /etc/ld.so.conf.d/name-of-lib-i686.conf
  Reload the library cache (/etc/ld.so.cache) with the command: sudo ldconfig
  or
• Add library and path explicitly to the compiler/linker command: -lname-of-lib -L/path/to/lib
  or
• Add the library path to the environment variable to fix run-time dependency:
  export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/path/to/lib

• Set path: export LD_LIBRARY_PATH=/opt/lib:$LD_LIBRARY_PATH
• Run: prog

Example with code:

Using the example code above for ctest1.c, ctest2.c and prog.c

1. Compile the library functions: gcc -Wall -fPIC -c ctest1.c ctest2.c
2. Generate the shared library: gcc -shared -Wl,-soname,libctest.so.1 -o libctest.so.1.0 ctest1.o ctest2.o
   This generates the library libctest.so.1.0
3. Move to lib/ directory:
   • sudo mv libctest.so.1.0 /opt/lib
   • sudo ln -sf /opt/lib/libctest.so.1.0 /opt/lib/libctest.so.1
   • sudo ln -sf /opt/lib/libctest.so.1 /opt/lib/libctest.so

   Compile program for use with a shared library: gcc -Wall -L/opt/lib prog.c -lctest -o prog
   [Potential Pitfall] : If the symbolic links are not created (above), you will get the following error: The reference to the library name -lctest refers to /opt/lib/libctest.so

4. Configure the library path (see below and choose one of three mechanisms).
   In this example we set the environment variable: export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/opt/lib
5. Run the program: ./prog
   [Potential Pitfall] : You get the following error if the library path is not set:

• gcc — GNU C compiler
• ld — The GNU Linker
• ldd — List library dependencies
• ldconfig — configure dynamic linker run time bindings (update cache /etc/ld.so.cache)

In order for an executable to find the required libraries to link with during run time, one must configure the system so that the libraries can be found. Methods available: (Do at least one of the following)

Add library directories to be included during dynamic linking to the file /etc/ld.so.conf

Add the library path to this file and then execute the command (as root) ldconfig to configure the linker run-time bindings.
You can use the «-f file-name» flag to reference another configuration file if you are developing for different environments.
See man page for command ldconfig.

Add specified directory to library cache: (as root)
ldconfig -n /opt/lib
Where /opt/lib is the directory containing your library libctest.so
(When developing and just adding your current directory: ldconfig -n . Link with -L.)

This will NOT permanently configure the system to include this directory. The information will be lost upon system reboot.

Specify the environment variable LD_LIBRARY_PATH to point to the directory paths containing the shared object library. This will specify to the run time loader that the library paths will be used during execution to resolve dependencies.
(Linux/Solaris: LD_LIBRARY_PATH, SGI: LD_LIBRARYN32_PATH, AIX: LIBPATH, Mac OS X: DYLD_LIBRARY_PATH, HP-UX: SHLIB_PATH)

Example (bash shell): export LD_LIBRARY_PATH=/opt/lib:$LD_LIBRARY_PATH or add to your

This instructs the run time loader to look in the path described by the environment variable LD_LIBRARY_PATH, to resolve shared libraries. This will include the path /opt/lib.

Library paths used should conform to the «Linux Standard Base» directory structure.

ar: list object files in archive library

This will list all of the object files held in the archive library: Also see: Man page for ar

nm: list symbols: object files, archive library and shared library

The command «nm» lists symbols contained in object files:

The command «nm» lists symbols contained in the archive library:

Object symbols in static archive libraries are categorized using the source and object file hierarchy of the library:

The command «nm» lists symbols contained in the object file or shared library.

Use the command nm -D libctest.so.1.0
(or nm --dynamic libctest.so.1.0)

Note that other platforms (Cygwin) may not respond to «-D». Try nm -gC libctest.so.1.0

Also see: Man page for nm

Symbol Type	Description
A	The symbol’s value is absolute, and will not be changed by further linking.
B	Un-initialized data section
D	Initialized data section
T	Normal code section
U	Undefined symbol used but not defined. Dependency on another library.
W	Doubly defined symbol. If found, allow definition in another library to resolve dependency.

Also see: objdump man page

readelf: list symbols in shared library

The command «readelf» command to list symbols contained in a shared library:

Use the command readelf -s /usr/lib64/libjpeg.so

Also see: readelf man page

Library versions should be specified for shared objects if the function interfaces are expected to change (C++ public/protected class definitions), more or fewer functions are included in the library, the function prototype changes (return data type (int, const int, . ) or argument list changes) or data type changes (object definitions: class data members, inheritance, virtual functions, . ).

The library version can be specified when the shared object library is created. If the library is expected to be updated, then a library version should be specified. This is especially important for shared object libraries which are dynamically linked. This also avoids the Microsoft «DLL hell» problem of conflicting libraries where a system upgrade which changes a standard library breaks an older application expecting an older version of the the shared object function.

Versioning occurs with the GNU C/C++ libraries as well. This often make binaries compiled with one version of the GNU tools incompatible with binaries compiled with other versions unless those versions also reside on the system. Multiple versions of the same library can reside on the same system due to versioning. The version of the library is included in the symbol name so the linker knows which version to link with.

One can look at the symbol version used: nm csub1.o

No version is specified in object code by default.

There is one GNU C/C++ compiler flag that explicitly deals with symbol versioning. Specify the version script to use at compile time with the flag: --version-script=your-version-script-file
Note: This is only useful when creating shared libraries. It is assumed that the programmer knows which libraries to link with when static linking. Run-time linking allows opportunity for library incompatibility.

GNU/Linux, see examples of version scripts here: sysdeps/unix/sysv/linux/Versions

Some symbols may also get version strings from assembler code which appears in glibc headers files. Look at include/libc-symbols.h.

Example: nm /lib/libc.so.6 | more

Note the use of a version script.

Library referencing a versioned library: nm /lib/libutil-2.2.5.so

These libraries are dynamically loaded / unloaded and linked during execution. Useful for creating a «plug-in» architecture.

Prototype include file for the library: ctest.h

Load and unload the library libctest.so (created above), dynamically:

gcc -rdynamic -o progdl progdl.c -ldl

• dlopen("/opt/lib/libctest.so", RTLD_LAZY);
  Open shared library named «libctest.so«.
  The second argument indicates the binding. See include file dlfcn.h.
  Returns NULL if it fails.
  Options:
  • RTLD_LAZY: If specified, Linux is not concerned about unresolved symbols until they are referenced.
  • RTLD_NOW: All unresolved symbols resolved when dlopen() is called.
  • RTLD_GLOBAL: Make symbol libraries visible.
• dlsym(lib_handle, "ctest1");
  Returns address to the function which has been loaded with the shared library..
  Returns NULL if it fails.
  Note: When using C++ functions, first use nm to find the «mangled» symbol name or use the extern "C" construct to avoid name mangling.
  i.e. extern "C" void function-name();

Object code location: Object code archive libraries can be located with either the executable or the loadable library. Object code routines used by both should not be duplicated in each. This is especially true for code which use static variables such as singleton classes. A static variable is global and thus can only be represented once. Including it twice will provide unexpected results. The programmer can specify that specific object code be linked with the executable by using linker commands which are passed on by the compiler.

Use the «-Wl» gcc/g++ compiler flag to pass command line arguments on to the GNU «ld» linker.

Example makefile statement: g++ -rdynamic -o appexe $(OBJ) $(LINKFLAGS) -Wl,--whole-archive -L -laa -Wl,--no-whole-archive $(LIBS)

• —whole-archive: This linker directive specifies that the libraries listed following this directive (in this case AA_libs) shall be included in the resulting output even though there may not be any calls requiring its presence. This option is used to specify libraries which the loadable libraries will require at run time.
• -no-whole-archive: This needs to be specified whether you list additional object files or not. The gcc/g++ compiler will add its own list of archive libraries and you would not want all the object code in the archive library linked in if not needed. It toggles the behavior back to normal for the rest of the archive libraries.

• dlopen() — gain access to an executable object file
• dclose() — close a dlopen object
• dlsym() — obtain the address of a symbol from a dlopen object
• dlvsym() — Programming interface to dynamic linking loader.
• dlerror() — get diagnostic information

C++ and name mangling:

When running the above «C» examples with the «C++» compiler one will quickly find that «C++» function names get mangled and thus will not work unless the function definitions are protected with extern "C"<>.

Note that the following are not equivalent:

The following are equivalent:

Dynamic loading of C++ classes:

The dynamic library loading routines enable the programmer to load «C» functions. In C++ we would like to load class member functions. In fact the entire class may be in the library and we may want to load and have access to the entire object and all of its member functions. Do this by passing a «C» class factory function which instantiates the class.

The class «.h» file:

The class «.cpp» file:

Main executable which calls the loadable libraries:

Pitfalls:

• The new/delete of the C++ class should both be provided by the executable or the library but not split. This is so that there is no surprise if one overloads new/delete in one or the other.

The Microsoft Windows equivalent to the Linux / Unix shared object («.so») is the «.dll». The Microsoft Windows DLL file usually has the extension «.dll», but may also use the extension «.ocx». On the old 16 bit windows, the dynamically linked libraries were also named with the «.exe» suffix. «Executing» the DLL will load it into memory.

The Visual C++ .NET IDE wizard will create a DLL framework through the GUI, and generates a «.def» file. This «module definition file» lists the functions to be exported. When exporting C++ functions, the C++ mangled names are used. Using the Visual C++ compiler to generate a «.map» file will allow you to discover the C++ mangled name to use in the «.def» file. The «SECTIONS» label in the «.def» file will define the portions which are «shared». Unfortunately the generation of DLLs are tightly coupled to the Microsoft IDE, so much so that I would not recommend trying to create one without it.

The Microsoft Windows C++ equivalent functions to libdl are the following functions:

• ::LoadLibrary() — dlopen()
• ::GetProcAddress() — dlsym()
• ::FreeLibrary() — dlclose()

[Potential Pitfall] : Microsoft Visual C++ .NET compilers do not allow the linking control that the GNU linker «ld» allows (i.e. —whole-archive, -no-whole-archive). All symbols need to be resolved by the VC++ compiler for both the loadable library and the application executable individually and thus it can cause duplication of libraries when the library is loaded. This is especially bad when using static variables (i.e. used in singleton patterns) as you will get two memory locations for the static variable, one used by the loadable library and the other used by the program executable. This breaks the whole static variable concept and the singleton pattern. Thus you can not use a static variable which is referenced by by both the loadable library and the application executable as they will be unique and different. To use a unique static variable, you must pass a pointer to that static variable to the other module so that each module (main executable and DLL library) can use the same instantiation. On MS/Windows you can use shared memory or a memory mapped file so that the main executable and DLL library can share a pointer to an address they both will use.

Cross platform (Linux and MS/Windows) C++ code snippet:

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