Dynamic Linking and Loading with glibc

This document describes how to create and deploy dynamically linked and loaded applications with the glibc library in the Native Client SDK. Before reading this document, we recommend reading Building Native Client Modules

C standard libraries: glibc and newlib

The Native Client SDK comes with two C standard libraries — glibc and newlib. These libraries are described in the table below.

For proprietary (closed-source) applications, your options are to either statically link to newlib, or dynamically link to glibc. We recommend dynamically linking to glibc, for a couple of reasons:

• The glibc library is widely distributed (it’s included in Linux distributions), and as such it’s mature, hardened, and feature-rich. Your code is more likely to compile out-of-the-box with glibc.
• Dynamic loading can provide a big performance benefit for your application if you can structure the application to defer loading of code that’s not needed for initial interaction with the user. It takes some work to put such code in shared libraries and to load the libraries at runtime, but the payoff is usually worth it. In future releases, Chrome may also support caching of common dynamically linked libraries such as libc.so between applications. This could significantly reduce download size and provide a further potential performance benefit (for example, the hello_world example would only require downloading a .nexe file that’s on the order of 30KB, rather than a .nexe file and several libraries, which are on the order of 1.5MB).

Native Client support for dynamic linking and loading is based on glibc. Thus, if your Native Client application must dynamically link and load code (e.g., due to licensing considerations), we recommend that you use the glibc library.

SDK toolchains

The Native Client SDK contains multiple toolchains, which are differentiated by target architecture and C library:

In the directories listed above, <platform> is the platform of your development machine (i.e., win, mac, or linux). For example, in the Windows SDK, the x86 toolchain that uses glibc is in toolchain/win_x86_glibc.

To use the glibc library and dynamic linking in your application, you must use a glibc toolchain. Note that you must build all code in your application with one toolchain. Code from multiple toolchains cannot be mixed.

Specifying and delivering shared libraries

One significant difference between newlib and glibc applications is that glibc applications must explicitly list and deploy the shared libraries that they use.

In a desktop environment, when the user launches a dynamically linked application, the operating system’s program loader determines the set of libraries the application requires by reading explicit inter-module dependencies from executable file headers, and loads the required libraries into the address space of the application process. Typically the required libraries will have been installed on the system as a part of the application’s installation process. Often the desktop application developer doesn’t know or think about the libraries that are required by an application, as those details are taken care of by the user’s operating system.

In the Native Client sandbox, dynamic linking can’t rely in the same way on the operating system or the local file system. Instead, the application developer must identify the set of libraries that are required by an application, list those libraries in a Native Client manifest file, and deploy the libraries along with the application. Instructions for how to build a dynamically linked Native Client application, generate a Native Client manifest (.nmf) file, and deploy an application are provided below.

Building a dynamically linked application

Applications built with the glibc toolchain will by dynamically linked by default. Application that load shared libraries at runtime using dlopen() must link with the libdl library (-ldl).

Like other gcc-based toolchains building a dynamic library for NaCl is normally done by linking with the -shared flag and compiling with the -fPIC flag. The SDK build system will do this automatically when the SO_RULE Makefile rule is used.

The Native Client SDK includes an example that demonstrates how to build a shared library, and how to use the dlopen() interface to load that library at runtime (after the application is already running). Many applications load and link shared libraries at launch rather than at runtime, and hence do not use the dlopen() interface. The SDK example is nevertheless instructive, as it demonstrates how to build Native Client modules (.nexe files) and shared libraries (.so files) with the x86 glibc toolchain, and how to generate a Native Client manifest file for glibc applications.

The SDK example, located in examples/tutorial/dlopen, includes three C++ files:

eightball.cc

This file implements the function Magic8Ball(), which is used to provide whimsical answers to user questions. This file is compiled into a shared library called libeightball.so. This library gets included in the .nmf file and is therefore directly loadable with dlopen().

reverse.cc

This file implements the function Reverse(), which returns reversed copies of strings that are passed to it. This file is compiled into a shared library called libreverse.so. This library is not included in the .nmf file and is loaded via an http mount using the nacl_io library.

dlopen.cc

This file implements the Native Client module, which loads the two shared libraries and handles communcation with with JavaScript. The file is compiled into a Native Client executable (.nexe).

Run make in the dlopen directory to see the commands the Makefile executes to build x86 32-bit and 64-bit .nexe and .so files, and to generate a .nmf file. These commands are described below.

Generating a Native Client manifest file for a dynamically linked application

The Native Client manifest file specifies the name of the executable to run and must also specify any shared libraries that the application directly depends on. For indirect dependencies (such as libraries opened via dlopen()) it is also convenient to list libraries in the manifest file. However it is possile to load arbitrary shared libraries at runtime that are not mentioned in the manifest by using the nacl_io library to mount a filesystem that contains the shared libraries which will then allow dlopen() to access them.

In this example we demonstrate both loading directly from via the manifest file (libeightball.so) and loading indirectly via a http mount (libreverse.so).

Take a look at the manifest file in the dlopen example to see how a glibc-style manifest file is structured. (Run make in the dlopen directory to generate the manifest file if you haven’t done so already.) Here is an excerpt from dlopen.nmf:

{
  "files": {
    "libeightball.so": {
      "x86-64": {
        "url": "lib64/libeightball.so"
      },
      "x86-32": {
        "url": "lib32/libeightball.so"
      }
    },
    "libstdc++.so.6": {
      "x86-64": {
        "url": "lib64/libstdc++.so.6"
      },
      "x86-32": {
        "url": "lib32/libstdc++.so.6"
      }
    },
    "libppapi_cpp.so": {
      "x86-64": {
        "url": "lib64/libppapi_cpp.so"
      },
      "x86-32": {
        "url": "lib32/libppapi_cpp.so"
      }
    },
... etc.

In most cases, you can use the create_nmf.py script in the SDK to generate a manifest file for your application. The script is located in the tools directory (e.g. pepper_28/tools).

The Makefile in the dlopen example generates the manifest automatically using the NMF_RULE provided by the SDK build system. Running make V=1 will show the full command line which is used to generate the nmf:

create_nmf.py -o dlopen.nmf glibc/Release/dlopen_x86_32.nexe \
   glibc/Release/dlopen_x86_64.nexe glibc/Release/libeightball_x86_32.so \
   glibc/Release/libeightball_x86_64.so  -s ./glibc/Release \
   -n libeightball_x86_32.so,libeightball.so \
   -n libeightball_x86_64.so,libeightball.so

Run python create_nmf.py --help to see a full description of the command-line flags. A few of the important flags are described below.

-s directory

use directory to stage libraries (libraries are added to lib32 and lib64 subfolders)

-L directory

add directory to the library search path. The default search path already includes the toolchain and SDK libraries directories.

As an alternative to using create_nmf, it is possible to manually calculate the list of shared library dependencies using tools such as objdump_.

Deploying a dynamically linked application

As described above, an application’s manifest file must explicitly list all the executable code modules that the application directly depends on, including modules from the application itself (.nexe and .so files), modules from the Native Client SDK (e.g., libppapi_cpp.so), and perhaps also modules from webports or from middleware systems that the application uses. You must provide all of those modules as part of the application deployment process.

As explained in Distributing Your Application, there are two basic ways to deploy a Chrome app:

• hosted application: all modules are hosted together on a web server of your choice
• packaged application: all modules are packaged into one file, hosted in the Chrome Web Store, and downloaded to the user’s machine

The web store documentation contains a handy guide to help you choose which to use.

You must deploy all the modules listed in your application’s manifest file for either the hosted application or the packaged application case. For hosted applications, you must upload the modules to your web server. For packaged applications, you must include the modules in the application’s Chrome Web Store .crx file. Modules should use URLs/names that are consistent with those in the Native Client manifest file, and be named relative to the location of the manifest file. Remember that some of the libraries named in the manifest file may be located in directories you specified with the -L option to create_nmf.py. You are free to rename/rearrange files and directories referenced by the Native Client manifest file, so long as the modules are available in the locations indicated by the manifest file. If you move or rename modules, it may be easier to re-run create_nmf.py to generate a new manifest file rather than edit the original manifest file. For hosted applications, you can check for name mismatches during testing by watching the request log of the web server hosting your test deployment.

Opening a shared library at runtime

Native Client supports a version of the POSIX standard dlopen() interface for opening libraries explicitly, after an application is already running. Calling dlopen() may cause a library download to occur, and automatically loads all libraries that are required by the named library.

The best practice for opening libraries with dlopen() is to use a worker thread to pre-load libraries asynchronously during initialization of your application, so that the libraries are available when they’re needed. You can call dlopen() a second time when you need to use a library – per the specification, subsequent calls to dlopen() return a handle to the previously loaded library. Note that you should only call dlclose() to close a library when you no longer need the library; otherwise, subsequent calls to dlopen() could cause the library to be fetched again.

The dlopen example in the SDK demonstrates how to open a shared libraries at runtime. To reiterate, the example includes three C++ files:

• eightball.cc: this is the shared library that implements the function Magic8Ball() (this file is compiled into libeightball.so)
• reverse.cc: this is the shared library that implements the function Reverse() (this file is compiled into libreverse.so)
• dlopen.cc: this is the Native Client module that loads the shared libraries and makes calls to Magic8Ball() and Reverse() in response to requests from JavaScript.

When the Native Client module starts, it kicks off a worker thread that calls dlopen() to load the two shared libraries. Once the module has a handle to the library, it fetches the addresses of the Magic8Ball() and Reverse() functions using dlsym(). When a user types in a query and clicks the ‘ASK!’ button, the module calls Magic8Ball() to generate an answer, and returns the result to the user. Likewise when the user clicks the ‘Reverse’ button it calls the Reverse() function to reverse the string.

Troubleshooting

If your .nexe isn’t loading, the best place to look for information that can help you troubleshoot the JavaScript console and standard output from Chrome. See Debugging for more information.

Here are a few common error messages and explanations of what they mean:

/main.nexe: error while loading shared libraries: /main.nexe: failed to allocate code and data space for executable

The .nexe may not have been compiled correctly (e.g., the .nexe may be statically linked). Try cleaning and recompiling with the glibc toolchain.

/main.nexe: error while loading shared libraries: libpthread.so.xxxx: cannot open shared object file: Permission denied

(xxxx is a version number, for example, 5055067a.) This error can result from having the wrong path in the .nmf file. Double-check that the path in the .nmf file is correct.

/main.nexe: error while loading shared libraries: /main.nexe: cannot open shared object file: No such file or directory

If there are no obvious problems with your main.nexe entry in the .nmf file, check where main.nexe is being requested from. Use Chrome’s Developer Tools: Click the menu icon , select Tools > Developer Tools, click the Network tab, and look at the path in the Name column.

NaCl module load failed: ELF executable text/rodata segment has wrong starting address

This error happens when using a newlib-style .nmf file instead of a glibc-style .nmf file. Make sure you build your application with the glic toolchain, and use the create_nmf.py script to generate your .nmf file.

NativeClient: NaCl module load failed: Nexe crashed during startup

This error message indicates that a module crashed while being loaded. You can determine which module crashed by looking at the Network tab in Chrome’s Developer Tools (see above). The module that crashed will be the last one that was loaded.

/lib/main.nexe: error while loading shared libraries: /lib/main.nexe: only ET_DYN and ET_EXEC can be loaded

This error message indicates that there is an error with the .so files listed in the .nmf file – either the files are the wrong type or kind, or an expected library is missing.

undefined reference to ‘dlopen’ collect2: ld returned 1 exit status

This is a linker ordering problem that usually results from improper ordering of command line flags when linking. Reconfigure your command line string to list libraries after the -o flag.