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And, there you have it, the gcc version 4.2.1 installed and working correctly on the latest version of Mac OS X 10.8.4. In Apple’s version of GCC, both cc and gcc are actually symbolic links to the llvm-gcc compiler. Thankfully I found three ways I could catch up with my classmates during our lab classes: REPL.it (an online tool for writing in a plethora of languages and seeing the STDOUT result), the CSC (C-Sharp compiler) command line tool in Windows and Mono (compiled from the Terminal or with MonoDevelop which is a cross-platform IDE).

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You have finally made the move to become a programmer. You’ve registered for a course, you have your texts and manuals, and you’ve fired up your trusty Mac. This is exciting! You think you are all set, and then it hits: they want you to have a compiler. What the heck is that? We’ll explain this and help you to get a C compiler for Mac up and running on your computer. If you are relatively new to the Mac, you can develop your skills with a course on getting started with a Mac.

The compiler is the last step in turning your code into a program that runs on your computer. You learn the C language to write source code. Source code cannot be understood and run by a computer in this state. It has to be converted to code that the computer can run. This is the job of the compiler. You feed your source code in to the compiler and it will either give you an executable program or a long list of error codes telling you why it couldn’t make the program. Source code can be written on any platform. It is meant for humans and is the same on any operating system. The compiler, on the other hand, has to be specific for the operating system where the program will run.

Compilers usually produce code that will run faster than the alternative, interpreters. The executable program can be distributed without the source code, which makes it harder for anyone to steal the programming ideas that went into the program. A disadvantage of compilers is that the compiling step adds time to the development process because the whole program must be compiled each time a change is made.

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The most recommended way to get a C compiler for your Mac is to use Xcode. This uses gcc, the popular open source C compiler. The details vary for each version of OS X. We’ll go through the recent versions here. You will have to register as an apple developer to get access to these tools. In order to do these installs, you will be using Terminal to work at the command line. Get a solid foundation on the Mac command line with this course.

For all of the versions of OS X, you will be downloading Xcode. Xcode is an Integrated Development Environment, or IDE. An IDE allows you to write, compile, and debug a program from one central interface. Xcode can act as an IDE for C programming. All of the install methods involve first getting Xcode, then making the gcc compiler available outside of Xcode, and then installing a newer version of gcc.

For OS X 10.6 Snow Leopard, download Xcode 3 from the Apple Developer Site. This will give you a working version of gcc, but it is an older version. If you want or need a more up to date version, that is available at High Performance Computing for Mac OS X. You can install this after installing Xcode. The files must be unzipped and installed at the command line. After that, you will need to update your Shell resource file so that the newer versions are used. Details can be found at Installing the GNU compilers on Mac OS X.

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For OS X 10.7 Lion, you must get Xcode 4 from the Mac App Store. It is free, but you need to supply credit card information in order to have an App Store account. For Xcode 4.2, what you download from the App Store is an installer, which you then run. For Xcode 4.3, it is installed automatically, but it does not have gcc in the correct location. To finish the job, start Xcode and go to Preferences, Downloads, Components. Click on the Install button that is next to Command Line Tools. This gives you older versions of gcc. For the newest versions, you can use High Performance Computing for Mac OS X, as described for OS X 10.6. The process is similar and details can also be found at Installing the GNU compilers on Mac OS X.

OS X 10.8 will be very similar to 10.7. Install Xcode, then install the command line tools from the preferences. You can then get the newer versions of gcc as described for version 10.7.

OS X 10.9 Mavericks will use Xcode 5 and a revised process. Xcode 5 does not have the option to install the command line version of gcc. Instead, ensure that Xcode 5 has all available updates installed by checking from within the program. Then go to the Apple Developer Site and find the latest version of Command Line Tools (OS X Mavericks) for Xcode. It is a standard installer package. Finally, you can update the version of gcc in a manner similar to the other versions of OS X.

Other C compilers for Mac

Apple has extended the gcc compiler with a version called llvm. It incorporates more modern functioning and has a different licensing model needed by Apple for its proprietary software. Clang is an IDE for this compiler. It is designed to give more user-friendly error messages. Clang will give you the latest tools used by Apple for development. The downside is that there is no installer. It has to be built from source code, which means that you will need gcc already. Details are given at the llvm site.

Another option is given by Eclipse. Eclipse is a popular IDE for Java. The CDT plugin for Eclipse gives it the ability to compile C programs and become an IDE for C. Details can be found at the CDT page of the Eclipse site.

Now that you have a C compiler for your Mac, you can try a tutorial to write a simple program. Then get a solid start in C programming with this course for beginners. If you already know one language, extend your skills with a course for intermediate coders.

Now that you have the basic pieces in place, it is time to build your application. This section covers some of the more common issues that you may encounter in bringing your UNIX application to OS X. These issues apply largely without regard to what type of development you are doing.

Using GNU Autoconf, Automake, and Autoheader

If you are bringing a preexisting command-line utility to OS X that uses GNU autoconf, automake, or autoheader, you will probably find that it configures itself without modification (though the resulting configuration may be insufficient). Just run configure and make as you would on any other UNIX-based system.

If running the configure script fails because it doesn’t understand the architecture, try replacing the project’s config.sub and config.guess files with those available in /usr/share/automake-1.6. If you are distributing applications that use autoconf, you should include an up-to-date version of config.sub and config.guess so that OS X users don’t have to do anything extra to build your project.

If that still fails, you may need to run /usr/bin/autoconf on your project to rebuild the configure script before it works. OS X includes autoconf in the BSD tools package. Beyond these basics, if the project does not build, you may need to modify your makefile using some of the tips provided in the following sections. After you do that, more extensive refactoring may be required.

Some programs may use autoconf macros that are not supported by the version of autoconf that shipped with OS X. Because autoconf changes periodically, you may actually need to get a new version of autoconf if you need to build the very latest sources for some projects. In general, most projects include a prebuilt configure script with releases, so this is usually not necessary unless you are building an open source project using sources obtained from CVS or from a daily source snapshot.

However, if you find it necessary to upgrade autoconf, you can get a current version from http://www.gnu.org/software/autoconf/. Note that autoconf, by default, installs in /usr/local/, so you may need to modify your PATH environment variable to use the newly updated version. Do not attempt to replace the version installed in /usr/.

For additional information about using the GNU autotoolset, see http://autotoolset.sourceforge.net/tutorial.html and the manual pages autoconf, automake, and autoheader.

Compiling for Multiple CPU Architectures

Because the Macintosh platform includes more than one processor family, it is often important to compile software for multiple processor architectures. For example, libraries should generally be compiled as universal binaries even if you are exclusively targeting an Intel-based Macintosh computer, as your library may be used by a PowerPC binary running under Rosetta. For executables, if you plan to distribute compiled versions, you should generally create universal binaries for convenience.

When compiling programs for architectures other than your default host architecture, such as compiling for a ppc64 or Intel-based Macintosh target on a PowerPC-based build host, there are a few common problems that you may run into. Most of these problems result from one of the following mistakes:

• Assuming that the build host is architecturally similar to the target architecture and will thus be capable of executing intermediate build products

• Trying to determine target-processor-specific information at configuration time (by compiling and executing small code snippets) rather than at compile time (using macro tests) or execution time (for example, by using conditional byte swap functions)

Whenever cross-compiling occurs, extra care must be taken to ensure that the target architecture is detected correctly. This is particularly an issue when generating a binary containing object code for more than one architecture.

In many cases, binaries containing object code for more than one architecture can be generated simply by running the normal configuration script, then overriding the architecture flags at compile time.

For example, you might run

followed by

to generate a universal binary (for Intel-based and PowerPC-based Macintosh computers). To generate a 4-way universal binary that includes 64-bit versions, you would add -arch ppc64 and -arch x86_64 to the CFLAGS and LDFLAGS.

Note: If you are using an older version of gcc and your makefile passes LDFLAGS to gcc instead of passing them directly to ld, you may need to specify the linker flags as -Wl,-syslibroot,/Developer/SDKs/MacOSX10.4u.sdk. This tells the compiler to pass the unknown flags to the linker without interpreting them. Do not pass the LDFLAGS in this form to ld, however; ld does not currently support the -Wl syntax.

If you need to support an older version of gcc and your makefile passes LDFLAGS to both gcc and ld, you may need to modify it to pass this argument in different forms, depending on which tool is being used. Fortunately, these cases are rare; most makefiles either pass LDFLAGS to gcc or ld, but not both. Newer versions of gcc support -syslibroot directly.

If your makefile does not explicitly pass the contents of LDFLAGS to gcc or ld, they may still be passed to one or the other by a make rule. If you are using the standard built-in make rules, the contents of LDFLAGS are passed directly to ld. If in doubt, assume that it is passed to ld. If you get an invalid flag error, you guessed incorrectly.

If your makefile uses gcc to run the linker instead of invoking it directly, you must specify a list of target architectures to link when working with universal binary object (.o) files even if you are not using all of the architectures of the object file. If you don't, you will not create a universal binary, and you may also get a linker error. For more information about 64-bit executables, see 64-Bit Transition Guide.

However, applications that make configuration-time decisions about the size of data structures will generally fail to build correctly in such an environment (since those sizes may need to be different depending on whether the compiler is executing a ppc pass, a ppc64 pass, or an i386 pass). When this happens, the tool must be configured and compiled for each architecture as separate executables, then glued together manually using lipo.

In rare cases, software not written with cross-compilation in mind will make configure-time decisions by executing code on the build host. In these cases, you will have to manually alter either the configuration scripts or the resulting headers to be appropriate for the actual target architecture (rather than the build architecture). In some cases, this can be solved by telling the configure script that you are cross-compiling using the --host, --build, and --target flags. However, this may simply result in defaults for the target platform being inserted, which doesn’t really solve the problem.

The best fix is to replace configure-time detection of endianness, data type sizes, and so on with compile-time or run-time detection. For example, instead of testing the architecture for endianness to obtain consistent byte order in a file, you should do one of the following:

• Use C preprocessor macros like __BIG_ENDIAN__ and __LITTLE_ENDIAN__ to test endianness at compile time.

• Use functions like htonl, htons, ntohl, and ntohs to guarantee a big-endian representation on any architecture.

• Extract individual bytes by bitwise masking and shifting (for example, lowbyte=word & 0xff; nextbyte = (word >> 8) & 0xff; and so on).

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Similarly, instead of performing elaborate tests to determine whether to use int or long for a 4-byte piece of data, you should simply use a standard sized type such as uint32_t.

Note: Not all script execution is incompatible with cross-compiling. A number of open source tools (GTK, for example) use script execution to determine the presence or absence of libraries, determine their versions and locations, and so on.

In those cases, you must be certain that the info script associated with the universal binary installation (or the target platform installation if you are strictly cross-compiling) is the one that executes during the configuration process, rather than the info script associated with an installation specific to your host architecture.

There are a few other caveats when working with universal binaries:

• The library archive utility, ar, cannot work with libraries containing code for more than one architecture (or single-architecture libraries generated with lipo) after ranlib has added a table of contents to them. Thus, if you need to add additional object files to a library, you must keep a separate copy without a TOC.

• The -M switch to gcc (to output dependency information) is not supported when multiple architectures are specified on the command line. Depending on your makefile, this may require substantial changes to your makefile rules. For autoconf-based configure scripts, the flag --disable-dependency-tracking should solve this problem.

  For projects using automake, it may be necessary to run automake with the -i flag to disable dependency checks or put no-dependencies in the AUTOMAKE_OPTIONS variable in each Makefile.am file.

• If you run into problems building a universal binary for an open source tool, the first thing you should do is to get the latest version of the source code. This does two things:

  • Ensures that the version of autoconf and automake used to generate the configuration scripts is reasonably current, reducing the likelihood of build failures, execution failures, backwards or forwards compatibility problems, and other idiosyncratic or downright broken behavior.

  • Reduces the likelihood of building a version of an open source tool that contains known security holes or other serious bugs.

• Older versions of autoconf do not handle the case where --target, --host, and --build are not handled gracefully. Different versions also behave differently when you specify only one or two of these flags. Thus, you should always specify all three of these options if you are running an autoconf-generated configure script with intent to cross-compile.

• Some earlier versions of autoconf handle cross-compiling poorly. If your tool contains a configure script generated by an early autoconf, you may be able to significantly improve things by replacing some of the config.* files (and config.guess in particular) with updated copies from the version of autoconf that comes with OS X.

  This will not always work, however, in which case it may be necessary to actually regenerate the configure script by running autoconf. To do this, simply change into the root directory of the project and run /usr/bin/autoconf. It will automatically detect and use the configure.in file and use it to generate a new configure script. If you get warnings, you should first try a web search for the error message, as someone else may have already run into the problem (possibly on a different tool) and found a solution.

  If you get errors about missing AC_ macros, you may need to download a copy of libraries on which your tool depends and copy their .m4 autoconf configuration files into /usr/share/autoconf. Alternately, you can add the macros to the file acinclude.m4 in your project’s main directory and autoconf should automatically pick up those macros.

  You may, in some cases, need to rerun automake and/or autoheader if your tool uses them. Be prepared to run into missing AM_ and AH_ macros if you do, however. Because of the added risk of missing macros, this should generally only be done if running autoconf by itself does not correct a build problem.

  Important: Be sure to make a backup copy of the original scripts, headers, and other generated files (or, ideally, the entire project directory) before running autoheader or automake.

• Different makefiles and configure scripts handle command-line overrides in different ways. The most consistent way to force these overrides is to specify them prior to the command. For example:

  should generally result in the above being added to CFLAGS during compilation. However, this behavior is not completely consistent across makefiles from different projects.

For additional information about autoconf, automake, and autoheader, you can view the autoconf documentation at http://www.gnu.org/software/autoconf/manual/index.html.

For additional information on compiler flags for Intel-based Macintosh computers, modifying code to support little-endian CPUs, and other porting concerns, you should read Universal Binary Programming Guidelines, Second Edition, available from the ADC Reference Library.

Cross-Compiling a Self-Bootstrapping Tool

Probably the most difficult situation you may experience is that of a self-bootstrapping tool—a tool that uses a (possibly stripped-down) copy of itself to either compile the final version of itself or to construct support files or libraries. Some examples include TeX, Perl, and gcc.

Ideally, you should be able to build the executable as a universal binary in a single build pass. If that is possible, everything “just works”, since the universal binary can execute on the host. However, this is not always possible. If you have to cross-compile and glue the pieces together with lipo, this obviously will not work.

If the build system is written well, the tool will bootstrap itself by building a version compiled for the host, then use that to build the pieces for the target, and finally compile a version of the binary for the target. In that case, you should not have to do anything special for the build to succeed.

In some cases, however, it is not possible to simultaneously compile for multiple architectures and the build system wasn’t designed for cross-compiling. In those cases, the recommended solution is to pre-install a version of the tool for the host architecture, then modify the build scripts to rename the target’s intermediate copy of the tool and copy the host’s copy in place of that intermediate build product (for example, mv miniperl miniperl-target; cp /usr/bin/perl miniperl).

By doing this, later parts of the build script will execute the version of the tool built for the host architecture. Assuming there are no architecture dependencies in the dependent tools or support files, they should build correctly using the host’s copy of the tool. Once the dependent build is complete, you should swap back in the original target copy in the final build phase. The trick is in figuring out when to have each copy in place.

Conditional Compilation on OS X

You will sometimes find it necessary to use conditional compilation to make your code behave differently depending on whether certain functionality is available.

Older code sometimes used conditional statements like #ifdef __MACH__ or #ifdef __APPLE__ to try to determine whether it was being compiled on OS X or not. While this seems appealing as a quick way of getting ported, it ultimately causes more work in the long run. For example, if you make the assumption that a particular function does not exist in OS X and conditionally replace it with your own version that implements the same functionality as a wrapper around a different API, your application may no longer compile or may be less efficient if Apple adds that function in a later version.

Apart from displaying or using the name of the OS for some reason (which you can more portably obtain from the uname API), code should never behave differently on OS X merely because it is running on OS X. Code should behave differently because OS X behaves differently in some way—offering an additional feature, not offering functionality specific to another operating system, and so on. Thus, for maximum portability and maintainability, you should focus on that difference and make the conditional compilation dependent upon detecting the difference rather than dependent upon the OS itself. This not only makes it easier to maintain your code as OS X evolves, but also makes it easier to port your code to other platforms that may support different but overlapping feature sets.

The most common reasons you might want to use such conditional statements are attempts to detect differences in:

• processor architecture

• byte order

• file system case sensitivity

• other file system properties

• compiler, linker, or toolchain differences

• availability of application frameworks

• availability of header files

• support for a function or feature

Instead it is better to figure out why your code needs to behave differently in OS X, then use conditional compilation techniques that are appropriate for the actual root cause.

The misuse of these conditionals often causes problems. For example, if you assume that certain frameworks are present if those macros are defined, you might get compile failures when building a 64-bit executable. If you instead test for the availability of the framework, you might be able to fall back on an alternative mechanism such as X11, or you might skip building the graphical portions of the application entirely.

For example, OS X provides preprocessor macros to determine the CPU architecture and byte order. These include:

• __i386__—Intel (32-bit)

• __x86_64__—Intel (64-bit)

• __ppc__—PowerPC (32-bit)

• __ppc64__—PowerPC (64-bit)

• __BIG_ENDIAN__—Big endian CPU

• __LITTLE_ENDIAN__—Little endian CPU

• __LP64__—The LP64 (64-bit) data model

In addition, using tools like autoconf, you can create arbitrary conditional compilation on nearly any practical feature of the installation, from testing to see if a file exists to seeing if you can successfully compile a piece of code.

For example, if a portion of your project requires a particular application framework, you can compile a small test program whose main function calls a function in that framework. If the test program compiles and links successfully, the application framework is present for the specified CPU architecture.

You can even use this technique to determine whether to include workarounds for known bugs in Apple or third-party libraries and frameworks, either by testing the versions of those frameworks or by providing a test case that reproduces the bug and checking the results.

For example, in OS X, poll does not support device files such as /dev/tty. If you just avoid poll if your code is running on OS X, you are making two assumptions that you should not make:

• You are assuming that what you are doing will always be unsupported. OS X is an evolving operating system that adds new features on a regular basis, so this is not necessarily a valid assumption.

• You are assuming that OS X is the only platform that does not support using poll on device files. While this is probably true for most device files, not all device files support poll in all operating systems, so this is also not necessarily a valid assumption.

A better solution is to use a configuration-time test that tries to use poll on a device file, and if the call fails, disables the use of poll. If using poll provides some significant advantage, it may be better to perform a runtime test early in your application execution, then use poll only if that test succeeds. By testing for support at runtime, your application can use the poll API if is supported by a particular version of any operating system, falling back gracefully if it is not supported.

A good rule is to always test for the most specific thing that is guaranteed to meet your requirements. If you need a framework, test for the framework. If you need a library, test for the library. If you need a particular compiler version, test the compiler version. And so on. By doing this, you increase your chances that your application will continue to work correctly without modification in the future.

Choosing a Compiler

OS X ships two compilers and their corresponding toolchains. The default compiler is based on GCC 4.2. In addition, a compiler based on GCC 4.0 is provided. Older versions of Xcode also provide prior versions. Compiling for 64-bit PowerPC and Intel-based Macintosh computers is only supported in version 4.0 and later. Compiling 64-bit kernel extensions is only supported in version 4.2 and later.

Always try to compile your software using GCC 4 because future toolchains will be based on GCC version 4 or later. However, because GCC 4 is a relatively new toolchain, you may find bugs that prevent compiling certain programs.

Use of the legacy GCC 2.95.2-based toolchain is strongly discouraged unless you have to maintain compatibility with OS X version 10.1.

If you run into a problem that looks like a compiler bug, try using a different version of GCC. You can run a different version by setting the CC environment variable in your Makefile. For example, CC=gcc-4.0 chooses GCC 4.0. In Xcode, you can change the compiler setting on a per-project basis or a per-file basis by selecting a different compiler version in the appropriate build settings inspector.

Setting Compiler Flags

When building your projects in OS X, simply supplying or modifying the compiler flags of a few key options is all you need to do to port most programs. These are usually specified by either the CFLAGS or LDFLAGS variable in your makefile, depending on which part of the compiler chain interprets the flags. Unless otherwise specified, you should add these flags to CFLAGS if needed.

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Note: The 64-bit toolchain in OS X v10.4 and later has additional compiler flags (and a few deprecated flags). These are described in more detail in 64-Bit Transition Guide.

Some common flags include:

Changes from a two-level to a single-level (flat) namespace. By default, OS X builds libraries and applications with a two-level namespace where references to dynamic libraries are resolved to a definition in a specific dynamic library when the image is built. Use of this flag is generally discouraged, but in some cases, is unavoidable. For more information, see Understanding Two-Level Namespaces.

Produces a Mach-O bundle format file, which is used for creating loadable plug-ins. See the ld man page for more discussion of this flag.

Specifies which executable will load a plug-in. Undefined symbols in that bundle are checked against the specified executable as if it were another dynamic library, thus ensuring that the bundle will actually be loadable without missing symbols.

Links the executable being built against the listed framework. For example, you might add -framework vecLib to include support for vector math.

Specifies the version of OS X you are targeting. You must target your compile for the oldest version of OS X on which you want to run the executable. In addition, you should install and use the cross-development SDK for that version of OS X. For more information, see SDK Compatibility Guide.

Note: OS X also uses this value to determine the UNIX conformance behavior of some APIs. For more information, read Unix 03 Conformance Release Notes.

Note: OS X uses a single-pass linker. Make sure that you put your framework and library options after the object(.o) files. To get more information about the Apple linker read the manual page for ld.

More extensive discussion for the compiler in general can be found at http://developer.apple.com/releasenotes/DeveloperTools/.

Understanding Two-Level Namespaces

By default, OS X builds libraries and applications with a two-level namespace. In a two-level namespace environment, when you compile a new dynamic library, any references that the library might make to other dynamic libraries are resolved to a definition in those specific dynamic libraries.

The two-level namespace design has many advantages for Carbon applications. However, it can cause problems for many traditional UNIX applications if they were designed to work in a flat namespace environment.

For example, suppose one library, call it libfoo, uses another library, libbar, for its implementation of the function barIt. Now suppose an application wants to override the use of libbar with a compressed version, called libzbar. Since libfoo was linked against libbar at compile time, this is not possible without recompiling libfoo.

To allow the application to override references made by libfoo to libbar, you would use the flag -flat_namespace. The ld man page has a more detailed discussion of this flag.

If you are writing libraries from scratch, it may be worth considering the two-level namespace issue in your design. If you expect that someone may want to override your library’s use of another library, you might have an initializer routine that takes pointers to the second library as its arguments, and then use those pointers for the calls instead of calling the second library directly.

Alternately, you might use a plug-in architecture in which the calls to the outside library are made from a plug-in that could be easily replaced with a different plug-in for a different outside library. See Dynamic Libraries and Plug-ins for more information.

For the most part, however, unless you are designing a library from scratch, it is not practical to avoid using -flat_namespace if you need to override a library’s references to another library.

If you are compiling an executable (as opposed to a library), you can also use -force_flat_namespace to tell dyld to use a flat namespace when loading any dynamic libraries and bundles loaded by the binary. This is usually not necessary, however.

Executable Format

The only executable format that the OS X kernel understands is the Mach-O format. Some bridging tools are provided for classic Macintosh executable formats, but Mach-O is the native format. It is very different from the commonly used Executable and Linking Format (ELF). For more information on Mach-O, see OS X ABI Mach-O File Format Reference.

Dynamic Libraries and Plug-ins

Dynamic libraries and plug-ins behave differently in OS X than in other operating systems. This section explains some of those differences.

Using Dynamic Libraries at Link Time

When linking an executable, OS X treats dynamic libraries just like libraries in any other UNIX-based or UNIX-like operating system. If you have a library called libmytool.a, libmytool.dylib, or libmytool.so, for example, all you have to do is this:

As a general rule, you should avoid creating static libraries (.a) except as a temporary side product of building an application. You must run ranlib on any archive file before you attempt to link against it.

Using Dynamic Libraries Programmatically

OS X makes heavy use of dynamically linked code. Unlike other binary formats such as ELF and xcoff, Mach-O treats plug-ins differently than it treats shared libraries.

The preferred mechanism for dynamic loading of shared code, beginning in OS X v10.4 and later, is the dlopen family of functions. These are described in the man page for dlopen. The ld and dyld man pages give more specific details of the dynamic linker’s implementation.

Note: By default, the names of dynamic libraries in OS X end in .dylib instead of .so. You should be aware of this when writing code to load shared code in OS X.

Libraries that you are familiar with from other UNIX-based systems might not be in the same location in OS X. This is because OS X has a single dynamically loadable framework, libSystem, that contains much of the core system functionality. This single module provides the standard C runtime environment, input/output routines, math libraries, and most of the normal functionality required by command-line applications and network services.

The libSystem library also includes functions that you would normally expect to find in libc and libm, RPC services, and a name resolver. Because libSystem is automatically linked into your application, you do not need to explicitly add it to the compiler’s link line. For your convenience, many of these libraries exist as symbolic links to libSystem, so while explicitly linking against -lm (for example) is not needed, it will not cause an error.

To learn more about how to use dynamic libraries, see Dynamic Library Programming Topics.

Compiling Dynamic Libraries and Plugins

For the most part, you can treat dynamic libraries and plugins the same way as on any other platform if you use GNU libtool. This tool is installed in OS X as glibtool to avoid a name conflict with NeXT libtool. For more information, see the manual page for glibtool.

You can also create dynamic libraries and plugins manually if desired. As mentioned in Using Dynamic Libraries Programmatically, dynamic libraries and plugins are not the same thing in OS X. Thus, you must pass different flags when you create them.

To create dynamic libraries in OS X, pass the -dynamiclib flag.

To create plugins, pass the -bundle flag.

Because plugins can be tailored to a particular application, the OS X compiler provides the ability to check these plugins for loadability at compile time. To take advantage of this feature, use the -bundle_loader flag. For example:

If the compiler finds symbol requests in the plugin that cannot be resolved in the application, you will get a link error. This means that you must use the -l flag to link against any libraries that the plugin requires as though you were building a complete executable.

Important: OS X does not support the concept of weak linking as it is found in systems like Linux. If you override one symbol, you must override all of the symbols in that object file.

To learn more about how to create and use dynamic libraries, see Dynamic Library Programming Topics.

Bundles

In the OS X file system, some directories store executable code and the software resources related to that code in one discrete package. These packages, known as bundles, come in two varieties: application bundles and frameworks.

There are two basic types of bundles that you should be familiar with during the basic porting process: application bundles and frameworks. In particular, you should be aware of how to use frameworks, since you may need to link against the contents of a framework when porting your application.

Application Bundles

Application bundles are special directories that appear in the Finder as a single entity. Having only one item allows a user to double-click it to get the application with all of its supporting resources. If you are building Mac apps, you should make application bundles. Xcode builds them by default if you select one of the application project types. More information on application bundles is available in Bundles vs. Installers and in Mac Technology Overview.

Frameworks

A framework is a type of bundle that packages a shared library with the resources that the library requires. Depending on the library, this bundle could include header files, images, and reference documentation. If you are trying to maintain cross-platform compatibility, you may not want to create your own frameworks, but you should be aware of them because you might need to link against them. For example, you might want to link against the Core Foundation framework. Since a framework is just one form of a bundle, you can do this by linking against /System/Library/Frameworks/CoreFoundation.framework with the -frameworkflag. A more thorough discussion of frameworks is in Mac Technology Overview.

For More Information

You can find additional information about bundles in Mac Technology Overview.

Handling Multiply Defined Symbols

A multiply defined symbol error occurs if there are multiple definitions for any symbol in the object files that you are linking together. You can specify the following flags to modify the handling of multiply defined symbols under certain circumstances:

Specifies how multiply defined symbols in dynamic libraries should be treated when -twolevel_namespace is in effect. The values for treatment must be one of:

• error—Treat multiply defined symbols as an error.

• warning—Treat multiply defined symbols as a warning.

• suppress—Ignore multiply defined symbols.

The default behavior is to treat multiply defined symbols in dynamic libraries as warnings when -twolevel_namespace is in effect.

Specifies how unused multiply defined symbols in dynamic libraries should be treated when -twolevel_namespace is in effect. An unused multiply defined symbol is a symbol defined in the output that is also defined in one of the dynamic libraries, but in which but the symbol in the dynamic library is not used by any reference in the output. The values for treatment must be error, warning, or suppress. The default for unused multiply defined symbols is to suppress these messages.

Predefined Macros

The following macros are predefined in OS X:

This macro is defined when your code is being compiled by the Objective-C compiler. By default, this occurs when compiling a .m file or any header included by a .m file. You can force the Objective-C compiler to be used for a .c or .h file by passing the -ObjC or -ObjC++ flags.

This macro is defined when your code is being compiled by the C++ compiler (either explicitly or by passing the -ObjC++ flag).

This macro is defined when compiling .s files.

This macro is defined on systems that use natural alignment. When using natural alignment, an int is aligned on sizeof(int) boundary, a short int is aligned on sizeof(short) boundary, and so on. It is defined by default when you're compiling code for PowerPC architecutres. It is not defined when you use the -malign-mac68k compiler switch, nor is it defined on Intel architectures.

This macro is defined if Mach system calls are supported.

This macro is defined in any Apple computer.

This macro is set to an integer that represents the version number ofthe compiler. This lets you distinguish, for example, between compilersbased on the same version of GCC, but with different bug fixes orfeatures. Larger values denote later compilers.

These macros tell whether the current architecture uses little endian or big endian byte ordering. For more information, see Compiling for Multiple CPU Architectures.

Note: To define a section of code to be compiled on OS X system, you should define a section using __APPLE__ with __MACH__ macros. The macro __UNIX__ is not defined in OS X.

Other Porting Tips

This section describes alternatives to certain commonly used APIs.

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Headers

The following headers commonly found in UNIX, BSD, or Linux operating systems are either unavailable or are unsupported in OS X:

This file does not exist in OS X, but the functionality does exist. You should include stdlib.h instead. Alternatively, you can define the prototypes yourself as follows:

The ftw function traverses through the directory hierarchy and calls a function to get information about each file. However, there isn't a function similar to ftw in fts.h.

One alternative is to use fts_open, fts_children, and fts_close to implement such a file traversal. To do this, use the fts_open function to get a handle to the file hierarchy, use fts_read to get information on each file, and use fts_children to get a link to a list of structures containing information about files in a directory.

Alternatively, you can use opendir, readdir and closedir with recursion to achieve the same result.

For example, in order to get a description of each file located in /usr/include using fts.h, then the code would be as follows:

See the manual page for fts to understand the structures and macros used in the code. The sample code above shows a very simplistic file traversal. For instance, it does not consider possible subdirectories existing in the directory being searched.

Not suported, use unistd.h instead.

Not supported, use unistd.h instead.

Not supported, use stdlib.h instead.

This header is supported in Linux for memory mapping, but is not supported in Max OS X. In OS X, you can use mmap to map files into memory. If you wish to map devices, use the I/O Kit framework instead.

Kernel modules should be loaded using the KextManager API in the I/O Kit framework. For more information, see KextManagerLoadKextWithURL and related functions. The modules themselves must be compiled against the Kernel framework. For more information, see IOKit Fundamentals.

Use the CFBundleCopyLocalizedString API in Core Foundation for similar localization functionality.

Although pseudo-TTYs are supported in OS X, this header is not. The implementation of pseudo-TTYs is very different from Linux. For more information, see the pty manual page.

This header file is not present in OS X. For file streaming, use iostream.h.

Not supported.

OS X does not support this header file. You can use the header file swap.h to implement swap functionality. The swap.h header file contains many functions that can be used for swap tuning.

This header file is obsolete, and has been replaced by termios.h, which is part of the POSIX standard. These two header files are very similar. However, the termios.h does not include the same header files as termio.h. Thus, you should be sure to look directly at the termios.h header to make sure it includes everything your application needs.

Deprecated, use utmpx.h instead.

Not supported, use limits.h instead.

Although this functionality is available, you should generally use the CFStringRef API in Core Foundation instead.

Functions

The following functions commonly seen in other Unix, Linux, or BSD operating systems are not supported or are discouraged in OS X.

Although OS X supports the wchar API, the preferred way to work with international strings is the CFStringRef API, which is part of the Core Foundation framework. Some of the APIs available in Core Foundation are CFStringGetSystemEncoding, CFStringCreateCopy, CFStringCreateMutable, and so on. See the Core Foundation documentation for a complete list of supported APIs.

nl_types.h is not supported, thus, these functions are not supported. These functions gives access to localized strings. There is no direct equivalent in OS X. In general, you should use Core Foundation to access localized resources (CFBundleCopyLocalizedString, for example).

The crypt function performs password encryption, based on the NBS Data Encryption Standard (DES). Additional code has been added to deter key search attempts.crypt. OS X's version of the function crypt behaves very similarly to the Linux version, except that it encrypts a 64-bit constant rather than a 128-bit constant. This function is located in the unistd.h header file rather than lcrypt.h.

Note: The linker flag -lcrypt is not supported in OS X.

This function is not supported in OS X. Its calculation for leap year is based on:

You can either use this code to implement this functionality, or you can use any of the existing APIs in time.h to do something similar.

Discouraged in OS X. Use sprintf, snprintf, and similar functions instead.

This function is an extension to fclose. Although OS X supports fclose, fcloseall is not supported. You can use fclose to implement fcloseall by storing the file pointers in an array and iterating through the array.

In general, volumes in OS X are not in /etc/fstab. However, to the extent that they are, you can get similar functionality from getfsent and related functions.

This API is partially supported in OS X. It does not support polling devices.

The brk and sbrk functions are historical curiosities left over from earlier days before the advent of virtual memory management. Although they are present on the system, they are not recommended.

This API is supported but is not recommended. shmget has a limited memory blocks allocation. When several applications use shmget, this limit may change and cause problems for the other applications. In general, you should either use mmap for mapping files into memory or use the POSIX shm_open function and related functions for creating non-file-backed shared memory.

These functions are not supported in OS X.

Utilities

The chapter Designing Scripts for Cross-Platform Deployment in Shell Scripting Primer describes a number of cross-platform compatibility issues that you may run into with command-line utilities that are commonly scripted.

This section lists several commands that are primarily of interest to people porting compiled applications and drivers, rather than general-purpose scripting.

The ldd command is not available in OS X. However, you can use the command otool -L to get the same functionality that ldd provides. The otool command displays specified parts of object files or libraries. The option -L displays the name and version numbers of the shared libraries that an object file uses. To see all the existing options, see the manual page for otool.

lsmod is not available on OS X, but other commands exist that offer similar functionality. The

Loads, diagnoses problems with, and generates symbols for kernel extensions.

Prints statistics about currently loaded drivers and other kernel extensions.

Loads the kernel module for a device driver or other kernel extensions. This command is a basic command intended for use in scripts. For developer purposes, use kextutil instead.

Unloads the kernel module for a device driver or other kernel extensions. This command is a basic command intended for use in scripts. For developer purposes, use kextutil instead.

For more information about loading kernel modules, see Kernel Extension Programming Topics.

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