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15.3.6 Cross compiling

Most build systems support the ability to compile libraries and applications on one platform for use on a different platform, provided a compiler capable of generating the appropriate output is available. In such cross compiling scenarios, the platform on which the libraries or applications are compiled is called the build platform, while the platform on which the libraries or applications are intended to be used or executed is called the host platform. The GNU Build System, of which libtool is a part, supports cross compiling via arguments passed to the configure script: --build=... and --host=.... However, when the build platform and host platform are very different, libtool is required to make certain accommodations to support these scenarios.

In most cases, because the build platform and host platform differ, the cross-compiled libraries and executables can't be executed or tested on the build platform where they were compiled. The testsuites of most build systems will often skip any tests that involve executing such foreign executables when cross-compiling. However, if the build platform and host platform are sufficiently similar, it is often possible to run cross-compiled applications. Libtool's own testsuite often attempts to execute cross-compiled tests, but will mark any failures as skipped since the failure might simply be due to the differences between the two platforms.

In addition to cases where the host platform and build platform are extremely similar (e.g. ‘i586-pc-linux-gnu’ and ‘i686-pc-linux-gnu’), there is another case in which cross-compiled host applications may be executed on the build platform. This is possible when the build platform supports an emulation or API-enhanced environment for the host platform. One example of this situation would be if the build platform were MinGW, and the host platform were Cygwin (or vice versa). Both of these platforms can actually operate within a single Windows instance, so Cygwin applications can be launched from a MinGW context, and vice versa—provided certain care is taken. Another example would be if the build platform were GNU/Linux on an x86 32bit processor, and the host platform were MinGW. In this situation, the Wine environment can be used to launch Windows applications from the GNU/Linux operating system; again, provided certain care is taken.

One particular issue occurs when a Windows platform such as MinGW, Cygwin, or MSYS is the host or build platform, while the other platform is a Unix-style system. In these cases, there are often conflicts between the format of the file names and paths expected within host platform libraries and executables, and those employed on the build platform.

This situation is best described using a concrete example: suppose the build platform is GNU/Linux with canonical triplet ‘i686-pc-linux-gnu’. Suppose further that the host platform is MinGW with canonical triplet ‘i586-pc-mingw32’. On the GNU/Linux platform there is a cross compiler following the usual naming conventions of such compilers, where the compiler name is prefixed by the host canonical triplet (or suitable alias). (For more information concerning canonical triplets and platform aliases, see Specifying Target Triplets and Canonicalizing) In this case, the C compiler is named ‘i586-pc-mingw32-gcc’.

As described in Wrapper executables, for the MinGW host platform libtool uses a wrapper executable to set various environment variables before launching the actual program executable. Like the program executable, the wrapper executable is cross-compiled for the host platform (that is, for MinGW). As described above, ordinarily a host platform executable cannot be executed on the build platform, but in this case the Wine environment could be used to launch the MinGW application from GNU/Linux. However, the wrapper executable, as a host platform (MinGW) application, must set the PATH variable so that the true application's dependent libraries can be located—but the contents of the PATH variable must be structured for MinGW. Libtool must use the Wine file name mapping facilities to determine the correct value so that the wrapper executable can set the PATH variable to point to the correct location.

For example, suppose we are compiling an application in /var/tmp on GNU/Linux, using separate source code and build directories:

     

/var/tmp/foo-1.2.3/app/ (application source code)
/var/tmp/foo-1.2.3/lib/ (library source code)
/var/tmp/BUILD/app/ (application build objects here)
/var/tmp/BUILD/lib/ (library build objects here)

Since the library will be built in /var/tmp/BUILD/lib, the wrapper executable (which will be in /var/tmp/BUILD/app) must add that directory to PATH (actually, it must add the directory named objdir under /var/tmp/BUILD/lib, but we'll ignore that detail for now). However, Windows does not have a concept of Unix-style file or directory names such as /var/tmp/BUILD/lib. Therefore, Wine provides a mapping from Windows file names such as C:\Program Files to specific Unix-style file names. Wine also provides a utility that can be used to map Unix-style file names to Windows file names.

In this case, the wrapper executable should actually add the value

     Z:\var\tmp\BUILD\lib

to the PATH. libtool contains support for path conversions of this type, for a certain limited set of build and host platform combinations. In this case, libtool will invoke Wine's winepath utility to ensure that the correct PATH value is used. For more information, see see File name conversion.