Command Line Options

The behavior of compilers are primarily controlled with ​switches. For example, -I${prefix}/include and -L${prefix}/lib tell the ​C ​preprocessor and ​linker to look in the ​MacPorts prefix for header files and libraries respectively. However, changing compiler behavior with switches can cause problems.

• Order matters.
  • -I${prefix}/include -I${worksrcpath}/include and -I${worksrcpath}/include -I${prefix}/include produce different results if different header files with the same name are in both location.
  • Such errors can be subtle and difficult to track down, especially when the header files are from different versions of the same packages (e.g. during the upgrade process).
  • The MacPorts construct conflicts_build can be used to prevent the conflicting header files from being simultaneously installed, but this not ideal.
  • Using -isystem instead of -I can more ​precisely control search order, but this also ​causes problems.
  • The search order for libraries has similar problems, but it is more difficult to precisely control library search order (there is just -L or using the full library path name).
  • The MacPorts ​buildbot is careful to build a port only with the minimal set of dependencies active, which prevents most search order issues.
• Switches can be “baked in” the installed files.
  • ​Some packages record the history of the switches they were built with.
  • Some packages use the same switches they were built with to build other packages.
  • At best, this is unnecessary noise. At worst, it causes build problems that can be difficult to diagnose.
  • The switches -isysroot and -Wl,-syslibroot are particularly problematic.
    • Major upgrades of Xcode usually remove old ​SDKs.
    • After a user upgrades Xcode, ports can ​mysteriously stop building correctly.
    • If the Xcode versions of the user and buildbot are different, ports can ​fail to build correctly.

Environment Variables

To a limited extent, the behavior of compilers can also be controlled with ​environment variables instead of switches.

Switch	Variable	Affect	Notes
-I	CPATH	treated as a delimited list of paths to be added to the default system include path list
-isystem	C_INCLUDE_PATH, OBJC_INCLUDE_PATH, CPLUS_INCLUDE_PATH, OBJCPLUS_INCLUDE_PATH	like CPATH except only used when processing the appropriate language
-L	LIBRARY_PATH	tries the directories specified when searching for special linker files
-mmacosx-version-min	MACOSX_DEPLOYMENT_TARGET	the default deployment target
-isysroot, -Wl,-syslibroot	SDKROOT	specifies the SDK
	DEVELOPER_DIR	tells xcrun where to search for development tools and properties
	CC_PRINT_OPTIONS	like -v but logs the commands	​interferes with-v
	CC_PRINT_OPTIONS_FILE	the file to log CC_PRINT_OPTIONS output to

Using environment variables can alleviate some of the problems with switches.

• They are less likely to be “baked in” the installed files.
• Directories in CPATH and LIBRARY_PATH are searched after those specified by the command line options.

However, an environment variable is no panacea.

• Not all build systems ​respect environment variables.
• Not all environment variables are supported on all compilers.
• Setting CC_PRINT_OPTIONS causes -v to behave differently. For example, it breaks FindOpenMP in CMake.