How to append values in xcconfig variables?

For reasons stated in other answers to this question, you can't inherit values easily.

I recommend defining your settings in cascade. Let's assume APP is your project prefix and make this simple defining only a few CFLAGS:

platform.xcconfig:

APP_PLATFORM_CFLAGS = -DMAS=1

project.xcconfig:

#include "platform.xcconfig"APP_PROJECT_CFLAGS = -DBETA=1

target-one.xcconfig:

#include "project.xcconfig"APP_TARGET_CFLAGS = -DSUPER_COOL=1#include "merge.xcconfig"

target-two.xcconfig:

#include "project.xcconfig"APP_TARGET_CFLAGS = -DULTRA_COOL=1#include "merge.xcconfig"

merge.xcconfig:

OTHER_CFLAGS = $(inherited) $(APP_PLATFORM_CFLAGS) $(APP_PROJECT_CFLAGS) $(APP_TARGET_CFLAGS)

Then, you'll base each of your targets build configurations on target-xxx.xcconfig. A real project will use more complex setups, using a configuration file for the project and a different one for the target, but you get the idea.

Also, remember that $(inherited) refers to higher level in the hierarchy, not earlier. For instance, it inherits from Project level at Target level. Not sure if this apply to Xcode 4 too.

This is a simplification of GTM, go there to learn more.

As stated in other answers, prior to Xcode 10, xcconfig files could not simply inherit and extend each other's values. But,

Since Xcode 10, xcconfig now work as one might expect them to : $(inherited) actually expand to the previously defined value of the variable.

When an .xcconfig file contains multiple assignments of the same build setting, later assignments using $(inherited) or $(<setting_name>) will inherit from earlier assignments in the .xcconfig. The legacy build system caused every use of $(inherited) or $(<setting_name>) to skip any other values defined within the .xcconfig. To detect whether your .xcconfig is affected by this improvement, running defaults write com.apple.dt.XCBuild EnableCompatibilityWarningsForXCBuildTransition -bool YES in Terminal will cause Xcode to generate a warning about this situation.

(Xcode 10 beta 1 release notes)

So for example, given two simple .xcconfig files:

// Generic.xcconfigOTHER_SWIFT_FLAGS = $(inherited) -DMY_GENERIC_FLAG

// Debug.xcconfig#include "Generic.xcconfig"OTHER_SWIFT_FLAGS = $(inherited) -DMY_DEBUG_FLAG

Assuming your project uses Debug.xcconfig for its Debug configuration, you'll get the expected value -DMY_GENERIC_FLAG -DMY_DEBUG_FLAG for OTHER_SWIFT_FLAGS.

(instead of just -DMY_DEBUG_FLAG in Xcode 9 and earlier releases)

The new behavior is pretty straightforward: $(inherited) is simply replaced by the previously defined value of the variable, if any.

So in the previous example, if we expand the #include statement, we'll get the following xcconfig file:

// Merged xcconfig files after resolving #includeOTHER_SWIFT_FLAGS = -DMY_GENERIC_FLAGOTHER_SWIFT_FLAGS = $(inherited) -DMY_DEBUG_FLAG

• On the first line OTHER_SWIFT_FLAGS value is -DMY_GENERIC_FLAG ( $(inherited) expands to nothing, because this is the first definition of OTHER_SWIFT_FLAGS we encounter¹).
• On the second line OTHER_SWIFT_FLAGS if overwritten, and its value is now -DMY_GENERIC_FLAG -DMY_DEBUG_FLAG (its previous value + the newly added flag).

On a more complex xcconfig setup, things could look like this:

// First.xcconfigOTHER_SWIFT_FLAGS = $(inherited) -DMY_FIRST_FLAG

// Second.xcconfigOTHER_SWIFT_FLAGS = $(inherited) -DMY_SECOND_FLAG

// Last.xcconfig#include "Generic.xcconfig"OTHER_SWIFT_FLAGS = $(inherited) -DMY_LAST_FLAG

// Merge.xcconfig#include "First.xcconfig"#include "Second.xcconfig"OTHER_SWIFT_FLAGS = $(inherited) -DMY_INTERMEDIATE_FLAG#include "Last.xcconfig"

We'll assume this time we're using the Merge.xcconfig in our configuration.

The resolved value for OTHER_SWIFT_FLAGS will then be -DMY_FIRST_FLAG -DMY_SECOND_FLAG -DMY_INTERMEDIATE_FLAG -DMY_GENERIC_FLAG -DMY_LAST_FLAG.

This might be surprising at first, but it actually makes sense: once the #include are resolved, we end up with this xcconfig:

OTHER_SWIFT_FLAGS = $(inherited) -DMY_FIRST_FLAGOTHER_SWIFT_FLAGS = $(inherited) -DMY_SECOND_FLAGOTHER_SWIFT_FLAGS = $(inherited) -DMY_INTERMEDIATE_FLAGOTHER_SWIFT_FLAGS = $(inherited) -DMY_GENERIC_FLAGOTHER_SWIFT_FLAGS = $(inherited) -DMY_LAST_FLAG

The final resolved value is then the one defined on the last line, which is -DMY_LAST_FLAG plus the value it inherited from the previous line -DDMY_GENERIC_FLAG (which comes from Generic.xcconfig, included in Last.xcconfig) etc etc.

Note that naturally, if you forget $(inherited) in one of the definitions, you'll break the inheritance chain and only get the values from the bottom definitions, up to the definition without $(inherited).

¹ One may expect the xcconfig file to inherit previous values defined at the Project level, but it doesn't seem to be the case

📡 As of Xcode 10 beta 1, it seems the build settings editor GUI doesn't properly resolve the correct value for variables defined in the xcconfig files, and displays the values as if resolved with the old pre-Xcode 10 behavior. I filed rdar://40873121 regarding this (https://openradar.appspot.com/radar?id=4925869923500032).

According to the Xcode Build System Guide:

When a configuration unit contains more than one definition for a particular build setting, Xcode uses the last definition in the unit. Keep in mind that configuration files do not have access to build setting definitions made in configuration files they include. That is, you cannot modify the definition made in an included configuration file; you can only replace it.

So, I guess this mean that it is not possible to append values to a given variable.