Code Coverage for iOS (gcov)

Create code coverage reports for iOS unit tests using gcov tool.

Note that this approach may not work with Xcode 7 and future Xcode updates. For information on how to generate test coverage reports using new Profdata format, check this article).

Calculating Code Coverage is a way to get more out of your unit tests, given that you have those and run on regular basis. With minor changes you can get coverage reports that include stats for

  • Packages
  • Files
  • Classes
  • Lines
  • Conditionals


Good staring point is documentation from Apple. Important takeaway from that article is that you need 2 sets of files to generate coverage reports. The .gcno files contain information to reconstruct the basic block graphs and assign source line numbers to blocks. The .gcda files are generated when the tests are executed and contain transition counts and some summary information. Check this link for more details.

The Apple’s article recommends to create a separate configuration and set the following LLVM 5.0 Code Generation options to YES.

  • Generate Test Coverage Files (GCC_GENERATE_TEST_COVERAGE_FILES)
    • This is required to generate .gcno files.
  • Instrument Program Flow (GCC_INSTRUMENT_PROGRAM_FLOW_ARCS)
    • This is required to get .gcda files.

I have also specified the GCC settings names so you’d know how to enable these flags when building from command line or when using xcconfigs. The benefit of building from command line is that you don’t have to create new configuration in the project, instead you can customize existing Debug configuration.

# Make sure tests are run against latest OS at all times.
export OS=$(xcrun --sdk iphonesimulator --show-sdk-platform-version)

xcodebuild test -project MyProject.xcodeproj -scheme MyScheme \
  -configuration Debug \
  -sdk iphonesimulator${OS} \
  -destination OS=${OS},name="iPad Retina" \

If you’ve just ran this command you will be surprised to find no .gcda files anywhere. Apparently, writing these files to disk is a costly operation so this data needs to be flushed with explicit command. The way to do that is to call __gcov_flush in your app’s code. Apple recommends to do that when the app is sent to background, here’s the quote

Set the UIApplicationExitsOnSuspend key to YES in your Info.plist file and send your running application to the background by pressing the home button.

This is very un-automatic and very un-CI, Apple. This readme provides a list of other options. Well, the first one is reacting to the event of app entering background, which is not good. The second option uses method swizzling and swizzles tearDown method of XCTest class. This way GCOV data will be flushed when all the test cases are completed.

I am using swizzling as well, but swizzle tearDown method of XCTestCase. This way data is flushed every time a test case finishes.


@import XCTest;
@import ObjectiveC.runtime;

// If you have to build for versions below iOS 7.0, use this code instead
// #import <XCTest/XCTest.h>
// #import <objc/runtime.h>

extern void __gcov_flush();

@implementation XCTestCase (GCovFlush)

+ (void)load {
    Method original, swizzled;

    original = class_getClassMethod(self, @selector(tearDown));
    swizzled = class_getClassMethod(self, @selector(_swizzledTearDown));
    method_exchangeImplementations(original, swizzled);

+ (void)_swizzledTearDown {
    if (__gcov_flush) {
    [self _swizzledTearDown];



Note that I don’t have a header file. There’s no need for it since you are not going to include it anywhere. Another notable difference is the use of ifdef guard ENABLE_GCOV_FLUSH which needs to be defined when building from command line, this is just me being over paranoid about including any kind of non-production code in the app. Save this file and name it XCTestCase+GCovFlush.m Before you add it to your project, one very important note

The file must be added to the main app target, also called “Test Host”, not to the unit tests target. More details.

OK, so now you can add this file to your Xcode project. There are ways to automate this injection as well, e.g. using xcodeproj Ruby gem. I plan to have a write up about it some time later. Anyway, you can now run your tests and have .gcno and .gcda files at your disposal.

export OS=$(xcrun --sdk iphonesimulator --show-sdk-platform-version)

xcodebuild test -project MyProject.xcodeproj -scheme MyScheme \
  -configuration Debug \
  -sdk iphonesimulator${OS} \
  -destination "OS=${OS},name=iPad Retina" \
  | tee test.log

Note that I’m redefining preprocessor definitions to enable ENABLE_GCOV_FLUSH and also want to reuse those already defined in Xcode project, that’s why I refer to them with help of escaping $ sign. I’m also capturing logs in test.log for further processing.

This paragraph has nothing to do with coverage reports, but you need to capture test results in a report format that most CI servers would understand. You can use ocunit2junit to convert OCUnit (now XCTest) report to JUnit format.

# install
[sudo] gem install ocunit2junit

# run
cat test.log | ocunit2junit

Default output is in test-reports directory, point your CI report plugin to this location to pick up XML and generate test reports.

Another option is xcpretty tool, which is yet another Ruby gem.

# install
[sudo] gem intall xcpretty

# generate JUnit XML reports (default location is build/reports directory)
cat test.log | xcpretty --report junit --color

# generate HTML reports (default location is build/reports directory)
cat test.log | xcpretty --report html --color

Note that you don’t have to re-run the tests. All that the tool needs is build log.

If you are OK with using something other than xcodebuild, check out Facebook’s xctool. It has few options to help you get test reports without the need of extra gems. It also supports parallel execution of tests, so definitely worth a look.


It’s time to report the coverage results. gcovr is the right tool for the job. Since it’s a Python utility, here’s how you install it

sudo -E easy_install pip
pip install gcovr

Use -E option to tell easy_install to pick up current user’s environment variables such as HTTP_PROXY while running as super user. Don’t bother with this option if you can run easy_install as a non-sudoer, e.g. when you have custom Python installation.

Use -x or --xml to generate XML report. gcovr has issues with generating HTML reports, we’ll have to use another tool for the job later.


gcovr \
    --filter=${GCOV_FILTER} \
    --exclude=${GCOV_EXCLUDE} \
    --object-directory=${BUILD_DIR} -x > ${COVERAGE_REPORT}

This example also demonstrates the use of filter and exclude options to filter unwanted files from reports. BUILD_DIR points to the directory you defined with help of CONFIGURATION_BUILD_DIR when running the tests.

The XML output is enough for CI tasks, but to have a sneak peek at readable HTML results on your computer you’ll end up nowhere with gcovr, the tool has a bug (unless it got fixed already). The tool to help at this time is lcov.

# Install.
brew install lcov

# Use.
LCOV_EXCLUDE="${LCOV_INFO} '*/*' '*Tests.m' '$(BUILD_DIR)/*'"

lcov --capture --directory ${BUILD_DIR} --output-file ${LCOV_INFO}
lcov --remove ${LCOV_EXCLUDE} --output-file ${LCOV_INFO}
genhtml ${LCOV_INFO} --output-directory ${LCOV_REPORTS_DIR}

Here you can see another use of exclude option to filter out unwanted results. genhtml is bundled with lcov installation. More documentation on lcov can be found here. You now have browsable coverage report in lcov-reports directory.


The real point of calculating code coverage reports is to use them as part of CI process to keep track of overall project health and mark it as unstable or failed if coverage is less than required. Check out Jenkins vs Bamboo article for more details.

Published: February 08 2015

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