Why is my C code failing to compile with the `` header after upgrading to Catalina?

Catalina C : Header Error: Undefined 'signbit' Member

Introduction

Upgrading to Catalina from Mojave can introduce issues when compiling programs that rely on the header. This is due to a change in how the system libraries are handled in Catalina.

Problem Statement

After the upgrade, attempting to compile code using the header results in errors such as:

error: no member named 'signbit' in the global namespace
error: no member named 'fpclassify' in the global namespace
error: no member named 'isfinite' in the global namespace

Cause

Catalina purges and protects the /usr/include directory, which is where the C headers are typically found. As a result, build systems may fail to locate the necessary headers.

Solution

To resolve this issue, it is necessary to point the build system to the correct headers using the -isysroot compiler flag. Here are the steps to do so:

1. Verify that Xcode is up to date.

2. Determine the system SDK path using xcrun --show-sdk-path, and modify the build system accordingly:

   • CMake: set(CMAKE_OSX_SYSROOT /sdk/path) or set(CMAKE_CXX_FLAGS "[...] -isysroot /sdk/path")
   • Other compilers: Add -isysroot /sdk/path to the compiler flags

Conclusion

Following these steps should resolve the issue and allow programs using the header to compile successfully under Catalina.

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