Hey there, sorry to hear you're stuck with this frustrating precompiled header issue—especially when stepping into someone else's project, it can feel like solving a mystery! Let’s break down practical, actionable fixes that usually resolve the Failed to emit precompiled header error:

• Clean up build caches first
  This is the first step for almost any Xcode build error. Close Xcode entirely, then delete the Derived Data folder (you can find it at ~/Library/Developer/Xcode/DerivedData—use Finder to navigate there, or run rm -rf ~/Library/Developer/Xcode/DerivedData in Terminal). Reopen Xcode, go to Product > Clean Build Folder (use the shortcut Cmd+Shift+K), then try building again.

• Verify your prefix header (.pch) configuration
  Precompiled header issues often stem from misconfigured or broken .pch files:

  1. Open your project’s Build Settings, search for Precompile Prefix Header—make sure it’s set to YES.
  2. Check the Prefix Header path: it should point correctly to your .pch file. Use a relative path like $(SRCROOT)/YourProjectName/PrefixHeader.pch to avoid absolute path issues.
  3. Open the .pch file itself and scan for errors: missing semicolons, broken import statements (like the missing FMDatabase.h you mentioned), or references to files that no longer exist in the project.

• Check third-party library integrations
  Since you linked issues related to route-me and FMDB, double-check these dependencies:

  • If the project uses CocoaPods, confirm you opened the .xcworkspace file (not .xcodeproj) and ran pod install to ensure all libraries are properly installed.
  • For manually added libraries, verify the library files are still in the project, that they’re included in Build Phases > Link Binary With Libraries, and that Header Search Paths in Build Settings includes the path to the library’s headers (enable Recursive if needed to catch nested headers).

• Fix Xcode compatibility issues
  If the previous developer used an older Xcode version, your current setup might have compatibility gaps:

  • Check the project’s Deployment Target in Build Settings—make sure it’s a version supported by your Xcode.
  • Try updating Xcode to the latest stable release, or switch the command line tool version in Xcode > Settings > Locations > Command Line Tools to match what the original project might have used.

• Force a fresh precompiled header generation
  Sometimes the existing precompiled header gets corrupted. Temporarily set Precompile Prefix Header to NO, build the project once, then switch it back to YES and rebuild. This forces Xcode to generate a brand new precompiled header file.

内容的提问来源于stack exchange，提问作者user1849661