Fix "Function Declaration Without a Prototype" Error in Xcode

Introduction

When integrating Firebase Analytics into an iOS project using CocoaPods, developers may encounter the build error: "A function declaration without a prototype is deprecated in all versions of C". This error typically occurs in Xcode when building projects including Firebase frameworks like GoogleDataTransport. This article explains the cause and provides optimized solutions to resolve the issue.

Problem Statement

The error occurs when building Firebase Analytics libraries in Xcode, specifically targeting the GoogleDataTransport framework. The problematic code appears as:

GDTCORNetworkType GDTCORNetworkTypeMessage() {  // Error highlighted here
    #if !TARGET_OS_WATCH
    // Network type detection logic
    #endif
    return GDTCORNetworkTypeUNKNOWN;
}

The core issue stems from how C language standards handle function declarations:

• Empty parentheses () indicate an unspecified argument list (K&R-style)
• Modern C standards require prototype declarations with explicit parameters
• This syntax is deprecated since C99 and enforced as an error in Xcode

The problem manifests in Firebase libraries but affects project builds globally. Simply updating pods might not resolve the issue without additional configuration changes.

Solutions

1. Modify Podfile with Post-Install Hook (Recommended)

Add a post-install hook to your Podfile to configure build settings specifically for GoogleDataTransport targets:

post_install do |installer|
    # Optional: React Native hook if applicable
    react_native_post_install(installer)
    
    installer.pods_project.targets.each do |target|
        # Required for Xcode 14+ builds
        if target.respond_to?(:product_type) && target.product_type == "com.apple.product-type.bundle"
            target.build_configurations.each do |config|
                config.build_settings['CODE_SIGNING_ALLOWED'] = 'NO'
            end
        end
        
        # Disable strict prototype checking for GoogleDataTransport
        if target.name.start_with? "GoogleDataTransport"
            target.build_configurations.each do |config|
                config.build_settings['CLANG_WARN_STRICT_PROTOTYPES'] = 'NO'
            end
        end
    end
end

Implementation steps:

1. Open your project's Podfile
2. Add the post-install hook after any existing content
3. Save and run pod install

Benefits

• Avoids manual source code changes
• Preserves settings across pod updates
• Targeted fix only impacts GoogleDataTransport

2. Modify Build Settings Manually

For non-automated fixes or immediate resolution:

1. Open your Xcode workspace
2. Select Pods project in left sidebar
3. Locate GoogleDataTransport target
4. Navigate to Build Settings tab
5. Search for "Strict Prototypes"
6. Set CLANG_WARN_STRICT_PROTOTYPES to NO

Limitation

This setting may reset when running pod install or pod update. Prefer the Podfile solution for persistent configuration.

3. Temporary Source Code Modification

If you need an immediate workaround before permanent fixes:

1. In error location, change:

   GDTCORNetworkType GDTCORNetworkTypeMessage() {

   →

   GDTCORNetworkType GDTCORNetworkTypeMessage(void) {

2. Xcode Quick Fix: Click the error icon and select "Replace 'O' with 'void'"

Warning

This approach:

• Is temporary (modifications lost in pod updates)
• Requires changes to multiple Firebase files
• Not recommended as permanent solution

Explanation and Background

Why This Error Occurs

• C language requirement: Function declarations must use (void) instead of () to indicate no parameters
• Firebase dependency: GoogleDataTransport contains non-prototyped function declarations
• Xcode strictness: Newer Xcode versions enforce strict C standards compliance

Technical Insights

• K&R-style declarations (()) are obsolete in modern C (deprecated since C99)
• Proper C99+ declaration uses explicit (void) for no-argument functions
• The fix has been merged into GoogleDataTransport source (see commit)
• C23 standard will resolve this ambiguity permanently

Conclusion

The most sustainable solution is updating your Podfile with the post-install hook configuration to disable strict prototype checking specifically for GoogleDataTransport. This targets the issue without impacting other build settings or requiring manual file modifications.

For React Native users, this solution also resolves the underlying issue as documented in the React Native GitHub discussion.

Prioritize the Podfile solution for long-term stability. The manual build settings change serves as a good temporary workaround, while direct source modification should be temporary due to maintainability concerns.