Implicit Declaration of Function is Invalid in C99

Photo by Max Chen

Implicit Declaration of Function is Invalid in C99

If you get the following error when attempting to install the thin gem on MacOS (gem install thin):

Fetching thin-1.7.2.gem
Building native extensions. This could take a while...
ERROR:  Error installing thin:
	ERROR: Failed to build gem native extension.

[Omitted for brevity]

thin.c:374:10: error: implicit declaration of function 'thin_http_parser_is_finished' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
  return thin_http_parser_is_finished(http) ? Qtrue : Qfalse;
         ^
9 errors generated.
make: *** [thin.o] Error 1

The solution:

Install the gem with the following C flags:

gem install thin -- --with-cflags="-Wno-error=implicit-function-declaration"

Same solution works if you are trying to install mailcatcher

  gem install mailcatcher -- --with-cflags="-Wno-error=implicit-function-declaration"

If you are installing the gem using bundler, then you can change bundler’s config to use the C flags

  bundle config build.thin --with-cflags="-Wno-error=implicit-function-declaration"

Credits for the solution:

Mike Szyndel on Github