Gem install fails on MacOS

[kride]

Just recently upgraded some Homebrew packages, including MySQL@8.0, and thus required a reinstall for mysql2-gem.

Compilation of extension fails with following error:

compiling client.c
In file included from client.c:15:
./mysql_enc_name_to_ruby.h:43:35: error: unknown type name 'str'
   43 | mysql2_mysql_enc_name_to_rb_hash (str, len)
      |                                   ^
./mysql_enc_name_to_ruby.h:43:40: error: unknown type name 'len'
   43 | mysql2_mysql_enc_name_to_rb_hash (str, len)
      |                                        ^
./mysql_enc_name_to_ruby.h:43:44: error: expected ';' after top level declarator
   43 | mysql2_mysql_enc_name_to_rb_hash (str, len)
      |                                            ^
      |                                            ;
./mysql_enc_name_to_ruby.h:44:27: error: illegal storage class on file-scoped variable
   44 |      register const char *str;
      |                           ^
./mysql_enc_name_to_ruby.h:45:28: error: illegal storage class on file-scoped variable
   45 |      register unsigned int len;
      |                            ^
./mysql_enc_name_to_ruby.h:46:1: error: expected identifier or '('
   46 | {
      | ^
./mysql_enc_name_to_ruby.h:86:30: error: unknown type name 'str'
   86 | mysql2_mysql_enc_name_to_rb (str, len)
      |                              ^
./mysql_enc_name_to_ruby.h:86:35: error: unknown type name 'len'
   86 | mysql2_mysql_enc_name_to_rb (str, len)
      |                                   ^
./mysql_enc_name_to_ruby.h:86:39: error: expected ';' after top level declarator
   86 | mysql2_mysql_enc_name_to_rb (str, len)
      |                                       ^
      |                                       ;
./mysql_enc_name_to_ruby.h:87:27: error: illegal storage class on file-scoped variable
   87 |      register const char *str;
      |                           ^
./mysql_enc_name_to_ruby.h:88:28: error: illegal storage class on file-scoped variable
   88 |      register unsigned int len;
      |                            ^
./mysql_enc_name_to_ruby.h:89:1: error: expected identifier or '('
   89 | {
      | ^
12 errors generated.
make: *** [client.o] Error 1

make failed, exit code 2

AI proposed following patch which worked on a forked branch

--- a/ext/mysql2/mysql_enc_name_to_ruby.h
+++ b/ext/mysql2/mysql_enc_name_to_ruby.h
@@ -40,9 +40,7 @@ inline
 #endif
 #endif
 static unsigned int
-mysql2_mysql_enc_name_to_rb_hash (str, len)
-     register const char *str;
-     register unsigned int len;
+mysql2_mysql_enc_name_to_rb_hash (const char *str, unsigned int len)
 {
   static const unsigned char asso_values[] =
     {
@@ -83,9 +81,7 @@ __attribute__ ((__gnu_inline__))
 #endif
 #endif
 const struct mysql2_mysql_enc_name_to_rb_map *
-mysql2_mysql_enc_name_to_rb (str, len)
-     register const char *str;
-     register unsigned int len;
+mysql2_mysql_enc_name_to_rb (const char *str, unsigned int len)
 {
   enum
     {

Currently using:
mysql2-gem 0.5.7
MacOS 26.4 (also on 26.3)
ruby 3.4.9 (also on 3.4.7)
MySQL 8.0.45 (tried also with 8.4 and with just mysql-client@8.0)
XCode 26.4 (also 26.2)
clang 21.0.0 (also 17.0.0)

[dbourguignon]

Add the same issue, I've patched locally and it looks fine

[nicholasdower]

This seems to have started for me after upgrading to Xcode / Command Line Tools 26.4 (released March 24, 2026).

When building Ruby via RVM, I now see:

cat ~/.rvm/log/1775570553_ruby-4.0.2/configure.log
[...]
checking for gcc option to enable C23 features... -std=gnu23
[...]

This is reflected in Ruby’s configuration:

ruby -rrbconfig -e 'p RbConfig::CONFIG["CC"]'
"gcc -std=gnu23"

Compare this to a Ruby installation built before the Xcode update:

ruby -rrbconfig -e 'p RbConfig::CONFIG["CC"]'
"gcc"

As a result, mysql2 is compiled with C23, where old-style (K&R) function definitions are no longer supported, causing the build to fail.

The issue reproduces with:

bundle exec rake clean compile -- --with-cflags="-std=gnu23"

but not with:

bundle exec rake clean compile -- --with-cflags="-std=gnu17"

Updating the code to use standard (prototype-style) function definitions, as suggested, fixes the issue. I’ve opened #1430.

[nicholasdower]

For now, I am using the following in my Gemfile (see b7907a0):

gem 'mysql2', github: 'nicholasdower/mysql2', ref: 'b7907a0'

Alternatively, building mysql2 with C17 also worked:

bundle config set --local build.mysql2 --with-cflags="-std=gnu17"

Alternatively, building Ruby with C17 also worked:

rvm install ruby-4.0.2 --disable-binary -E CFLAGS=-std=gnu17