Issue #20585 has been reported by os (Shigeki OHARA). ---------------------------------------- Bug #20585: Size of memory allocated by String.new(:capacity) is different from the specified value https://bugs.ruby-lang.org/issues/20585 * Author: os (Shigeki OHARA) * Status: Open * ruby -v: ruby 3.3.2 (2024-05-30 revision e5a195edf6) [x86_64-freebsd14.0] * Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN ---------------------------------------- IMHO, if :capacity is specified in String.new, capa will be its value. In fact, Ruby 3.2 seems to allocate the size as specified. ``` % cat string_capacity.rb unless /\A3\.[23]\./ =~ RUBY_VERSION raise NotImplementedError, 'Not Supported Ruby Version' end require 'inline' class String def super_inspect self.class.superclass.instance_method(:inspect).bind(self).call end inline do |builder| builder.include '<stdio.h>' builder.add_compile_flags '-Wall' builder.c_raw <<~CODE VALUE capacity(int argc, VALUE *argv, VALUE self) { struct RString *rstring = RSTRING(self); if (! (RBASIC(self)->flags & RSTRING_NOEMBED)) { return rb_to_symbol(rb_str_new_cstr("EMBED")); } else { if (RBASIC(self)->flags & ELTS_SHARED) { return rb_to_symbol(rb_str_new_cstr("SHARED")); } else { return LONG2NUM(rstring->as.heap.aux.capa); } } return Qnil; /* NOTREACHED */ } CODE end end ``` ``` % irb -I. -rstring_capacity irb(main):001:0> [RUBY_PLATFORM, RUBY_VERSION] => ["x86_64-freebsd14.0", "3.2.4"] irb(main):002:0> String.new('', capacity: 1024).capacity => 1024 irb(main):003:0> String.new('*'*1024, capacity: 1024).capacity => 1024 irb(main):004:0> ``` This is what I expect. However, Ruby 3.3 seems to behave differently. ``` % irb -I. -rstring_capacity irb(main):001> [RUBY_PLATFORM, RUBY_VERSION] => ["x86_64-freebsd14.0", "3.3.2"] irb(main):002> String.new('', capacity: 1024).capacity => 1023 irb(main):003> String.new('*'*1024, capacity: 1024).capacity => 2047 irb(main):004> ``` * If only :capacity is specified, one byte less is allocated. * If the initial string and its bytesize are specified, about twice the size is allocated. Is this intentional? -- https://bugs.ruby-lang.org/