[ruby-core:118349] [Ruby master Bug#20585] Size of memory allocated by String.new(:capacity) is different from the specified value

Issue #20585 has been updated by byroot (Jean Boussier).


Most of this comes from: https://github.com/ruby/ruby/pull/8825

Long story short, `capacity` is a bit confusing because since Ruby strings are null terminated, there is always at least one extra byte needed. So it's debatable whether the terminating byte is accounted for in the capacity.

I see how when using `String.new(capacity:)`, the goal is to avoid reallocation, so if you precomputed the final string size, that might defeat the purpose. The other side of the coin though, is that if you use sizes like `4096` hoping to fit in a specific size in memory, the extra terminator byte make it not behave as you'd hoped.

> If the initial string and its bytesize are specified, about twice the size is allocated.

I need to dig more to answer this one.

----------------------------------------
Bug #20585: Size of memory allocated by String.new(:capacity) is different from the specified value
https://bugs.ruby-lang.org/issues/20585#change-108854

* 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?




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