Issue #9709 has been reported by Hiroshi Shirosaki.

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Bug #9709: Large string causes SEGV with x64-mingw32
https://bugs.ruby-lang.org/issues/9709

* Author: Hiroshi Shirosaki
* Status: Open
* Priority: Normal
* Assignee: 
* Category: 
* Target version: 
* ruby -v: ruby 2.2.0dev (2014-04-07 trunk 45529) [x64-mingw32]
* Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN
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Creating large string causes SEGV with x64-mingw32 on Windows.

test.rb

~~~
A = ""
1000000.times do |i|
  A << "a" * 100000
end
~~~

gdb backtrace of `./miniruby test.rb`

~~~
Program received signal SIGSEGV, Segmentation fault.
0x000007fefe88120b in msvcrt!memmove () from C:\Windows\system32\msvcrt.dll
(gdb) bt
#0  0x000007fefe88120b in msvcrt!memmove () from C:\Windows\system32\msvcrt.dll
#1  0x000000000054e404 in str_buf_cat (str=str@entry=115691040, ptr=ptr@entry=0x7b510e0 'a' <repeats 200 times>...,
    len=len@entry=100000) at ../../../ruby/string.c:2042
#2  0x000000000054e90a in rb_enc_cr_str_buf_cat (str=str@entry=115691040, ptr=0x7b510e0 'a' <repeats 200 times>...,
    len=100000, ptr_encindex=<optimized out>, ptr_cr=ptr_cr@entry=1048576, ptr_cr_ret=0x22eb10,
    ptr_cr_ret@entry=0x22eaf0) at ../../../ruby/string.c:2164
#3  0x0000000000553c6c in rb_str_buf_append (str=115691040, str2=115660360) at ../../../ruby/string.c:2207
#4  0x0000000000553d9f in rb_str_append (str2=115660360, str=115691040) at ../../../ruby/string.c:2220
#5  rb_str_concat (str1=115691040, str2=115660360) at ../../../ruby/string.c:2256
#6  0x00000000005ac743 in vm_exec_core (th=0x768ce00, th@entry=0x0, initial=initial@entry=0)
    at ../../../ruby/insns.def:1824
#7  0x00000000005ad661 in vm_exec (th=0x0) at ../../../ruby/vm.c:1328
#8  0x0000000000000000 in ?? ()
~~~

`capa` setting looks wrong in the following code. Here is a patch.

~~~
diff --git a/string.c b/string.c
index 511374c..8abfc25 100644
--- a/string.c
+++ b/string.c
@@ -2029,7 +2029,7 @@ str_buf_cat(VALUE str, const char *ptr, long len)
     if (capa <= total) {
        while (total > capa) {
            if (capa + termlen >= LONG_MAX / 2) {
-               capa = (total + 4095) / 4096;
+               capa = LONG_MAX - termlen;
                break;
            }
            capa = (capa + termlen) * 2;
~~~



-- 
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