一次偶然,发现完全同一份代码,在不同机器上find出现两个不同执行结果,本文旨在研究find的“诡异”行为,找出背后的原因。
测试代码:
// g++ -g -o x x.cpp
#include
#include
extern "C" int main()
{
std::string::size_type n = std::string::npos;
std::string str = "123";
std::string::size_type m = str.find("2", n); // 按照期望,m值应为npos
std::cout << "n=" << n << ", m=" << m << std::endl;
return 0;
}
i386输出结果(gcc (GCC) 4.1.2):
n=4294967295, m=1
这里m值为1,是一个非期望的值。
i86_64编译成64位输出结果(gcc (GCC) 4.8.5):
n=18446744073709551615, m=18446744073709551615
i86_64编译成32位输出结果(gcc (GCC) 4.8.5):
n=4294967295, m=4294967295
i386上编译放到i86_64上执行的输出结果:
n=4294967295, m=4294967295
i386上编译成共享库后放到i86_64上执行的输出结果:
// g++ -g -o libx.so -fPIC -shared x.cpp
n=4294967295, m=4294967295
测试代码:
// g++ -g -o x x.cpp
#include
#include
extern "C" int main()
{
std::string::size_type n = std::string::npos;
std::string str = "123";
std::string::size_type m = str.find('2', n);
std::cout << "n=" << n << ", m=" << m << std::endl;
return 0;
}
i386输出结果(gcc (GCC) 4.1.2):
n=4294967295, m=4294967295
i86_64编译成64位输出结果(gcc (GCC) 4.8.5):
n=18446744073709551615, m=18446744073709551615
i86_64编译成32位输出结果(gcc (GCC) 4.8.5):
n=4294967295, m=4294967295
i386上编译放到i86_64上执行的输出结果:
n=4294967295, m=4294967295
i386上编译成共享库后放到i86_64上执行的输出结果:
n=4294967295, m=4294967295
对于字符串版本的find,出现不同的结果。小技巧:加上编译选项“-D_GLIBCXX_DEBUG”,方可DEBUG进入find。
以下为i386环境。
g++ -g -o x x.cpp -D_GLIBCXX_DEBUG
Breakpoint 2, main () at x.cpp:6
6 std::string::size_type n = std::string::npos;
(gdb) n
7 std::string str = "123";
(gdb)
8 std::string::size_type m = str.find("2", n);
(gdb) s
std::string::find (this=0xbfb54a10, __s=0x804b8f2 "2", __pos=4294967295) at /usr/include/c++/4.1.2/bits/basic_string.h:1579
1579 return this->find(__s, __pos, traits_type::length(__s));
(gdb) s
std::char_traits::length (__s=0x804b8f2 "2") at /usr/include/c++/4.1.2/bits/char_traits.h:257
257 { return strlen(__s); }
(gdb) finish
Run till exit from #0 std::char_traits::length (__s=0x804b8f2 "2") at /usr/include/c++/4.1.2/bits/char_traits.h:257
0x0804a8d9 in std::string::find (this=0xbfb54a10, __s=0x804b8f2 "2", __pos=4294967295) at /usr/include/c++/4.1.2/bits/basic_string.h:1579
1579 return this->find(__s, __pos, traits_type::length(__s));
Value returned is $1 = 1
(gdb) s
std::string::find (this=0xbfb54a10, __s=0x804b8f2 "2", __pos=4294967295, __n=1) at /usr/include/c++/4.1.2/bits/basic_string.tcc:721
721 size_type __ret = npos;
723 if (__pos + __n <= __size)
(gdb) p __pos
$2 = 4294967295
(gdb) p __n
$3 = 1
(gdb) p __size
$4 = 3
(gdb) p __pos + __n
$5 = 0
以下为x86_64环境。
// g++ -g -o x x.cpp -m32 -D_GLIBCXX_DEBUG
Breakpoint 1, main () at x.cpp:6
6 std::string::size_type n = std::string::npos;
Missing separate debuginfos, use: debuginfo-install glibc-2.17-196.tl2.3.i686 libgcc-4.8.5-4.el7.i686 libstdc++-4.8.5-4.el7.i686
(gdb) n
7 std::string str = "123";
(gdb)
8 std::string::size_type m = str.find("2", n);
(gdb) s
std::string::find (this=0xffffd300, __s=0x80499d8 "2", __pos=4294967295) at /usr/include/c++/4.8.2/bits/basic_string.h:1864
1864 return this->find(__s, __pos, traits_type::length(__s));
(gdb) s
std::char_traits::length (__s=0x80499d8 "2") at /usr/include/c++/4.8.2/bits/char_traits.h:259
259 { return __builtin_strlen(__s); }
(gdb) finish
Run till exit from #0 std::char_traits::length (__s=0x80499d8 "2") at /usr/include/c++/4.8.2/bits/char_traits.h:259
0x0804931f in std::string::find (this=0xffffd300, __s=0x80499d8 "2", __pos=4294967295) at /usr/include/c++/4.8.2/bits/basic_string.h:1864
1864 return this->find(__s, __pos, traits_type::length(__s));
Value returned is $1 = 1
(gdb) s
std::string::find (this=0xffffd300, __s=0x80499d8 "2", __pos=4294967295, __n=1) at /usr/include/c++/4.8.2/bits/basic_string.tcc:740
740 const size_type __size = this->size();
// g++ -g -o a a.cpp -ldl -m32
#include
#include
int main()
{
typedef int (*X)();
void* p = dlopen("./libx.so", RTLD_NOW);
if (!p)
printf("dlopen failed: %s\n", dlerror());
else {
X x = (X)dlsym(p, "main");
(*x)();
}
return 0;
}
gcc-4.1.2版本的find源码,gcc-4.8.2的实现相同。
// basic_string.tcc
template
typename basic_string<_CharT, _Traits, _Alloc>::size_type
basic_string<_CharT, _Traits, _Alloc>::
find(_CharT __c, size_type __pos) const _GLIBCXX_NOEXCEPT
{
size_type __ret = npos;
const size_type __size = this->size();
if (__pos < __size)
{
const _CharT* __data = _M_data();
const size_type __n = __size - __pos;
const _CharT* __p = traits_type::find(__data + __pos, __n, __c);
if (__p)
__ret = __p - __data;
}
return __ret;
}
// /usr/include/c++/4.1.2/bits/basic_string.h
1575 size_type
1576 find(const _CharT* __s, size_type __pos = 0) const
1577 {
1578 __glibcxx_requires_string(__s);
1579 return this->find(__s, __pos, traits_type::length(__s));
1580 }
// /usr/include/c++/4.1.2/bits/basic_string.tcc
715 template
716 typename basic_string<_CharT, _Traits, _Alloc>::size_type
717 basic_string<_CharT, _Traits, _Alloc>::
718 find(const _CharT* __s, size_type __pos, size_type __n) const
719 {
720 __glibcxx_requires_string_len(__s, __n);
721 size_type __ret = npos;
722 const size_type __size = this->size();
723 if (__pos + __n <= __size) // 这里溢出
724 {
725 const _CharT* __data = _M_data();
726 const _CharT* __p = std::search(__data + __pos, __data + __size,
727 __s, __s + __n, traits_type::eq);
728 if (__p != __data + __size || __n == 0)
729 __ret = __p - __data;
730 }
731 return __ret;
732 }
实现和gcc-4.1.2不同了,新的实现不存在溢出漏洞。
// /usr/include/c++/4.8.2/bits/basic_string.h
1860 size_type
1861 find(const _CharT* __s, size_type __pos = 0) const
1862 {
1863 __glibcxx_requires_string(__s);
1864 return this->find(__s, __pos, traits_type::length(__s));
1865 }
// /usr/include/c++/4.8.2/bits/basic_string.tcc
734 template
735 typename basic_string<_CharT, _Traits, _Alloc>::size_type
736 basic_string<_CharT, _Traits, _Alloc>::
737 find(const _CharT* __s, size_type __pos, size_type __n) const
738 {
739 __glibcxx_requires_string_len(__s, __n);
740 const size_type __size = this->size();
741 const _CharT* __data = _M_data();
742
743 if (__n == 0)
744 return __pos <= __size ? __pos : npos;
745
746 if (__n <= __size) // 这里不存在溢出
747 {
748 for (; __pos <= __size - __n; ++__pos)
749 if (traits_type::eq(__data[__pos], __s[0])
750 && traits_type::compare(__data + __pos + 1,
751 __s + 1, __n - 1) == 0)
752 return __pos;
753 }
754 return npos;
755 }
一些低版本的find实现存在bug,存在溢出。注:std::string::size_type实际为size_t,是一个无符号整数类型,在i386上为4字节无符号整数类型,在x86_84上为8字节无符号整数类型,对应的有符号类型为ssize_t。
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