the 1st edition: 20131228
the 2nd edition: 20160219
the 3rd edition: 20180919
這篇不是在說明 overloaded operator new, 這件事情所有的 c++ 入門書籍都會告知你, 若你看的那本沒有, 可以考慮換掉它。
本範例一樣是作業系統等級的程式碼 (bare-metal), 在完成了 global/static object 的研究之後, 來看看怎麼實作 new/delete, 在 c++ 若不能使用 new/delete, 好像說不過去。
測試方式:
在 linux compile 成 dos .com 檔, 然後放到 qemu/dos 上執行。
no_impl_new_delete.cpp 1 __asm__ ( ".code16gcc\n " ) ;
2 #include "io.h" 3 #include "obj.h" 4
5 typedef signed char s8 ;
6 typedef signed short s16 ;
7 typedef signed int s32 ;
8
9 typedef unsigned char u8 ;
10 typedef unsigned short u16 ;
11 typedef unsigned int u32 ;
12
13 #define BOCHS_MB __asm__ __volatile__("xchg %bx, %bx" ); 14
15 extern int _start_ctors ;
16 extern int _end_ctors ;
17
18 void s16_print_int ( int i , int radix ) ;
19 void print_str ( const char * s ) ;
20
21 extern int obj_count ;
22
23
24 const int SOME_BASE_ADDRESS = 0x500 ;
25
26 void myfree ( void * ptr )
27 { 28 print_str ( "myfree\r \n " ) ;
29 } 30
31 void * mymalloc ( int size )
32 { 33 print_str ( "size: " ) ;
34 s16_print_int ( size , 10 ) ;
35 print_str ( "\r \n " ) ;
36 static char mem [ 256 ] ;
37 return mem ;
38 } 39
52
53 extern "C" int cpp_main ( void )
54 { 55 print_str ( "cpp_main\r \n " ) ;
56 s16_print_int ( obj_count , 10 ) ;
57 print_str ( "\r \n " ) ;
58
59 Io * io_p = new Io ; 60 const char * ver = __VERSION__ ;
61 io_p - > print ( "io_p g++ version: " ) ;
62 io_p - > print ( ver ) ;
63 io_p - > print ( "\r \n " ) ;
64 delete io_p ;
65
66 return 0 ;
67 }
L59 和我們平常使用的 c++ new 用法一模一樣, new 會做類似 malloc 的動作, 先去要一塊記憶體, 並呼叫 ctor, 可是在作業系統之前的程式, 根本沒有記憶體管理的程式碼 (記得我們沒有 c++ runtime 來幫我們做這件事情嗎?), 來看看編譯會怎麼樣?
descent@debian-vm:dos_cpp$ make new_test.bin
ld -m elf_i386 -static -Tcpp.ld -nostdlib -M -o new_test.elf cpp_init.o new_test.o io.o cpp_abi.o obj.o dos_io.o > new_test.elf.map
new_test.o: In function `cpp_main':
/home/descent/git/simple_os/cpp_runtime/global_object/dos_cpp/new_test.cpp:59: undefined reference to `operator new(unsigned int)'
/home/descent/git/simple_os/cpp_runtime/global_object/dos_cpp/new_test.cpp:64: undefined reference to `operator delete(void*)'
make: *** [new_test.elf] Error 1
呵呵, 果然有預期的錯誤, 要是沒錯誤反而令我害怕。訊息很容易理解, 沒定義 operator new, operator delete, 所以該怎麼辦? 不難, 自己定義就好了。
new_test.cpp L76, L81 提供了 operator new, operator delete, 比想像中還要簡單吧! 這時候再編譯就沒問題了。有問題的是那個 mymalloc 要怎麼寫是吧?
這個的確是個問題, 得先搞定記憶體管理才能把這部份寫好, 一開始也困擾我許久, 因為我這部份還沒做得很好 (有趣吧! 我的 simple os 竟然沒有記憶體管理機制), 不過我只想搞定 c++ runtime new/delete, 並不是要實作記憶體管理機制 (這個就不容易了), 所以想了個簡單的辦法來模擬 malloc 的行為。
L26, L31 的 mymalloc/myfree 應該很簡單, 簡單到不用說明吧?
new_test.cpp 1 __asm__(".code16gcc\n");
2 #include "io.h"
3 #include "obj.h"
4
5 typedef signed char s8;
6 typedef signed short s16;
7 typedef signed int s32;
8
9 typedef unsigned char u8;
10 typedef unsigned short u16;
11 typedef unsigned int u32;
12
13
14 #define BOCHS_MB __asm__ __volatile__("xchg %bx, %bx");
15
16 #if 0
17 Ab ab(7);
18 Io io;
19 Obj1 g_obj1;
20 #endif
21
22 extern int _start_ctors;
23 extern int _end_ctors;
24
25 void s16_print_int(int i, int radix);
26 void print_str(const char *s);
27
28 extern int obj_count;
29
30 u16 asm_get_ds(void)
31 {
32 u16 v=0;
33 __asm__ __volatile__
34 (
35 "mov %%ds, %%ax\n"
36 : "=a"(v)// output
37 );
38 return v;
39 }
40
41 u16 asm_get_cs(void)
42 {
43 u16 v=0;
44 __asm__ __volatile__
45 (
46 "mov %%cs, %%ax\n"
47 : "=a"(v)// output
48 );
49 return v;
50 }
51
52 const int SOME_BASE_ADDRESS = 0x500;
53
54 void myfree(void *ptr)
55 {
56 print_str("myfree\r\n");
57 }
58
59 void *mymalloc(int size)
60 {
61 print_str("mymalloc ## size: ");
62 s16_print_int(size, 10);
63 print_str("\r\n");
64 static char mem[256];
65 return mem;
66 #if 0
67 // ref: http://wiki.osdev.org/C%2B%2B_Exception_Support
68 static char* freeMemoryBase = reinterpret_cast<char *>(SOME_BASE_ADDRESS);
69 size = (size + 7) / 8 * 8;
70 freeMemoryBase += size;
71 return freeMemoryBase - size;
72 #endif
73 }
74
75 // ref: http://wiki.osdev.org/C%2B%2B#The_Operators_.27new.27_and_.27delete.27
76 void *operator new(unsigned int s)
77 {
78 return mymalloc(s);
79 }
80
81 void operator delete(void *p)
82 {
83 myfree(p);
84 }
85
86 // new/delete: array version
87
88 void *operator new[] (unsigned int s)
89 {
90 return mymalloc(s);
91 }
92
93 void operator delete[] (void *p)
94 {
95 myfree(p);
96 }
97
98 // new/delete: no exception version
99
100
101 struct nothrow_t { };
102
103 const nothrow_t nothrow;
104
105 void *operator new (unsigned int s, const nothrow_t&)
106 {
107 return mymalloc(s);
108 }
109
110
111 extern "C" int cpp_main(void)
112 {
113 print_str("cpp_main\r\n");
114 s16_print_int(obj_count, 10);
115 print_str("\r\n");
116
117 Io *io_p = new Io;
118 const char *ver=__VERSION__;
119 io_p->print("io_p g++ version: ");
120 io_p->print(ver);
121 io_p->print("\r\n");
122 delete io_p;
123
124 // test new array
125 int *pia = new int[1024];
126 delete [] pia;
127
128 // test no exception new
129
130 Io *ne_io_p = new(nothrow) Io;
131
153 return 0;
154 }
new_test.cpp L117 new 發動時, L76 operator new 會被 c++ 編譯器傳入正確的 class size, 在 Io class 這個例子, size 是 4, list 1 便是在 dos 下的執行結果 (qemu)。可以看到除了呼叫配置記憶體的程式碼之外, new 還會「順便」呼叫 constructor, 這才是 c++ 的 new 和 malloc 不一樣的地方, c++ compiler 會知道要喚起那些 class 的 constructor, 很神奇吧! c++ compiler 難寫是有道理的。
你寫了一行的 new, 但 c++ compiler 卻產生兩行 code, c++ 的黑箱作業可不是說假的。
list 1. qemu-system-i386 -fda dos622.img -nographic (使用 -nographic 需要 qemu 2.12 以上的版本) 1 iPXE (http://ipxe.org) 00:03.0 C980 PCI2.10 PnP PMM+07F92340+07EF2340 C980
2
3
4
5 Booting from Hard Disk...
6 Boot failed: could not read the boot disk
7
8 Booting from Floppy...
9 Starting MS-DOS...
10
11 A:\>new_test
12 cpp_main
13 0
14 mymalloc ## size: 4
15 Io ctor: data member
16 io_p g++ version: 7.3.0
17 io dtor
18 myfree
19 mymalloc ## size: 4096
20 myfree
21 mymalloc ## size: 4
22 Io ctor: data member
23 g_dtors
24 0
比起 global/static object, operator new/delete 簡單到令我驚訝 (當然, 真的要處理記憶體的分配與回收就沒那麼簡單了)。所以現在你知道在作業系統之下寫 c++ 程式, c++ runtime library 的辛勞了。
source code:
https://github.com/descent/simple_os
commit 93e327234d153636e315721ca66f73df0e7c0887
cpp_runtime branch
這是 g++ 裡頭的實作, 不難懂。
1 // Support routines for the -*- C++ -*- dynamic memory management.
2
3 // Copyright (C) 1997-2014 Free Software Foundation, Inc.
4 //
5 // This file is part of GCC.
6 //
7 // GCC is free software; you can redistribute it and/or modify
8 // it under the terms of the GNU General Public License as published by
9 // the Free Software Foundation; either version 3, or (at your option)
10 // any later version.
11 //
12 // GCC is distributed in the hope that it will be useful,
13 // but WITHOUT ANY WARRANTY; without even the implied warranty of
14 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 // GNU General Public License for more details.
16 //
17 // Under Section 7 of GPL version 3, you are granted additional
18 // permissions described in the GCC Runtime Library Exception, version
19 // 3.1, as published by the Free Software Foundation.
20
21 // You should have received a copy of the GNU General Public License and
22 // a copy of the GCC Runtime Library Exception along with this program;
23 // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
24 // <http://www.gnu.org/licenses/>.
25
26 #include <bits/c++config.h>
27 #include <cstdlib>
28 #include <bits/exception_defines.h>
29 #include "new"
30
31 using std::new_handler;
32 using std::bad_alloc;
33 #if _GLIBCXX_HOSTED
34 using std::malloc;
35 #else
36 // A freestanding C runtime may not provide "malloc" -- but there is no
37 // other reasonable way to implement "operator new".
38 extern "C" void *malloc (std::size_t);
39 #endif
40
41 _GLIBCXX_WEAK_DEFINITION void *
42 operator new (std::size_t sz) _GLIBCXX_THROW (std::bad_alloc)
43 {
44 void *p;
45
46 /* malloc (0) is unpredictable; avoid it. */
47 if (sz == 0)
48 sz = 1;
49 p = (void *) malloc (sz);
50 while (p == 0)
51 {
52 new_handler handler = std::get_new_handler ();
53 if (! handler)
54 _GLIBCXX_THROW_OR_ABORT(bad_alloc());
55 handler ();
56 p = (void *) malloc (sz);
57 }
58
59 return p;
60 }
除了最基本的 new, 還有另外一個版本, 不拋出 exception 的版本。這個 operator new 多了一個 const std::nothrow_t& 參數。
gcc-5.5.0/libstdc++-v3/libsupc++/new_opnt.cc 1 // Support routines for the -*- C++ -*- dynamic memory management.
2 // Copyright (C) 1997-2015 Free Software Foundation, Inc.
3 //
4 // This file is part of GCC.
5 //
6 // GCC is free software; you can redistribute it and/or modify
7 // it under the terms of the GNU General Public License as published by
8 // the Free Software Foundation; either version 3, or (at your option)
9 // any later version.
10 //
11 // GCC is distributed in the hope that it will be useful,
12 // but WITHOUT ANY WARRANTY; without even the implied warranty of
13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 // GNU General Public License for more details.
15 //
16 // Under Section 7 of GPL version 3, you are granted additional
17 // permissions described in the GCC Runtime Library Exception, version
18 // 3.1, as published by the Free Software Foundation.
19
20 // You should have received a copy of the GNU General Public License and
21 // a copy of the GCC Runtime Library Exception along with this program;
22 // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
23 // <http://www.gnu.org/licenses/>.
24
25 #include <bits/c++config.h>
26 #include <bits/exception_defines.h>
27 #include "new"
28
29 using std::new_handler;
30 using std::bad_alloc;
31
32 extern "C" void *malloc (std::size_t);
33
34 _GLIBCXX_WEAK_DEFINITION void *
35 operator new (std::size_t sz, const std::nothrow_t&) _GLIBCXX_USE_NOEXCEPT
36 {
37 void *p;
38
39 /* malloc (0) is unpredictable; avoid it. */
40 if (sz == 0)
41 sz = 1;
42
43 while (__builtin_expect ((p = malloc (sz)) == 0, false))
44 {
45 new_handler handler = std::get_new_handler ();
46 if (! handler)
47 return 0;
48 __try
49 {
50 handler ();
51 }
52 __catch(const bad_alloc&)
53 {
54 return 0;
55 }
56 }
57
58 return p;
59 }
gcc-5.5.0/libstdc++-v3/libsupc++/new 1 // The -*- C++ -*- dynamic memory management header.
2
3 // Copyright (C) 1994-2015 Free Software Foundation, Inc.
4
5 // This file is part of GCC.
6 //
7 // GCC is free software; you can redistribute it and/or modify
8 // it under the terms of the GNU General Public License as published by
9 // the Free Software Foundation; either version 3, or (at your option)
10 // any later version.
11 //
12 // GCC is distributed in the hope that it will be useful,
13 // but WITHOUT ANY WARRANTY; without even the implied warranty of
14 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 // GNU General Public License for more details.
16 //
17 // Under Section 7 of GPL version 3, you are granted additional
18 // permissions described in the GCC Runtime Library Exception, version
19 // 3.1, as published by the Free Software Foundation.
20
21 // You should have received a copy of the GNU General Public License and
22 // a copy of the GCC Runtime Library Exception along with this program;
23 // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
24 // <http://www.gnu.org/licenses/>.
25
26 /** @file new
27 * This is a Standard C++ Library header.
28 *
29 * The header @c new defines several functions to manage dynamic memory and
30 * handling memory allocation errors; see
31 * http://gcc.gnu.org/onlinedocs/libstdc++/18_support/howto.html#4 for more.
32 */
33
34 #ifndef _NEW
35 #define _NEW
36
37 #pragma GCC system_header
38
39 #include <bits/c++config.h>
40 #include <exception>
41
42 #pragma GCC visibility push(default)
43
44 extern "C++" {
45
46 namespace std
47 {
48 /**
49 * @brief Exception possibly thrown by @c new.
50 * @ingroup exceptions
51 *
52 * @c bad_alloc (or classes derived from it) is used to report allocation
53 * errors from the throwing forms of @c new. */
54 class bad_alloc : public exception
55 {
56 public:
57 bad_alloc() throw() { }
58
59 // This declaration is not useless:
60 // http://gcc.gnu.org/onlinedocs/gcc-3.0.2/gcc_6.html#SEC118
61 virtual ~bad_alloc() throw();
62
63 // See comment in eh_exception.cc.
64 virtual const char* what() const throw();
65 };
66
67 #if __cplusplus >= 201103L
68 class bad_array_new_length : public bad_alloc
69 {
70 public:
71 bad_array_new_length() throw() { };
72
73 // This declaration is not useless:
74 // http://gcc.gnu.org/onlinedocs/gcc-3.0.2/gcc_6.html#SEC118
75 virtual ~bad_array_new_length() throw();
76
77 // See comment in eh_exception.cc.
78 virtual const char* what() const throw();
79 };
80 #endif
81
82 struct nothrow_t { };
83
84 extern const nothrow_t nothrow;
85
86 /** If you write your own error handler to be called by @c new, it must
87 * be of this type. */
88 typedef void (*new_handler)();
89
90 /// Takes a replacement handler as the argument, returns the
91 /// previous handler.
92 new_handler set_new_handler(new_handler) throw();
93
94 #if __cplusplus >= 201103L
95 /// Return the current new handler.
96 new_handler get_new_handler() noexcept;
97 #endif
98 } // namespace std
99
100 //@{
101 /** These are replaceable signatures:
102 * - normal single new and delete (no arguments, throw @c bad_alloc on error)
103 * - normal array new and delete (same)
104 * - @c nothrow single new and delete (take a @c nothrow argument, return
105 * @c NULL on error)
106 * - @c nothrow array new and delete (same)
107 *
108 * Placement new and delete signatures (take a memory address argument,
109 * does nothing) may not be replaced by a user's program.
110 */
111 void* operator new(std::size_t) _GLIBCXX_THROW (std::bad_alloc)
112 __attribute__((__externally_visible__));
113 void* operator new[](std::size_t) _GLIBCXX_THROW (std::bad_alloc)
114 __attribute__((__externally_visible__));
115 void operator delete(void*) _GLIBCXX_USE_NOEXCEPT
116 __attribute__((__externally_visible__));
117 void operator delete[](void*) _GLIBCXX_USE_NOEXCEPT
118 __attribute__((__externally_visible__));
119 void* operator new(std::size_t, const std::nothrow_t&) _GLIBCXX_USE_NOEXCEPT
120 __attribute__((__externally_visible__));
121 void* operator new[](std::size_t, const std::nothrow_t&) _GLIBCXX_USE_NOEXCEPT
122 __attribute__((__externally_visible__));
123 void operator delete(void*, const std::nothrow_t&) _GLIBCXX_USE_NOEXCEPT
124 __attribute__((__externally_visible__));
125 void operator delete[](void*, const std::nothrow_t&) _GLIBCXX_USE_NOEXCEPT
126 __attribute__((__externally_visible__));
127
128 // Default placement versions of operator new.
129 inline void* operator new(std::size_t, void* __p) _GLIBCXX_USE_NOEXCEPT
130 { return __p; }
131 inline void* operator new[](std::size_t, void* __p) _GLIBCXX_USE_NOEXCEPT
132 { return __p; }
133
134 // Default placement versions of operator delete.
135 inline void operator delete (void*, void*) _GLIBCXX_USE_NOEXCEPT { }
136 inline void operator delete[](void*, void*) _GLIBCXX_USE_NOEXCEPT { }
137 //@}
138 } // extern "C++"
139
140 #pragma GCC visibility pop
141
142 #endif
再來還有一個 new array 的版本:
new_opv.cc 1 // Boilerplate support routines for -*- C++ -*- dynamic memory management.
2
3 // Copyright (C) 1997-2015 Free Software Foundation, Inc.
4 //
5 // This file is part of GCC.
6 //
7 // GCC is free software; you can redistribute it and/or modify
8 // it under the terms of the GNU General Public License as published by
9 // the Free Software Foundation; either version 3, or (at your option)
10 // any later version.
11 //
12 // GCC is distributed in the hope that it will be useful,
13 // but WITHOUT ANY WARRANTY; without even the implied warranty of
14 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 // GNU General Public License for more details.
16 //
17 // Under Section 7 of GPL version 3, you are granted additional
18 // permissions described in the GCC Runtime Library Exception, version
19 // 3.1, as published by the Free Software Foundation.
20
21 // You should have received a copy of the GNU General Public License and
22 // a copy of the GCC Runtime Library Exception along with this program;
23 // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
24 // <http://www.gnu.org/licenses/>.
25
26 #include <bits/c++config.h>
27 #include "new"
28
29 _GLIBCXX_WEAK_DEFINITION void*
30 operator new[] (std::size_t sz) _GLIBCXX_THROW (std::bad_alloc)
31 {
32 return ::operator new(sz);
33 }
delete 當然也有一個對應的 array 版本:
del_opv.cc 1 // Boilerplate support routines for -*- C++ -*- dynamic memory management.
2
3 // Copyright (C) 1997-2015 Free Software Foundation, Inc.
4 //
5 // This file is part of GCC.
6 //
7 // GCC is free software; you can redistribute it and/or modify
8 // it under the terms of the GNU General Public License as published by
9 // the Free Software Foundation; either version 3, or (at your option)
10 // any later version.
11 //
12 // GCC is distributed in the hope that it will be useful,
13 // but WITHOUT ANY WARRANTY; without even the implied warranty of
14 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 // GNU General Public License for more details.
16 //
17 // Under Section 7 of GPL version 3, you are granted additional
18 // permissions described in the GCC Runtime Library Exception, version
19 // 3.1, as published by the Free Software Foundation.
20
21 // You should have received a copy of the GNU General Public License and
22 // a copy of the GCC Runtime Library Exception along with this program;
23 // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
24 // <http://www.gnu.org/licenses/>.
25
26 #include <bits/c++config.h>
27 #include "new"
28
29 _GLIBCXX_WEAK_DEFINITION void
30 operator delete[] (void *ptr) _GLIBCXX_USE_NOEXCEPT
31 {
32 ::operator delete (ptr);
33 }
new_test.cpp L88, 93, 105 是補上這些版本。
ref:
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使用 google 的 reCAPTCHA 驗證碼, 總算可以輕鬆留言了。
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