昨天晚上下載了一份GCC V1.42的代碼，不知道是源代碼本身有問題，還是下載的源代碼有問題，看的第一個C文件就存在一些很
奇怪的情況。
首先要說的是： alloca.c 文件的作用，alloca.c文件的函數實現動態堆空間的分配，即運行時堆棧空間分配。
【1】源代碼
1 /* 2 alloca -- (mostly) portable public-domain implementation -- D A Gwyn 3 4 last edit: 86/05/30 rms 5 include config.h, since on VMS it renames some symbols. 6 Use xmalloc instead of malloc. 7 8 This implementation of the PWB library alloca() function, 9 which is used to allocate space off the run-time stack so 10 that it is automatically reclaimed upon procedure exit, 11 was inspired by discussions with J. Q. Johnson of Cornell. 12 13 It should work under any C implementation that uses an 14 actual procedure stack (as opposed to a linked list of 15 frames). There are some preprocessor constants that can 16 be defined when compiling for your specific system, for 17 improved efficiency; however, the defaults should be okay. 18 19 The general concept of this implementation is to keep 20 track of all alloca()-allocated blocks, and reclaim any 21 that are found to be deeper in the stack than the current 22 invocation. This heuristic does not reclaim storage as 23 soon as it becomes invalid, but it will do so eventually. 24 25 As a special case, alloca(0) reclaims storage without 26 allocating any. It is a good idea to use alloca(0) in 27 your main control loop, etc. to force garbage collection. 28 */ 29 30 31 #ifndef lint 32 static char SCCSid[] = "@(#)alloca.c 1.1"; /* for the "what" utility */ 33 #endif 34 35 #ifdef emacs 36 #include "config.h" 37 #ifdef static 38 /* actually, only want this if static is defined as " " 39 -- this is for usg, in which emacs must undefine static 40 in order to make unexec workable 41 */ 42 #ifndef STACK_DIRECTION 43 you 44 lose 45 -- must know STACK_DIRECTION at compile-time 46 #endif /* STACK_DIRECTION undefined */ 47 #endif static 48 #endif emacs 49 50 51 //很顯然這里是對 X3J11 規范的定義 52 #ifdef X3J11 53 typedef void *pointer; /* generic pointer type */ 54 #else 55 typedef char *pointer; /* generic pointer type */ 56 #endif 57 58 //為什么這個地方不定義為 (void*) 59 #define NULL 0 /* null pointer constant */ 60 61 extern void free(); 62 extern pointer xmalloc(); 63 64 /* 65 Define STACK_DIRECTION if you know the direction of stack 66 growth for your system; otherwise it will be automatically 67 deduced at run-time. 68 69 STACK_DIRECTION > 0 => grows toward higher addresses 70 STACK_DIRECTION < 0 => grows toward lower addresses 71 STACK_DIRECTION = 0 => direction of growth unknown 72 */ 73 74 75 //查看是否定義了棧增長方向宏定義 76 #ifndef STACK_DIRECTION 77 #define STACK_DIRECTION 0 /* direction unknown */ 78 #endif 79 80 //定義棧空間的增長方向宏 STACK_DIR 81 #if STACK_DIRECTION != 0 82 #define STACK_DIR STACK_DIRECTION /* known at compile-time */ 83 #else /* STACK_DIRECTION == 0; need run-time code */ 84 static int stack_dir; /* 1 or -1 once known */ 85 #define STACK_DIR stack_dir 86 //下面的函數用來判斷棧的增長方向 87 static void 88 find_stack_direction (/* void */) 89 { 90 static char *addr = NULL; /* address of first `dummy', once known */ 91 auto char dummy; /* to get stack address */ 92 93 if (addr == NULL) 94 { /* initial entry */ 95 addr = &dummy; 96 find_stack_direction (); /* recurse once */ 97 } 98 else /* second entry */ 99 if (&dummy > addr) 100 stack_dir = 1; /* stack grew upward */ 101 else 102 stack_dir = -1; /* stack grew downward */ 103 } 104 #endif /* STACK_DIRECTION == 0 */ 105 106 107 /* 108 An "alloca header" is used to: 109 (a) chain together all alloca()ed blocks; 110 (b) keep track of stack depth. 111 112 It is very important that sizeof(header) agree with malloc() 113 alignment chunk size. The following default should work okay. 114 */ 115 116 #ifndef ALIGN_SIZE 117 #define ALIGN_SIZE sizeof(double) 118 #endif 119 120 typedef union hdr 121 { 122 char align[ALIGN_SIZE]; /* to force sizeof(header) */ 123 struct 124 { 125 union hdr *next; /* for chaining headers */ 126 char *deep; /* for stack depth measure */ 127 } h; 128 } header; 129 130 /* 131 alloca( size ) returns a pointer to at least `size' bytes of 132 storage which will be automatically reclaimed upon exit from 133 the procedure that called alloca(). Originally, this space 134 was supposed to be taken from the current stack frame of the 135 caller, but that method cannot be made to work for some 136 implementations of C, for example under Gould's UTX/32. 137 */ 138 139 //全局變量用來存儲 棧 指針 140 static header *last_alloca_header = NULL; /* -> last alloca header */ 141 142 //動態堆分配函數， 這個函數有點類似 malloc 函數 143 //但是這個函數具有垃圾回收機制 144 pointer 145 alloca (size) /* returns pointer to storage */ 146 unsigned size; /* # bytes to allocate */ 147 { 148 auto char probe; /* probes stack depth: */ 149 register char *depth = &probe; 150 151 #if STACK_DIRECTION == 0 152 if (STACK_DIR == 0) /* unknown growth direction */ 153 find_stack_direction (); 154 #endif 155 156 /* Reclaim garbage, defined as all alloca()ed storage that 157 was allocated from deeper in the stack than currently. */ 158 { 159 register header *hp; /* traverses linked list */ 160 161 //可以發現這里for循環的語法非常特殊 162 for (hp = last_alloca_header; hp != NULL;) 163 if (STACK_DIR > 0 && hp->h.deep > depth || STACK_DIR < 0 && hp->h.deep < depth) 164 { 165 register header *np = hp->h.next; 166 free ((pointer) hp); /* collect garbage */ 167 hp = np; /* -> next header */ 168 } 169 else 170 break; /* rest are not deeper */ 171 172 last_alloca_header = hp; /* -> last valid storage */ 173 } 174 175 if (size == 0) 176 return NULL; /* no allocation required */ 177 178 /* Allocate combined header + user data storage. */ 179 { 180 register pointer new = xmalloc (sizeof (header) + size); 181 /* address of header */ 182 183 ((header *)new)->h.next = last_alloca_header; 184 ((header *)new)->h.deep = depth; 185 186 last_alloca_header = (header *)new; 187 188 /* User storage begins just after header. */ 189 return (pointer)((char *)new + sizeof(header)); 190 } 191 192 }
【2】第一個預處理
#ifndef lint static char SCCSid[] = "@(#)alloca.c 1.1"; /* for the "what" utility */ #endif
這個語句的作用就是定義一個描述源文件作用的數組，同時這個定義在V1.42中是一定定義的；通過代碼分析工具查看交叉索引可以知道：
---- lint Matches (4 in 2 files) ---- Alloca.c (g:\15_gcc\gcc_v1_42\gcc-1.42):#ifndef lint Va-mips.h (g:\15_gcc\gcc_v1_42\gcc-1.42):#ifdef lint /* complains about constant in conditional context */ Va-mips.h (g:\15_gcc\gcc_v1_42\gcc-1.42):#else /* !lint */ Va-mips.h (g:\15_gcc\gcc_v1_42\gcc-1.42):#endif /* lint */
可以發現在V1.42中並沒有定義這個宏符號：lint
因此可以確定，SCCSid數組必定定義，而且具有文件內可引用屬性。
【3】第二個預處理
#ifdef emacs #include "config.h" #ifdef static /* actually, only want this if static is defined as " " -- this is for usg, in which emacs must undefine static in order to make unexec workable */ #ifndef STACK_DIRECTION you lose -- must know STACK_DIRECTION at compile-time #endif /* STACK_DIRECTION undefined */ #endif static #endif emacs
同樣，在V1.42中並沒有emacs宏符號的定義，因此這個預處理語句是不會被解釋的
【4】第三個預處理
//很顯然這里是對 X3J11 規范的定義判斷 #ifdef X3J11 typedef void *pointer; /* generic pointer type */ #else typedef char *pointer; /* generic pointer type */ #endif
同樣在V1.42中並沒有定義X3J11宏符號，因此 這里定義一個新的數據類型pointer, 其實質數據類型為 char*
這里用X3J11表示的是ANSI C標准。
【5】定義宏，和聲明外部符號
//為什么這個地方不定義為 (void*) #define NULL 0 /* null pointer constant */ extern void free(); extern pointer xmalloc();
這里，xmalloc 函數相當於malloc函數，用來分配存儲空間
【6】判斷系統棧空間增加方向
//查看是否定義了棧增長方向宏定義 #ifndef STACK_DIRECTION #define STACK_DIRECTION 0 /* direction unknown */ #endif //定義棧空間的增長方向宏 STACK_DIR #if STACK_DIRECTION != 0 #define STACK_DIR STACK_DIRECTION /* known at compile-time */ #else /* STACK_DIRECTION == 0; need run-time code */ static int stack_dir; /* 1 or -1 once known */ #define STACK_DIR stack_dir //下面的函數用來判斷棧的增長方向 static void find_stack_direction (/* void */) { static char *addr = NULL; /* address of first `dummy', once known */ auto char dummy; /* to get stack address */ if (addr == NULL) { /* initial entry */ addr = &dummy; find_stack_direction (); /* recurse once */ } else /* second entry */ if (&dummy > addr) stack_dir = 1; /* stack grew upward */ else stack_dir = -1; /* stack grew downward */ } #endif /* STACK_DIRECTION == 0 */
這里利用一個遞歸函數，find_stack_direction 來判斷棧增長方向，是向高地址方向增長，還是地址方向增長。這里利用了一個static
local variable addr 和一個 auto local variable dummy 來判斷增長方向。
思路比較巧妙。
static：
1、global 變量，則將變量作用域限制在單個源文件
2、函數，則函數不能被定義函數源文件外的函數調用
3、local 變量， 延長變量生命周期
【7】堆 空間指針類型
#ifndef ALIGN_SIZE #define ALIGN_SIZE sizeof(double) #endif typedef union hdr { char align[ALIGN_SIZE]; /* to force sizeof(header) */ struct { union hdr *next; /* for chaining headers */ char *deep; /* for stack depth measure */ } h; } header;
定義了數據類型 header ，並且是用遞歸的聯合體定義的， 聯合體中定義成語域 align的目的是為了數據對齊；在下面的源代碼中並沒有
對這個成員域的引用
【8】堆 空間指針變量
//全局變量用來存儲 棧 指針 static header *last_alloca_header = NULL; /* -> last alloca header */
這里注釋： last_alloca_header 用來指向最新分配的空間頭（基指針）
【9】alloca函數
1 //動態堆分配函數， 這個函數有點類似 malloc 函數 2 //但是這個函數具有垃圾回收機制 3 pointer 4 alloca (size) /* returns pointer to storage */ 5 unsigned size; /* # bytes to allocate */ 6 { 7 auto char probe; /* probes stack depth: */ 8 register char *depth = &probe; 9 10 #if STACK_DIRECTION == 0 11 if (STACK_DIR == 0) /* unknown growth direction */ 12 find_stack_direction (); 13 #endif 14 15 /* Reclaim garbage, defined as all alloca()ed storage that 16 was allocated from deeper in the stack than currently. */ 17 { 18 register header *hp; /* traverses linked list */ 19 20 //可以發現這里for循環的語法非常特殊 21 for (hp = last_alloca_header; hp != NULL;) 22 if (STACK_DIR > 0 && hp->h.deep > depth || STACK_DIR < 0 && hp->h.deep < depth) 23 { 24 register header *np = hp->h.next; 25 free ((pointer) hp); /* collect garbage */ 26 hp = np; /* -> next header */ 27 } 28 else 29 break; /* rest are not deeper */ 30 31 last_alloca_header = hp; /* -> last valid storage */ 32 } 33 34 if (size == 0) 35 return NULL; /* no allocation required */ 36 37 /* Allocate combined header + user data storage. */ 38 { 39 register pointer new = xmalloc (sizeof (header) + size); 40 /* address of header */ 41 42 ((header *)new)->h.next = last_alloca_header; 43 ((header *)new)->h.deep = depth; 44 45 last_alloca_header = (header *)new; 46 47 /* User storage begins just after header. */ 48 return (pointer)((char *)new + sizeof(header)); 49 } 50 51 }
函數原型：
pointer alloca (size) /* returns pointer to storage */ unsigned size;
可以發現這里采用的老式的Ｃ語言函數定義形式，或者函數原型聲明形式。
局部變量：
auto char probe; /* probes stack depth: */ register char *depth = &probe;
局部變量用來檢測堆棧的深度，
register:
1、聲明為register的變量，表示要存儲在CPU的寄存器中，提高存儲效率。
判斷堆棧增長方向：
#if STACK_DIRECTION == 0 if (STACK_DIR == 0) /* unknown growth direction */ find_stack_direction (); #endif
增長結果存儲在stack_dir全局變量中。
回收內存：
/* Reclaim garbage, defined as all alloca()ed storage that was allocated from deeper in the stack than currently. */ { register header *hp; /* traverses linked list */ //可以發現這里for循環的語法非常特殊 for (hp = last_alloca_header; hp != NULL;) if (STACK_DIR > 0 && hp->h.deep > depth || STACK_DIR < 0 && hp->h.deep < depth) { register header *np = hp->h.next; free ((pointer) hp); /* collect garbage */ hp = np; /* -> next header */ } else break; /* rest are not deeper */ last_alloca_header = hp; /* -> last valid storage */ }
分配空間
如果申請的空間為0，則返回空指針。
if (size == 0) return NULL; /* no allocation required */
如果申請的空間不為0 ，則返回申請空間的地址
/* Allocate combined header + user data storage. */ { register pointer new = xmalloc (sizeof (header) + size); /* address of header */ ((header *)new)->h.next = last_alloca_header; ((header *)new)->h.deep = depth; last_alloca_header = (header *)new; /* User storage begins just after header. */ return (pointer)((char *)new + sizeof(header)); } }
調用的主要函數：xmalloc的定義如下
int xmalloc (size) { register int val = malloc (size); if (val == 0) fatal ("virtual memory exhausted"); return val; }
可以發現，這個函數還是調用的庫函數malloc來實現的，但是這個函數有一個特點，其返回值是一個int類型的值，也就是說這個函數
返回的可能是一個負值。
反正我下載的這個代碼庫的源代碼感覺是不對的，但是這里面一些思想和內容大體是對的，因此可以學習。
下面是我看到的bash 4.0 中關於這個函數alloca的源代碼：
/** alloca.c -- allocate automatically reclaimed memory (Mostly) portable public-domain implementation -- D A Gwyn This implementation of the PWB library alloca function, which is used to allocate space off the run-time stack so that it is automatically reclaimed upon procedure exit, was inspired by discussions with J. Q. Johnson of Cornell. J.Otto Tennant <jot@cray.com> contributed the Cray support. There are some preprocessor constants that can be defined when compiling for your specific system, for improved efficiency; however, the defaults should be okay. The general concept of this implementation is to keep track of all alloca-allocated blocks, and reclaim any that are found to be deeper in the stack than the current invocation. This heuristic does not reclaim storage as soon as it becomes invalid, but it will do so eventually. As a special case, alloca(0) reclaims storage without allocating any. It is a good idea to use alloca(0) in your main control loop, etc. to force garbage collection. */ #ifdef HAVE_CONFIG_H #include "config.h" #endif /** If compiling with GCC 2, this file's not needed. */ #if !defined (__GNUC__) || __GNUC__ < 2 #include <bashtypes.h> /** for size_t */ /** If alloca is defined somewhere, this file is not needed. */ #ifndef alloca #ifdef emacs #ifdef static /** actually, only want this if static is defined as "" -- this is for usg, in which emacs must undefine static in order to make unexec workable */ #ifndef STACK_DIRECTION you lose -- must know STACK_DIRECTION at compile-time #endif /** STACK_DIRECTION undefined */ #endif /** static */ #endif /** emacs */ /** If your stack is a linked list of frames, you have to provide an "address metric" ADDRESS_FUNCTION macro. */ #if defined (CRAY) && defined (CRAY_STACKSEG_END) long i00afunc (); #define ADDRESS_FUNCTION(arg) (char *) i00afunc (&(arg)) #else #define ADDRESS_FUNCTION(arg) &(arg) #endif /** CRAY && CRAY_STACKSEG_END */ #if __STDC__ typedef void *pointer; #else typedef char *pointer; #endif #define NULL 0 /** Different portions of Emacs need to call different versions of malloc. The Emacs executable needs alloca to call xmalloc, because ordinary malloc isn't protected from input signals. On the other hand, the utilities in lib-src need alloca to call malloc; some of them are very simple, and don't have an xmalloc routine. Non-Emacs programs expect this to call use xmalloc. Callers below should use malloc. */ #ifndef emacs #define malloc xmalloc extern pointer xmalloc (); #endif /** Define STACK_DIRECTION if you know the direction of stack growth for your system; otherwise it will be automatically deduced at run-time. STACK_DIRECTION > 0 => grows toward higher addresses STACK_DIRECTION < 0 => grows toward lower addresses STACK_DIRECTION = 0 => direction of growth unknown */ #ifndef STACK_DIRECTION #define STACK_DIRECTION 0 /** Direction unknown. */ #endif #if STACK_DIRECTION != 0 #define STACK_DIR STACK_DIRECTION /** Known at compile-time. */ #else /** STACK_DIRECTION == 0; need run-time code. */ static int stack_dir; /** 1 or -1 once known. */ #define STACK_DIR stack_dir static void find_stack_direction () { static char *addr = NULL; /** Address of first `dummy', once known. */ auto char dummy; /** To get stack address. */ if (addr == NULL) { /** Initial entry. */ addr = ADDRESS_FUNCTION (dummy); find_stack_direction (); /** Recurse once. */ } else { /** Second entry. */ if (ADDRESS_FUNCTION (dummy) > addr) stack_dir = 1; /** Stack grew upward. */ else stack_dir = -1; /** Stack grew downward. */ } } #endif /** STACK_DIRECTION == 0 */ /** An "alloca header" is used to: (a) chain together all alloca'ed blocks; (b) keep track of stack depth. It is very important that sizeof(header) agree with malloc alignment chunk size. The following default should work okay. */ #ifndef ALIGN_SIZE #define ALIGN_SIZE sizeof(double) #endif typedef union hdr { char align[ALIGN_SIZE]; /** To force sizeof(header). */ struct { union hdr *next; /** For chaining headers. */ char *deep; /** For stack depth measure. */ } h; } header; static header *last_alloca_header = NULL; /** -> last alloca header. */ /** Return a pointer to at least SIZE bytes of storage, which will be automatically reclaimed upon exit from the procedure that called alloca. Originally, this space was supposed to be taken from the current stack frame of the caller, but that method cannot be made to work for some implementations of C, for example under Gould's UTX/32. */ pointer alloca (size) size_t size; { auto char probe; /** Probes stack depth: */ register char *depth = ADDRESS_FUNCTION (probe); #if STACK_DIRECTION == 0 if (STACK_DIR == 0) /** Unknown growth direction. */ find_stack_direction (); #endif /** Reclaim garbage, defined as all alloca'd storage that was allocated from deeper in the stack than currently. */ { register header *hp; /** Traverses linked list. */ for (hp = last_alloca_header; hp != NULL;) if ((STACK_DIR > 0 && hp->h.deep > depth) || (STACK_DIR < 0 && hp->h.deep < depth)) { register header *np = hp->h.next; free ((pointer) hp); /** Collect garbage. */ hp = np; /** -> next header. */ } else break; /** Rest are not deeper. */ last_alloca_header = hp; /** -> last valid storage. */ } if (size == 0) return NULL; /** No allocation required. */ /** Allocate combined header + user data storage. */ { register pointer new = malloc (sizeof (header) + size); /** Address of header. */ ((header *) new)->h.next = last_alloca_header; ((header *) new)->h.deep = depth; last_alloca_header = (header *) new; /** User storage begins just after header. */ return (pointer) ((char *) new + sizeof (header)); } } #if defined (CRAY) && defined (CRAY_STACKSEG_END) #ifdef DEBUG_I00AFUNC #include <stdio.h> #endif #ifndef CRAY_STACK #define CRAY_STACK #ifndef CRAY2 /** Stack structures for CRAY-1, CRAY X-MP, and CRAY Y-MP */ struct stack_control_header { long shgrow:32; /** Number of times stack has grown. */ long shaseg:32; /** Size of increments to stack. */ long shhwm:32; /** High water mark of stack. */ long shsize:32; /** Current size of stack (all segments). */ }; /** The stack segment linkage control information occurs at the high-address end of a stack segment. (The stack grows from low addresses to high addresses.) The initial part of the stack segment linkage control information is 0200 (octal) words. This provides for register storage for the routine which overflows the stack. */ struct stack_segment_linkage { long ss[0200]; /** 0200 overflow words. */ long sssize:32; /** Number of words in this segment. */ long ssbase:32; /** Offset to stack base. */ long:32; long sspseg:32; /** Offset to linkage control of previous segment of stack. */ long:32; long sstcpt:32; /** Pointer to task common address block. */ long sscsnm; /** Private control structure number for microtasking. */ long ssusr1; /** Reserved for user. */ long ssusr2; /** Reserved for user. */ long sstpid; /** Process ID for pid based multi-tasking. */ long ssgvup; /** Pointer to multitasking thread giveup. */ long sscray[7]; /** Reserved for Cray Research. */ long ssa0; long ssa1; long ssa2; long ssa3; long ssa4; long ssa5; long ssa6; long ssa7; long sss0; long sss1; long sss2; long sss3; long sss4; long sss5; long sss6; long sss7; }; #else /** CRAY2 */ /** The following structure defines the vector of words returned by the STKSTAT library routine. */ struct stk_stat { long now; /** Current total stack size. */ long maxc; /** Amount of contiguous space which would be required to satisfy the maximum stack demand to date. */ long high_water; /** Stack high-water mark. */ long overflows; /** Number of stack overflow ($STKOFEN) calls. */ long hits; /** Number of internal buffer hits. */ long extends; /** Number of block extensions. */ long stko_mallocs; /** Block allocations by $STKOFEN. */ long underflows; /** Number of stack underflow calls ($STKRETN). */ long stko_free; /** Number of deallocations by $STKRETN. */ long stkm_free; /** Number of deallocations by $STKMRET. */ long segments; /** Current number of stack segments. */ long maxs; /** Maximum number of stack segments so far. */ long pad_size; /** Stack pad size. */ long current_address; /** Current stack segment address. */ long current_size; /** Current stack segment size. This number is actually corrupted by STKSTAT to include the fifteen word trailer area. */ long initial_address; /** Address of initial segment. */ long initial_size; /** Size of initial segment. */ }; /** The following structure describes the data structure which trails any stack segment. I think that the description in 'asdef' is out of date. I only describe the parts that I am sure about. */ struct stk_trailer { long this_address; /** Address of this block. */ long this_size; /** Size of this block (does not include this trailer). */ long unknown2; long unknown3; long link; /** Address of trailer block of previous segment. */ long unknown5; long unknown6; long unknown7; long unknown8; long unknown9; long unknown10; long unknown11; long unknown12; long unknown13; long unknown14; }; #endif /** CRAY2 */ #endif /** not CRAY_STACK */ #ifdef CRAY2 /** Determine a "stack measure" for an arbitrary ADDRESS. I doubt that "lint" will like this much. */ static long i00afunc (long *address) { struct stk_stat status; struct stk_trailer *trailer; long *block, size; long result = 0; /** We want to iterate through all of the segments. The first step is to get the stack status structure. We could do this more quickly and more directly, perhaps, by referencing the $LM00 common block, but I know that this works. */ STKSTAT (&status); /** Set up the iteration. */ trailer = (struct stk_trailer *) (status.current_address + status.current_size - 15); /** There must be at least one stack segment. Therefore it is a fatal error if "trailer" is null. */ if (trailer == 0) abort (); /** Discard segments that do not contain our argument address. */ while (trailer != 0) { block = (long *) trailer->this_address; size = trailer->this_size; if (block == 0 || size == 0) abort (); trailer = (struct stk_trailer *) trailer->link; if ((block <= address) && (address < (block + size))) break; } /** Set the result to the offset in this segment and add the sizes of all predecessor segments. */ result = address - block; if (trailer == 0) { return result; } do { if (trailer->this_size <= 0) abort (); result += trailer->this_size; trailer = (struct stk_trailer *) trailer->link; } while (trailer != 0); /** We are done. Note that if you present a bogus address (one not in any segment), you will get a different number back, formed from subtracting the address of the first block. This is probably not what you want. */ return (result); } #else /** not CRAY2 */ /** Stack address function for a CRAY-1, CRAY X-MP, or CRAY Y-MP. Determine the number of the cell within the stack, given the address of the cell. The purpose of this routine is to linearize, in some sense, stack addresses for alloca. */ static long i00afunc (long address) { long stkl = 0; long size, pseg, this_segment, stack; long result = 0; struct stack_segment_linkage *ssptr; /** Register B67 contains the address of the end of the current stack segment. If you (as a subprogram) store your registers on the stack and find that you are past the contents of B67, you have overflowed the segment. B67 also points to the stack segment linkage control area, which is what we are really interested in. */ /** This might be _getb67() or GETB67 () or getb67 () */ stkl = CRAY_STACKSEG_END (); ssptr = (struct stack_segment_linkage *) stkl; /** If one subtracts 'size' from the end of the segment, one has the address of the first word of the segment. If this is not the first segment, 'pseg' will be nonzero. */ pseg = ssptr->sspseg; size = ssptr->sssize; this_segment = stkl - size; /** It is possible that calling this routine itself caused a stack overflow. Discard stack segments which do not contain the target address. */ while (!(this_segment <= address && address <= stkl)) { #ifdef DEBUG_I00AFUNC fprintf (stderr, "%011o %011o %011o\n", this_segment, address, stkl); #endif if (pseg == 0) break; stkl = stkl - pseg; ssptr = (struct stack_segment_linkage *) stkl; size = ssptr->sssize; pseg = ssptr->sspseg; this_segment = stkl - size; } result = address - this_segment; /** If you subtract pseg from the current end of the stack, you get the address of the previous stack segment's end. This seems a little convoluted to me, but I'll bet you save a cycle somewhere. */ while (pseg != 0) { #ifdef DEBUG_I00AFUNC fprintf (stderr, "%011o %011o\n", pseg, size); #endif stkl = stkl - pseg; ssptr = (struct stack_segment_linkage *) stkl; size = ssptr->sssize; pseg = ssptr->sspseg; result += size; } return (result); } #endif /** not CRAY2 */ #endif /** CRAY && CRAY_STACKSEG_END */ #endif /** no alloca */ #endif /** !__GNUC__ || __GNUC__ < 2 */
可以發現兩個的寫法還是有很多的區別。