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1 @node Obstacks,Licenses,Functions,Top
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2 @chapter Obstacks
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3 @cindex obstacks
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4
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5 An @dfn{obstack} is a pool of memory containing a stack of objects. You
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6 can create any number of separate obstacks, and then allocate objects in
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7 specified obstacks. Within each obstack, the last object allocated must
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8 always be the first one freed, but distinct obstacks are independent of
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9 each other.
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10
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11 Aside from this one constraint of order of freeing, obstacks are totally
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12 general: an obstack can contain any number of objects of any size. They
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13 are implemented with macros, so allocation is usually very fast as long as
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14 the objects are usually small. And the only space overhead per object is
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15 the padding needed to start each object on a suitable boundary.
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16
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17 @menu
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18 * Creating Obstacks:: How to declare an obstack in your program.
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19 * Preparing for Obstacks:: Preparations needed before you can
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20 use obstacks.
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21 * Allocation in an Obstack:: Allocating objects in an obstack.
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22 * Freeing Obstack Objects:: Freeing objects in an obstack.
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23 * Obstack Functions:: The obstack functions are both
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24 functions and macros.
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25 * Growing Objects:: Making an object bigger by stages.
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26 * Extra Fast Growing:: Extra-high-efficiency (though more
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27 complicated) growing objects.
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28 * Status of an Obstack:: Inquiries about the status of an obstack.
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29 * Obstacks Data Alignment:: Controlling alignment of objects in obstacks.
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30 * Obstack Chunks:: How obstacks obtain and release chunks;
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31 efficiency considerations.
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32 * Summary of Obstacks::
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33 @end menu
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34
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35 @node Creating Obstacks
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36 @section Creating Obstacks
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37
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38 The utilities for manipulating obstacks are declared in the header
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39 file @file{obstack.h}.
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40 @pindex obstack.h
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41
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42 @comment obstack.h
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43 @comment GNU
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44 @deftp {Data Type} {struct obstack}
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45 An obstack is represented by a data structure of type @code{struct
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46 obstack}. This structure has a small fixed size; it records the status
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47 of the obstack and how to find the space in which objects are allocated.
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48 It does not contain any of the objects themselves. You should not try
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49 to access the contents of the structure directly; use only the functions
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50 described in this chapter.
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51 @end deftp
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52
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53 You can declare variables of type @code{struct obstack} and use them as
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54 obstacks, or you can allocate obstacks dynamically like any other kind
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55 of object. Dynamic allocation of obstacks allows your program to have a
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56 variable number of different stacks. (You can even allocate an
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57 obstack structure in another obstack, but this is rarely useful.)
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58
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59 All the functions that work with obstacks require you to specify which
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60 obstack to use. You do this with a pointer of type @code{struct obstack
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61 *}. In the following, we often say ``an obstack'' when strictly
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62 speaking the object at hand is such a pointer.
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63
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64 The objects in the obstack are packed into large blocks called
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65 @dfn{chunks}. The @code{struct obstack} structure points to a chain of
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66 the chunks currently in use.
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67
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68 The obstack library obtains a new chunk whenever you allocate an object
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69 that won't fit in the previous chunk. Since the obstack library manages
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70 chunks automatically, you don't need to pay much attention to them, but
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71 you do need to supply a function which the obstack library should use to
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72 get a chunk. Usually you supply a function which uses @code{malloc}
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73 directly or indirectly. You must also supply a function to free a chunk.
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74 These matters are described in the following section.
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75
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76 @node Preparing for Obstacks
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77 @section Preparing for Using Obstacks
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78
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79 Each source file in which you plan to use the obstack functions
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80 must include the header file @file{obstack.h}, like this:
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81
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82 @smallexample
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83 #include <obstack.h>
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84 @end smallexample
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85
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86 @findex obstack_chunk_alloc
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87 @findex obstack_chunk_free
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88 Also, if the source file uses the macro @code{obstack_init}, it must
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89 declare or define two functions or macros that will be called by the
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90 obstack library. One, @code{obstack_chunk_alloc}, is used to allocate
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91 the chunks of memory into which objects are packed. The other,
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92 @code{obstack_chunk_free}, is used to return chunks when the objects in
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93 them are freed. These macros should appear before any use of obstacks
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94 in the source file.
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95
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96 Usually these are defined to use @code{malloc} via the intermediary
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97 @code{xmalloc} (@pxref{Unconstrained Allocation, , , libc, The GNU C Library Reference Manual}). This is done with
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98 the following pair of macro definitions:
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99
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100 @smallexample
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101 #define obstack_chunk_alloc xmalloc
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102 #define obstack_chunk_free free
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103 @end smallexample
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104
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105 @noindent
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106 Though the memory you get using obstacks really comes from @code{malloc},
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107 using obstacks is faster because @code{malloc} is called less often, for
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108 larger blocks of memory. @xref{Obstack Chunks}, for full details.
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109
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110 At run time, before the program can use a @code{struct obstack} object
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111 as an obstack, it must initialize the obstack by calling
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112 @code{obstack_init}.
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113
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114 @comment obstack.h
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115 @comment GNU
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116 @deftypefun int obstack_init (struct obstack *@var{obstack-ptr})
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117 Initialize obstack @var{obstack-ptr} for allocation of objects. This
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118 function calls the obstack's @code{obstack_chunk_alloc} function. If
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119 allocation of memory fails, the function pointed to by
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120 @code{obstack_alloc_failed_handler} is called. The @code{obstack_init}
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121 function always returns 1 (Compatibility notice: Former versions of
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122 obstack returned 0 if allocation failed).
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123 @end deftypefun
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124
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125 Here are two examples of how to allocate the space for an obstack and
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126 initialize it. First, an obstack that is a static variable:
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127
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128 @smallexample
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129 static struct obstack myobstack;
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130 @dots{}
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131 obstack_init (&myobstack);
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132 @end smallexample
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133
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134 @noindent
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135 Second, an obstack that is itself dynamically allocated:
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136
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137 @smallexample
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138 struct obstack *myobstack_ptr
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139 = (struct obstack *) xmalloc (sizeof (struct obstack));
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140
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141 obstack_init (myobstack_ptr);
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142 @end smallexample
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143
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144 @comment obstack.h
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145 @comment GNU
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146 @defvar obstack_alloc_failed_handler
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147 The value of this variable is a pointer to a function that
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148 @code{obstack} uses when @code{obstack_chunk_alloc} fails to allocate
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149 memory. The default action is to print a message and abort.
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150 You should supply a function that either calls @code{exit}
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151 (@pxref{Program Termination, , , libc, The GNU C Library Reference Manual}) or @code{longjmp} (@pxref{Non-Local
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152 Exits, , , libc, The GNU C Library Reference Manual}) and doesn't return.
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153
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154 @smallexample
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155 void my_obstack_alloc_failed (void)
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156 @dots{}
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157 obstack_alloc_failed_handler = &my_obstack_alloc_failed;
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158 @end smallexample
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159
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160 @end defvar
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161
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162 @node Allocation in an Obstack
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163 @section Allocation in an Obstack
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164 @cindex allocation (obstacks)
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165
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166 The most direct way to allocate an object in an obstack is with
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167 @code{obstack_alloc}, which is invoked almost like @code{malloc}.
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168
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169 @comment obstack.h
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170 @comment GNU
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171 @deftypefun {void *} obstack_alloc (struct obstack *@var{obstack-ptr}, int @var{size})
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172 This allocates an uninitialized block of @var{size} bytes in an obstack
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173 and returns its address. Here @var{obstack-ptr} specifies which obstack
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174 to allocate the block in; it is the address of the @code{struct obstack}
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175 object which represents the obstack. Each obstack function or macro
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176 requires you to specify an @var{obstack-ptr} as the first argument.
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177
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178 This function calls the obstack's @code{obstack_chunk_alloc} function if
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179 it needs to allocate a new chunk of memory; it calls
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180 @code{obstack_alloc_failed_handler} if allocation of memory by
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181 @code{obstack_chunk_alloc} failed.
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182 @end deftypefun
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183
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184 For example, here is a function that allocates a copy of a string @var{str}
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185 in a specific obstack, which is in the variable @code{string_obstack}:
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186
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187 @smallexample
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188 struct obstack string_obstack;
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189
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190 char *
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191 copystring (char *string)
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192 @{
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193 size_t len = strlen (string) + 1;
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194 char *s = (char *) obstack_alloc (&string_obstack, len);
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195 memcpy (s, string, len);
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196 return s;
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197 @}
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198 @end smallexample
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199
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200 To allocate a block with specified contents, use the function
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201 @code{obstack_copy}, declared like this:
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202
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203 @comment obstack.h
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204 @comment GNU
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205 @deftypefun {void *} obstack_copy (struct obstack *@var{obstack-ptr}, void *@var{address}, int @var{size})
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206 This allocates a block and initializes it by copying @var{size}
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207 bytes of data starting at @var{address}. It calls
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208 @code{obstack_alloc_failed_handler} if allocation of memory by
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209 @code{obstack_chunk_alloc} failed.
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210 @end deftypefun
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211
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212 @comment obstack.h
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213 @comment GNU
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214 @deftypefun {void *} obstack_copy0 (struct obstack *@var{obstack-ptr}, void *@var{address}, int @var{size})
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215 Like @code{obstack_copy}, but appends an extra byte containing a null
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216 character. This extra byte is not counted in the argument @var{size}.
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217 @end deftypefun
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218
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219 The @code{obstack_copy0} function is convenient for copying a sequence
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220 of characters into an obstack as a null-terminated string. Here is an
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221 example of its use:
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222
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223 @smallexample
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224 char *
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225 obstack_savestring (char *addr, int size)
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226 @{
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227 return obstack_copy0 (&myobstack, addr, size);
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228 @}
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229 @end smallexample
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230
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231 @noindent
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232 Contrast this with the previous example of @code{savestring} using
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233 @code{malloc} (@pxref{Basic Allocation, , , libc, The GNU C Library Reference Manual}).
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234
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235 @node Freeing Obstack Objects
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236 @section Freeing Objects in an Obstack
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237 @cindex freeing (obstacks)
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238
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239 To free an object allocated in an obstack, use the function
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240 @code{obstack_free}. Since the obstack is a stack of objects, freeing
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241 one object automatically frees all other objects allocated more recently
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242 in the same obstack.
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243
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244 @comment obstack.h
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245 @comment GNU
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246 @deftypefun void obstack_free (struct obstack *@var{obstack-ptr}, void *@var{object})
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247 If @var{object} is a null pointer, everything allocated in the obstack
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248 is freed. Otherwise, @var{object} must be the address of an object
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249 allocated in the obstack. Then @var{object} is freed, along with
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250 everything allocated in @var{obstack} since @var{object}.
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251 @end deftypefun
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252
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253 Note that if @var{object} is a null pointer, the result is an
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254 uninitialized obstack. To free all memory in an obstack but leave it
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255 valid for further allocation, call @code{obstack_free} with the address
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256 of the first object allocated on the obstack:
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257
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258 @smallexample
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259 obstack_free (obstack_ptr, first_object_allocated_ptr);
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260 @end smallexample
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261
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262 Recall that the objects in an obstack are grouped into chunks. When all
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263 the objects in a chunk become free, the obstack library automatically
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264 frees the chunk (@pxref{Preparing for Obstacks}). Then other
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265 obstacks, or non-obstack allocation, can reuse the space of the chunk.
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266
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267 @node Obstack Functions
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268 @section Obstack Functions and Macros
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269 @cindex macros
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270
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271 The interfaces for using obstacks may be defined either as functions or
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272 as macros, depending on the compiler. The obstack facility works with
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273 all C compilers, including both @w{ISO C} and traditional C, but there are
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274 precautions you must take if you plan to use compilers other than GNU C.
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275
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276 If you are using an old-fashioned @w{non-ISO C} compiler, all the obstack
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277 ``functions'' are actually defined only as macros. You can call these
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278 macros like functions, but you cannot use them in any other way (for
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279 example, you cannot take their address).
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280
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281 Calling the macros requires a special precaution: namely, the first
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282 operand (the obstack pointer) may not contain any side effects, because
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283 it may be computed more than once. For example, if you write this:
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284
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285 @smallexample
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286 obstack_alloc (get_obstack (), 4);
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287 @end smallexample
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288
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289 @noindent
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290 you will find that @code{get_obstack} may be called several times.
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291 If you use @code{*obstack_list_ptr++} as the obstack pointer argument,
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292 you will get very strange results since the incrementation may occur
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293 several times.
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294
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295 In @w{ISO C}, each function has both a macro definition and a function
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296 definition. The function definition is used if you take the address of the
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297 function without calling it. An ordinary call uses the macro definition by
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298 default, but you can request the function definition instead by writing the
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299 function name in parentheses, as shown here:
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300
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301 @smallexample
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302 char *x;
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303 void *(*funcp) ();
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304 /* @r{Use the macro}. */
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305 x = (char *) obstack_alloc (obptr, size);
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306 /* @r{Call the function}. */
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307 x = (char *) (obstack_alloc) (obptr, size);
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308 /* @r{Take the address of the function}. */
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309 funcp = obstack_alloc;
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310 @end smallexample
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311
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312 @noindent
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313 This is the same situation that exists in @w{ISO C} for the standard library
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314 functions. @xref{Macro Definitions, , , libc, The GNU C Library Reference Manual}.
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315
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316 @strong{Warning:} When you do use the macros, you must observe the
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317 precaution of avoiding side effects in the first operand, even in @w{ISO C}.
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318
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319 If you use the GNU C compiler, this precaution is not necessary, because
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320 various language extensions in GNU C permit defining the macros so as to
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321 compute each argument only once.
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322
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323 @node Growing Objects
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324 @section Growing Objects
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325 @cindex growing objects (in obstacks)
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326 @cindex changing the size of a block (obstacks)
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327
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328 Because memory in obstack chunks is used sequentially, it is possible to
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329 build up an object step by step, adding one or more bytes at a time to the
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330 end of the object. With this technique, you do not need to know how much
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331 data you will put in the object until you come to the end of it. We call
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332 this the technique of @dfn{growing objects}. The special functions
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333 for adding data to the growing object are described in this section.
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334
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335 You don't need to do anything special when you start to grow an object.
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336 Using one of the functions to add data to the object automatically
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337 starts it. However, it is necessary to say explicitly when the object is
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338 finished. This is done with the function @code{obstack_finish}.
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339
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340 The actual address of the object thus built up is not known until the
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341 object is finished. Until then, it always remains possible that you will
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342 add so much data that the object must be copied into a new chunk.
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343
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344 While the obstack is in use for a growing object, you cannot use it for
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345 ordinary allocation of another object. If you try to do so, the space
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346 already added to the growing object will become part of the other object.
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347
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348 @comment obstack.h
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349 @comment GNU
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350 @deftypefun void obstack_blank (struct obstack *@var{obstack-ptr}, int @var{size})
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351 The most basic function for adding to a growing object is
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352 @code{obstack_blank}, which adds space without initializing it.
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353 @end deftypefun
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354
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355 @comment obstack.h
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356 @comment GNU
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357 @deftypefun void obstack_grow (struct obstack *@var{obstack-ptr}, void *@var{data}, int @var{size})
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358 To add a block of initialized space, use @code{obstack_grow}, which is
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359 the growing-object analogue of @code{obstack_copy}. It adds @var{size}
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360 bytes of data to the growing object, copying the contents from
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361 @var{data}.
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362 @end deftypefun
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363
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364 @comment obstack.h
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365 @comment GNU
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366 @deftypefun void obstack_grow0 (struct obstack *@var{obstack-ptr}, void *@var{data}, int @var{size})
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367 This is the growing-object analogue of @code{obstack_copy0}. It adds
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368 @var{size} bytes copied from @var{data}, followed by an additional null
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369 character.
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370 @end deftypefun
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371
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372 @comment obstack.h
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373 @comment GNU
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374 @deftypefun void obstack_1grow (struct obstack *@var{obstack-ptr}, char @var{c})
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|
375 To add one character at a time, use the function @code{obstack_1grow}.
|
|
|
376 It adds a single byte containing @var{c} to the growing object.
|
|
|
377 @end deftypefun
|
|
|
378
|
|
|
379 @comment obstack.h
|
|
|
380 @comment GNU
|
|
|
381 @deftypefun void obstack_ptr_grow (struct obstack *@var{obstack-ptr}, void *@var{data})
|
|
|
382 Adding the value of a pointer one can use the function
|
|
|
383 @code{obstack_ptr_grow}. It adds @code{sizeof (void *)} bytes
|
|
|
384 containing the value of @var{data}.
|
|
|
385 @end deftypefun
|
|
|
386
|
|
|
387 @comment obstack.h
|
|
|
388 @comment GNU
|
|
|
389 @deftypefun void obstack_int_grow (struct obstack *@var{obstack-ptr}, int @var{data})
|
|
|
390 A single value of type @code{int} can be added by using the
|
|
|
391 @code{obstack_int_grow} function. It adds @code{sizeof (int)} bytes to
|
|
|
392 the growing object and initializes them with the value of @var{data}.
|
|
|
393 @end deftypefun
|
|
|
394
|
|
|
395 @comment obstack.h
|
|
|
396 @comment GNU
|
|
|
397 @deftypefun {void *} obstack_finish (struct obstack *@var{obstack-ptr})
|
|
|
398 When you are finished growing the object, use the function
|
|
|
399 @code{obstack_finish} to close it off and return its final address.
|
|
|
400
|
|
|
401 Once you have finished the object, the obstack is available for ordinary
|
|
|
402 allocation or for growing another object.
|
|
|
403
|
|
|
404 This function can return a null pointer under the same conditions as
|
|
|
405 @code{obstack_alloc} (@pxref{Allocation in an Obstack}).
|
|
|
406 @end deftypefun
|
|
|
407
|
|
|
408 When you build an object by growing it, you will probably need to know
|
|
|
409 afterward how long it became. You need not keep track of this as you grow
|
|
|
410 the object, because you can find out the length from the obstack just
|
|
|
411 before finishing the object with the function @code{obstack_object_size},
|
|
|
412 declared as follows:
|
|
|
413
|
|
|
414 @comment obstack.h
|
|
|
415 @comment GNU
|
|
|
416 @deftypefun int obstack_object_size (struct obstack *@var{obstack-ptr})
|
|
|
417 This function returns the current size of the growing object, in bytes.
|
|
|
418 Remember to call this function @emph{before} finishing the object.
|
|
|
419 After it is finished, @code{obstack_object_size} will return zero.
|
|
|
420 @end deftypefun
|
|
|
421
|
|
|
422 If you have started growing an object and wish to cancel it, you should
|
|
|
423 finish it and then free it, like this:
|
|
|
424
|
|
|
425 @smallexample
|
|
|
426 obstack_free (obstack_ptr, obstack_finish (obstack_ptr));
|
|
|
427 @end smallexample
|
|
|
428
|
|
|
429 @noindent
|
|
|
430 This has no effect if no object was growing.
|
|
|
431
|
|
|
432 @cindex shrinking objects
|
|
|
433 You can use @code{obstack_blank} with a negative size argument to make
|
|
|
434 the current object smaller. Just don't try to shrink it beyond zero
|
|
|
435 length---there's no telling what will happen if you do that.
|
|
|
436
|
|
|
437 @node Extra Fast Growing
|
|
|
438 @section Extra Fast Growing Objects
|
|
|
439 @cindex efficiency and obstacks
|
|
|
440
|
|
|
441 The usual functions for growing objects incur overhead for checking
|
|
|
442 whether there is room for the new growth in the current chunk. If you
|
|
|
443 are frequently constructing objects in small steps of growth, this
|
|
|
444 overhead can be significant.
|
|
|
445
|
|
|
446 You can reduce the overhead by using special ``fast growth''
|
|
|
447 functions that grow the object without checking. In order to have a
|
|
|
448 robust program, you must do the checking yourself. If you do this checking
|
|
|
449 in the simplest way each time you are about to add data to the object, you
|
|
|
450 have not saved anything, because that is what the ordinary growth
|
|
|
451 functions do. But if you can arrange to check less often, or check
|
|
|
452 more efficiently, then you make the program faster.
|
|
|
453
|
|
|
454 The function @code{obstack_room} returns the amount of room available
|
|
|
455 in the current chunk. It is declared as follows:
|
|
|
456
|
|
|
457 @comment obstack.h
|
|
|
458 @comment GNU
|
|
|
459 @deftypefun int obstack_room (struct obstack *@var{obstack-ptr})
|
|
|
460 This returns the number of bytes that can be added safely to the current
|
|
|
461 growing object (or to an object about to be started) in obstack
|
|
|
462 @var{obstack} using the fast growth functions.
|
|
|
463 @end deftypefun
|
|
|
464
|
|
|
465 While you know there is room, you can use these fast growth functions
|
|
|
466 for adding data to a growing object:
|
|
|
467
|
|
|
468 @comment obstack.h
|
|
|
469 @comment GNU
|
|
|
470 @deftypefun void obstack_1grow_fast (struct obstack *@var{obstack-ptr}, char @var{c})
|
|
|
471 The function @code{obstack_1grow_fast} adds one byte containing the
|
|
|
472 character @var{c} to the growing object in obstack @var{obstack-ptr}.
|
|
|
473 @end deftypefun
|
|
|
474
|
|
|
475 @comment obstack.h
|
|
|
476 @comment GNU
|
|
|
477 @deftypefun void obstack_ptr_grow_fast (struct obstack *@var{obstack-ptr}, void *@var{data})
|
|
|
478 The function @code{obstack_ptr_grow_fast} adds @code{sizeof (void *)}
|
|
|
479 bytes containing the value of @var{data} to the growing object in
|
|
|
480 obstack @var{obstack-ptr}.
|
|
|
481 @end deftypefun
|
|
|
482
|
|
|
483 @comment obstack.h
|
|
|
484 @comment GNU
|
|
|
485 @deftypefun void obstack_int_grow_fast (struct obstack *@var{obstack-ptr}, int @var{data})
|
|
|
486 The function @code{obstack_int_grow_fast} adds @code{sizeof (int)} bytes
|
|
|
487 containing the value of @var{data} to the growing object in obstack
|
|
|
488 @var{obstack-ptr}.
|
|
|
489 @end deftypefun
|
|
|
490
|
|
|
491 @comment obstack.h
|
|
|
492 @comment GNU
|
|
|
493 @deftypefun void obstack_blank_fast (struct obstack *@var{obstack-ptr}, int @var{size})
|
|
|
494 The function @code{obstack_blank_fast} adds @var{size} bytes to the
|
|
|
495 growing object in obstack @var{obstack-ptr} without initializing them.
|
|
|
496 @end deftypefun
|
|
|
497
|
|
|
498 When you check for space using @code{obstack_room} and there is not
|
|
|
499 enough room for what you want to add, the fast growth functions
|
|
|
500 are not safe. In this case, simply use the corresponding ordinary
|
|
|
501 growth function instead. Very soon this will copy the object to a
|
|
|
502 new chunk; then there will be lots of room available again.
|
|
|
503
|
|
|
504 So, each time you use an ordinary growth function, check afterward for
|
|
|
505 sufficient space using @code{obstack_room}. Once the object is copied
|
|
|
506 to a new chunk, there will be plenty of space again, so the program will
|
|
|
507 start using the fast growth functions again.
|
|
|
508
|
|
|
509 Here is an example:
|
|
|
510
|
|
|
511 @smallexample
|
|
|
512 @group
|
|
|
513 void
|
|
|
514 add_string (struct obstack *obstack, const char *ptr, int len)
|
|
|
515 @{
|
|
|
516 while (len > 0)
|
|
|
517 @{
|
|
|
518 int room = obstack_room (obstack);
|
|
|
519 if (room == 0)
|
|
|
520 @{
|
|
|
521 /* @r{Not enough room. Add one character slowly,}
|
|
|
522 @r{which may copy to a new chunk and make room.} */
|
|
|
523 obstack_1grow (obstack, *ptr++);
|
|
|
524 len--;
|
|
|
525 @}
|
|
|
526 else
|
|
|
527 @{
|
|
|
528 if (room > len)
|
|
|
529 room = len;
|
|
|
530 /* @r{Add fast as much as we have room for.} */
|
|
|
531 len -= room;
|
|
|
532 while (room-- > 0)
|
|
|
533 obstack_1grow_fast (obstack, *ptr++);
|
|
|
534 @}
|
|
|
535 @}
|
|
|
536 @}
|
|
|
537 @end group
|
|
|
538 @end smallexample
|
|
|
539
|
|
|
540 @node Status of an Obstack
|
|
|
541 @section Status of an Obstack
|
|
|
542 @cindex obstack status
|
|
|
543 @cindex status of obstack
|
|
|
544
|
|
|
545 Here are functions that provide information on the current status of
|
|
|
546 allocation in an obstack. You can use them to learn about an object while
|
|
|
547 still growing it.
|
|
|
548
|
|
|
549 @comment obstack.h
|
|
|
550 @comment GNU
|
|
|
551 @deftypefun {void *} obstack_base (struct obstack *@var{obstack-ptr})
|
|
|
552 This function returns the tentative address of the beginning of the
|
|
|
553 currently growing object in @var{obstack-ptr}. If you finish the object
|
|
|
554 immediately, it will have that address. If you make it larger first, it
|
|
|
555 may outgrow the current chunk---then its address will change!
|
|
|
556
|
|
|
557 If no object is growing, this value says where the next object you
|
|
|
558 allocate will start (once again assuming it fits in the current
|
|
|
559 chunk).
|
|
|
560 @end deftypefun
|
|
|
561
|
|
|
562 @comment obstack.h
|
|
|
563 @comment GNU
|
|
|
564 @deftypefun {void *} obstack_next_free (struct obstack *@var{obstack-ptr})
|
|
|
565 This function returns the address of the first free byte in the current
|
|
|
566 chunk of obstack @var{obstack-ptr}. This is the end of the currently
|
|
|
567 growing object. If no object is growing, @code{obstack_next_free}
|
|
|
568 returns the same value as @code{obstack_base}.
|
|
|
569 @end deftypefun
|
|
|
570
|
|
|
571 @comment obstack.h
|
|
|
572 @comment GNU
|
|
|
573 @deftypefun int obstack_object_size (struct obstack *@var{obstack-ptr})
|
|
|
574 This function returns the size in bytes of the currently growing object.
|
|
|
575 This is equivalent to
|
|
|
576
|
|
|
577 @smallexample
|
|
|
578 obstack_next_free (@var{obstack-ptr}) - obstack_base (@var{obstack-ptr})
|
|
|
579 @end smallexample
|
|
|
580 @end deftypefun
|
|
|
581
|
|
|
582 @node Obstacks Data Alignment
|
|
|
583 @section Alignment of Data in Obstacks
|
|
|
584 @cindex alignment (in obstacks)
|
|
|
585
|
|
|
586 Each obstack has an @dfn{alignment boundary}; each object allocated in
|
|
|
587 the obstack automatically starts on an address that is a multiple of the
|
|
|
588 specified boundary. By default, this boundary is 4 bytes.
|
|
|
589
|
|
|
590 To access an obstack's alignment boundary, use the macro
|
|
|
591 @code{obstack_alignment_mask}, whose function prototype looks like
|
|
|
592 this:
|
|
|
593
|
|
|
594 @comment obstack.h
|
|
|
595 @comment GNU
|
|
|
596 @deftypefn Macro int obstack_alignment_mask (struct obstack *@var{obstack-ptr})
|
|
|
597 The value is a bit mask; a bit that is 1 indicates that the corresponding
|
|
|
598 bit in the address of an object should be 0. The mask value should be one
|
|
|
599 less than a power of 2; the effect is that all object addresses are
|
|
|
600 multiples of that power of 2. The default value of the mask is 3, so that
|
|
|
601 addresses are multiples of 4. A mask value of 0 means an object can start
|
|
|
602 on any multiple of 1 (that is, no alignment is required).
|
|
|
603
|
|
|
604 The expansion of the macro @code{obstack_alignment_mask} is an lvalue,
|
|
|
605 so you can alter the mask by assignment. For example, this statement:
|
|
|
606
|
|
|
607 @smallexample
|
|
|
608 obstack_alignment_mask (obstack_ptr) = 0;
|
|
|
609 @end smallexample
|
|
|
610
|
|
|
611 @noindent
|
|
|
612 has the effect of turning off alignment processing in the specified obstack.
|
|
|
613 @end deftypefn
|
|
|
614
|
|
|
615 Note that a change in alignment mask does not take effect until
|
|
|
616 @emph{after} the next time an object is allocated or finished in the
|
|
|
617 obstack. If you are not growing an object, you can make the new
|
|
|
618 alignment mask take effect immediately by calling @code{obstack_finish}.
|
|
|
619 This will finish a zero-length object and then do proper alignment for
|
|
|
620 the next object.
|
|
|
621
|
|
|
622 @node Obstack Chunks
|
|
|
623 @section Obstack Chunks
|
|
|
624 @cindex efficiency of chunks
|
|
|
625 @cindex chunks
|
|
|
626
|
|
|
627 Obstacks work by allocating space for themselves in large chunks, and
|
|
|
628 then parceling out space in the chunks to satisfy your requests. Chunks
|
|
|
629 are normally 4096 bytes long unless you specify a different chunk size.
|
|
|
630 The chunk size includes 8 bytes of overhead that are not actually used
|
|
|
631 for storing objects. Regardless of the specified size, longer chunks
|
|
|
632 will be allocated when necessary for long objects.
|
|
|
633
|
|
|
634 The obstack library allocates chunks by calling the function
|
|
|
635 @code{obstack_chunk_alloc}, which you must define. When a chunk is no
|
|
|
636 longer needed because you have freed all the objects in it, the obstack
|
|
|
637 library frees the chunk by calling @code{obstack_chunk_free}, which you
|
|
|
638 must also define.
|
|
|
639
|
|
|
640 These two must be defined (as macros) or declared (as functions) in each
|
|
|
641 source file that uses @code{obstack_init} (@pxref{Creating Obstacks}).
|
|
|
642 Most often they are defined as macros like this:
|
|
|
643
|
|
|
644 @smallexample
|
|
|
645 #define obstack_chunk_alloc malloc
|
|
|
646 #define obstack_chunk_free free
|
|
|
647 @end smallexample
|
|
|
648
|
|
|
649 Note that these are simple macros (no arguments). Macro definitions with
|
|
|
650 arguments will not work! It is necessary that @code{obstack_chunk_alloc}
|
|
|
651 or @code{obstack_chunk_free}, alone, expand into a function name if it is
|
|
|
652 not itself a function name.
|
|
|
653
|
|
|
654 If you allocate chunks with @code{malloc}, the chunk size should be a
|
|
|
655 power of 2. The default chunk size, 4096, was chosen because it is long
|
|
|
656 enough to satisfy many typical requests on the obstack yet short enough
|
|
|
657 not to waste too much memory in the portion of the last chunk not yet used.
|
|
|
658
|
|
|
659 @comment obstack.h
|
|
|
660 @comment GNU
|
|
|
661 @deftypefn Macro int obstack_chunk_size (struct obstack *@var{obstack-ptr})
|
|
|
662 This returns the chunk size of the given obstack.
|
|
|
663 @end deftypefn
|
|
|
664
|
|
|
665 Since this macro expands to an lvalue, you can specify a new chunk size by
|
|
|
666 assigning it a new value. Doing so does not affect the chunks already
|
|
|
667 allocated, but will change the size of chunks allocated for that particular
|
|
|
668 obstack in the future. It is unlikely to be useful to make the chunk size
|
|
|
669 smaller, but making it larger might improve efficiency if you are
|
|
|
670 allocating many objects whose size is comparable to the chunk size. Here
|
|
|
671 is how to do so cleanly:
|
|
|
672
|
|
|
673 @smallexample
|
|
|
674 if (obstack_chunk_size (obstack_ptr) < @var{new-chunk-size})
|
|
|
675 obstack_chunk_size (obstack_ptr) = @var{new-chunk-size};
|
|
|
676 @end smallexample
|
|
|
677
|
|
|
678 @node Summary of Obstacks
|
|
|
679 @section Summary of Obstack Functions
|
|
|
680
|
|
|
681 Here is a summary of all the functions associated with obstacks. Each
|
|
|
682 takes the address of an obstack (@code{struct obstack *}) as its first
|
|
|
683 argument.
|
|
|
684
|
|
|
685 @table @code
|
|
|
686 @item void obstack_init (struct obstack *@var{obstack-ptr})
|
|
|
687 Initialize use of an obstack. @xref{Creating Obstacks}.
|
|
|
688
|
|
|
689 @item void *obstack_alloc (struct obstack *@var{obstack-ptr}, int @var{size})
|
|
|
690 Allocate an object of @var{size} uninitialized bytes.
|
|
|
691 @xref{Allocation in an Obstack}.
|
|
|
692
|
|
|
693 @item void *obstack_copy (struct obstack *@var{obstack-ptr}, void *@var{address}, int @var{size})
|
|
|
694 Allocate an object of @var{size} bytes, with contents copied from
|
|
|
695 @var{address}. @xref{Allocation in an Obstack}.
|
|
|
696
|
|
|
697 @item void *obstack_copy0 (struct obstack *@var{obstack-ptr}, void *@var{address}, int @var{size})
|
|
|
698 Allocate an object of @var{size}+1 bytes, with @var{size} of them copied
|
|
|
699 from @var{address}, followed by a null character at the end.
|
|
|
700 @xref{Allocation in an Obstack}.
|
|
|
701
|
|
|
702 @item void obstack_free (struct obstack *@var{obstack-ptr}, void *@var{object})
|
|
|
703 Free @var{object} (and everything allocated in the specified obstack
|
|
|
704 more recently than @var{object}). @xref{Freeing Obstack Objects}.
|
|
|
705
|
|
|
706 @item void obstack_blank (struct obstack *@var{obstack-ptr}, int @var{size})
|
|
|
707 Add @var{size} uninitialized bytes to a growing object.
|
|
|
708 @xref{Growing Objects}.
|
|
|
709
|
|
|
710 @item void obstack_grow (struct obstack *@var{obstack-ptr}, void *@var{address}, int @var{size})
|
|
|
711 Add @var{size} bytes, copied from @var{address}, to a growing object.
|
|
|
712 @xref{Growing Objects}.
|
|
|
713
|
|
|
714 @item void obstack_grow0 (struct obstack *@var{obstack-ptr}, void *@var{address}, int @var{size})
|
|
|
715 Add @var{size} bytes, copied from @var{address}, to a growing object,
|
|
|
716 and then add another byte containing a null character. @xref{Growing
|
|
|
717 Objects}.
|
|
|
718
|
|
|
719 @item void obstack_1grow (struct obstack *@var{obstack-ptr}, char @var{data-char})
|
|
|
720 Add one byte containing @var{data-char} to a growing object.
|
|
|
721 @xref{Growing Objects}.
|
|
|
722
|
|
|
723 @item void *obstack_finish (struct obstack *@var{obstack-ptr})
|
|
|
724 Finalize the object that is growing and return its permanent address.
|
|
|
725 @xref{Growing Objects}.
|
|
|
726
|
|
|
727 @item int obstack_object_size (struct obstack *@var{obstack-ptr})
|
|
|
728 Get the current size of the currently growing object. @xref{Growing
|
|
|
729 Objects}.
|
|
|
730
|
|
|
731 @item void obstack_blank_fast (struct obstack *@var{obstack-ptr}, int @var{size})
|
|
|
732 Add @var{size} uninitialized bytes to a growing object without checking
|
|
|
733 that there is enough room. @xref{Extra Fast Growing}.
|
|
|
734
|
|
|
735 @item void obstack_1grow_fast (struct obstack *@var{obstack-ptr}, char @var{data-char})
|
|
|
736 Add one byte containing @var{data-char} to a growing object without
|
|
|
737 checking that there is enough room. @xref{Extra Fast Growing}.
|
|
|
738
|
|
|
739 @item int obstack_room (struct obstack *@var{obstack-ptr})
|
|
|
740 Get the amount of room now available for growing the current object.
|
|
|
741 @xref{Extra Fast Growing}.
|
|
|
742
|
|
|
743 @item int obstack_alignment_mask (struct obstack *@var{obstack-ptr})
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744 The mask used for aligning the beginning of an object. This is an
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745 lvalue. @xref{Obstacks Data Alignment}.
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746
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747 @item int obstack_chunk_size (struct obstack *@var{obstack-ptr})
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748 The size for allocating chunks. This is an lvalue. @xref{Obstack Chunks}.
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749
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750 @item void *obstack_base (struct obstack *@var{obstack-ptr})
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751 Tentative starting address of the currently growing object.
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752 @xref{Status of an Obstack}.
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753
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754 @item void *obstack_next_free (struct obstack *@var{obstack-ptr})
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755 Address just after the end of the currently growing object.
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756 @xref{Status of an Obstack}.
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757 @end table
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758
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