author | bugtracker-ml@nttdocomo.com |
Tue, 06 Apr 2010 17:00:22 +0100 | |
changeset 56 | f24810eebc6b |
parent 1 | 2fb8b9db1c86 |
permissions | -rw-r--r-- |
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.. highlightlang:: c |
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.. _memory: |
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***************** |
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Memory Management |
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***************** |
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.. sectionauthor:: Vladimir Marangozov <Vladimir.Marangozov@inrialpes.fr> |
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.. _memoryoverview: |
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Overview |
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======== |
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Memory management in Python involves a private heap containing all Python |
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objects and data structures. The management of this private heap is ensured |
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internally by the *Python memory manager*. The Python memory manager has |
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different components which deal with various dynamic storage management aspects, |
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like sharing, segmentation, preallocation or caching. |
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At the lowest level, a raw memory allocator ensures that there is enough room in |
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the private heap for storing all Python-related data by interacting with the |
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memory manager of the operating system. On top of the raw memory allocator, |
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several object-specific allocators operate on the same heap and implement |
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distinct memory management policies adapted to the peculiarities of every object |
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type. For example, integer objects are managed differently within the heap than |
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strings, tuples or dictionaries because integers imply different storage |
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requirements and speed/space tradeoffs. The Python memory manager thus delegates |
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some of the work to the object-specific allocators, but ensures that the latter |
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operate within the bounds of the private heap. |
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It is important to understand that the management of the Python heap is |
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performed by the interpreter itself and that the user has no control over it, |
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even if she regularly manipulates object pointers to memory blocks inside that |
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heap. The allocation of heap space for Python objects and other internal |
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buffers is performed on demand by the Python memory manager through the Python/C |
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API functions listed in this document. |
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.. index:: |
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single: malloc() |
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single: calloc() |
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single: realloc() |
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single: free() |
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To avoid memory corruption, extension writers should never try to operate on |
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Python objects with the functions exported by the C library: :cfunc:`malloc`, |
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:cfunc:`calloc`, :cfunc:`realloc` and :cfunc:`free`. This will result in mixed |
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calls between the C allocator and the Python memory manager with fatal |
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consequences, because they implement different algorithms and operate on |
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different heaps. However, one may safely allocate and release memory blocks |
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with the C library allocator for individual purposes, as shown in the following |
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example:: |
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PyObject *res; |
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char *buf = (char *) malloc(BUFSIZ); /* for I/O */ |
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if (buf == NULL) |
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return PyErr_NoMemory(); |
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...Do some I/O operation involving buf... |
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res = PyString_FromString(buf); |
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free(buf); /* malloc'ed */ |
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return res; |
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In this example, the memory request for the I/O buffer is handled by the C |
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library allocator. The Python memory manager is involved only in the allocation |
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of the string object returned as a result. |
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In most situations, however, it is recommended to allocate memory from the |
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Python heap specifically because the latter is under control of the Python |
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memory manager. For example, this is required when the interpreter is extended |
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with new object types written in C. Another reason for using the Python heap is |
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the desire to *inform* the Python memory manager about the memory needs of the |
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extension module. Even when the requested memory is used exclusively for |
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internal, highly-specific purposes, delegating all memory requests to the Python |
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memory manager causes the interpreter to have a more accurate image of its |
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memory footprint as a whole. Consequently, under certain circumstances, the |
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Python memory manager may or may not trigger appropriate actions, like garbage |
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collection, memory compaction or other preventive procedures. Note that by using |
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the C library allocator as shown in the previous example, the allocated memory |
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for the I/O buffer escapes completely the Python memory manager. |
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.. _memoryinterface: |
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Memory Interface |
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================ |
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The following function sets, modeled after the ANSI C standard, but specifying |
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behavior when requesting zero bytes, are available for allocating and releasing |
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memory from the Python heap: |
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.. cfunction:: void* PyMem_Malloc(size_t n) |
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Allocates *n* bytes and returns a pointer of type :ctype:`void\*` to the |
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allocated memory, or *NULL* if the request fails. Requesting zero bytes returns |
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a distinct non-*NULL* pointer if possible, as if :cfunc:`PyMem_Malloc(1)` had |
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been called instead. The memory will not have been initialized in any way. |
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.. cfunction:: void* PyMem_Realloc(void *p, size_t n) |
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Resizes the memory block pointed to by *p* to *n* bytes. The contents will be |
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108 |
unchanged to the minimum of the old and the new sizes. If *p* is *NULL*, the |
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109 |
call is equivalent to :cfunc:`PyMem_Malloc(n)`; else if *n* is equal to zero, |
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110 |
the memory block is resized but is not freed, and the returned pointer is |
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111 |
non-*NULL*. Unless *p* is *NULL*, it must have been returned by a previous call |
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112 |
to :cfunc:`PyMem_Malloc` or :cfunc:`PyMem_Realloc`. If the request fails, |
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:cfunc:`PyMem_Realloc` returns *NULL* and *p* remains a valid pointer to the |
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114 |
previous memory area. |
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|
115 |
|
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116 |
|
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.. cfunction:: void PyMem_Free(void *p) |
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118 |
|
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Frees the memory block pointed to by *p*, which must have been returned by a |
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120 |
previous call to :cfunc:`PyMem_Malloc` or :cfunc:`PyMem_Realloc`. Otherwise, or |
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if :cfunc:`PyMem_Free(p)` has been called before, undefined behavior occurs. If |
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*p* is *NULL*, no operation is performed. |
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123 |
|
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The following type-oriented macros are provided for convenience. Note that |
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*TYPE* refers to any C type. |
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126 |
|
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127 |
|
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.. cfunction:: TYPE* PyMem_New(TYPE, size_t n) |
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129 |
|
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Same as :cfunc:`PyMem_Malloc`, but allocates ``(n * sizeof(TYPE))`` bytes of |
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131 |
memory. Returns a pointer cast to :ctype:`TYPE\*`. The memory will not have |
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been initialized in any way. |
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133 |
|
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134 |
|
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.. cfunction:: TYPE* PyMem_Resize(void *p, TYPE, size_t n) |
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136 |
|
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137 |
Same as :cfunc:`PyMem_Realloc`, but the memory block is resized to ``(n * |
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138 |
sizeof(TYPE))`` bytes. Returns a pointer cast to :ctype:`TYPE\*`. On return, |
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139 |
*p* will be a pointer to the new memory area, or *NULL* in the event of |
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failure. This is a C preprocessor macro; p is always reassigned. Save |
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141 |
the original value of p to avoid losing memory when handling errors. |
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142 |
|
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143 |
|
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.. cfunction:: void PyMem_Del(void *p) |
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145 |
|
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146 |
Same as :cfunc:`PyMem_Free`. |
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147 |
|
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148 |
In addition, the following macro sets are provided for calling the Python memory |
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149 |
allocator directly, without involving the C API functions listed above. However, |
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150 |
note that their use does not preserve binary compatibility across Python |
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151 |
versions and is therefore deprecated in extension modules. |
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152 |
|
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153 |
:cfunc:`PyMem_MALLOC`, :cfunc:`PyMem_REALLOC`, :cfunc:`PyMem_FREE`. |
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154 |
|
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155 |
:cfunc:`PyMem_NEW`, :cfunc:`PyMem_RESIZE`, :cfunc:`PyMem_DEL`. |
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|
156 |
|
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157 |
|
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158 |
.. _memoryexamples: |
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159 |
|
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160 |
Examples |
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161 |
======== |
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162 |
|
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163 |
Here is the example from section :ref:`memoryoverview`, rewritten so that the |
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I/O buffer is allocated from the Python heap by using the first function set:: |
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165 |
|
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166 |
PyObject *res; |
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167 |
char *buf = (char *) PyMem_Malloc(BUFSIZ); /* for I/O */ |
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168 |
|
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169 |
if (buf == NULL) |
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170 |
return PyErr_NoMemory(); |
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171 |
/* ...Do some I/O operation involving buf... */ |
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172 |
res = PyString_FromString(buf); |
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|
173 |
PyMem_Free(buf); /* allocated with PyMem_Malloc */ |
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174 |
return res; |
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175 |
|
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176 |
The same code using the type-oriented function set:: |
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177 |
|
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178 |
PyObject *res; |
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179 |
char *buf = PyMem_New(char, BUFSIZ); /* for I/O */ |
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180 |
|
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181 |
if (buf == NULL) |
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182 |
return PyErr_NoMemory(); |
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|
183 |
/* ...Do some I/O operation involving buf... */ |
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184 |
res = PyString_FromString(buf); |
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185 |
PyMem_Del(buf); /* allocated with PyMem_New */ |
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186 |
return res; |
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187 |
|
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188 |
Note that in the two examples above, the buffer is always manipulated via |
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|
189 |
functions belonging to the same set. Indeed, it is required to use the same |
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190 |
memory API family for a given memory block, so that the risk of mixing different |
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|
191 |
allocators is reduced to a minimum. The following code sequence contains two |
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|
192 |
errors, one of which is labeled as *fatal* because it mixes two different |
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193 |
allocators operating on different heaps. :: |
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194 |
|
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195 |
char *buf1 = PyMem_New(char, BUFSIZ); |
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196 |
char *buf2 = (char *) malloc(BUFSIZ); |
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197 |
char *buf3 = (char *) PyMem_Malloc(BUFSIZ); |
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198 |
... |
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199 |
PyMem_Del(buf3); /* Wrong -- should be PyMem_Free() */ |
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|
200 |
free(buf2); /* Right -- allocated via malloc() */ |
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201 |
free(buf1); /* Fatal -- should be PyMem_Del() */ |
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202 |
|
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203 |
In addition to the functions aimed at handling raw memory blocks from the Python |
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204 |
heap, objects in Python are allocated and released with :cfunc:`PyObject_New`, |
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205 |
:cfunc:`PyObject_NewVar` and :cfunc:`PyObject_Del`. |
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206 |
|
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207 |
These will be explained in the next chapter on defining and implementing new |
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208 |
object types in C. |
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209 |