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36 .\" @(#)malloc.3 8.1 (Berkeley) 6/4/93
37 .\" $FreeBSD: src/lib/libc/stdlib/malloc.3,v 1.25.2.16 2003/01/06 17:10:45 trhodes Exp $
38 .\" $DragonFly: src/lib/libcr/stdlib/Attic/malloc.3,v 1.3 2004/03/11 12:28:51 hmp Exp $
44 .Nm malloc , calloc , realloc , free , reallocf
45 .Nd general purpose memory allocation functions
51 .Fn malloc "size_t size"
53 .Fn calloc "size_t number" "size_t size"
55 .Fn realloc "void *ptr" "size_t size"
57 .Fn reallocf "void *ptr" "size_t size"
68 The allocated space is suitably aligned (after possible pointer coercion)
69 for storage of any type of object.
70 If the space is at least
74 the returned memory will be page boundary aligned as well.
85 normally initialize the returned memory to zero bytes.
89 function allocates space for
95 The result is identical to calling
99 with the exception that the allocated memory is explicitly initialized
104 function changes the size of the previously allocated memory referenced by
109 The contents of the memory are unchanged up to the lesser of the new and
111 If the new size is larger,
112 the value of the newly allocated portion of the memory is undefined.
113 If the requested memory cannot be allocated,
116 the memory referenced by
118 is valid and unchanged.
125 function behaves identically to
127 for the specified size.
131 function call is identical to the realloc function call, except that it
132 will free the passed pointer when the requested memory cannot be allocated.
135 specific API designed to ease the problems with traditional coding styles
136 for realloc causing memory leaks in libraries.
140 function causes the allocated memory referenced by
142 to be made available for future allocations.
149 Once, when the first call is made to one of these memory allocation
150 routines, various flags will be set or reset, which affect the
151 workings of this allocation implementation.
153 The ``name'' of the file referenced by the symbolic link named
154 .Pa /etc/malloc.conf ,
155 the value of the environment variable
157 and the string pointed to by the global variable
159 will be interpreted, in that order, character by character as flags.
161 Most flags are single letters,
162 where uppercase indicates that the behavior is set, or on,
163 and lowercase means that the behavior is not set, or off.
164 .Bl -tag -width indent
166 All warnings (except for the warning about unknown
167 flags being set) become fatal.
168 The process will call
172 Each byte of new memory allocated by
177 as well as all memory returned by
182 will be initialized to 0xd0.
183 This options also sets the
186 This is intended for debugging and will impact performance negatively.
188 Pass a hint to the kernel about pages unused by the allocation functions.
189 This will help performance if the system is paging excessively. This
190 option is off by default.
196 functions to always reallocate memory even if the initial allocation was
198 This can substantially aid in compacting memory.
205 Consult the source for details on this option.
207 Attempting to allocate zero bytes will return a
211 (The default behavior is to make a minimal allocation and return a
213 This option is provided for System V compatibility.
214 This option is incompatible with the
218 Rather than return failure for any allocation function,
219 display a diagnostic message on stderr and cause the program to drop
222 This option should be set at compile time by including the following in
224 .Bd -literal -offset indent
225 extern char *malloc_options;
226 malloc_options = "X";
229 This option implicitly sets the
233 options, and then zeros out the bytes that were requested.
234 This is intended for debugging and will impact performance negatively.
236 Reduce the size of the cache by a factor of two.
237 The default cache size is 16 pages.
238 This option can be specified multiple times.
240 Double the size of the cache by a factor of two.
241 The default cache size is 16 pages.
242 This option can be specified multiple times.
249 options are intended for testing and debugging.
250 An application which changes its behavior when these options are used
253 To set a systemwide reduction of cache size, and to dump core whenever
256 .Bd -literal -offset indent
257 ln -s 'A<' /etc/malloc.conf
260 To specify in the source that a program does no return value checking
261 on calls to these functions:
262 .Bd -literal -offset indent
263 extern char *malloc_options;
264 malloc_options = "X";
267 The following environment variables affect the execution of the allocation
269 .Bl -tag -width ".Ev MALLOC_OPTIONS"
270 .It Ev MALLOC_OPTIONS
271 If the environment variable
273 is set, the characters it contains will be interpreted as flags to the
274 allocation functions.
281 functions return a pointer to the allocated memory if successful; otherwise
284 pointer is returned and
293 functions return a pointer, possibly identical to
295 to the allocated memory
296 if successful; otherwise a
298 pointer is returned, and
302 if the error was the result of an allocation failure.
305 function always leaves the original buffer intact
306 when an error occurs, whereas
308 deallocates it in this case.
312 function returns no value.
313 .Sh DEBUGGING MALLOC PROBLEMS
314 The major difference between this implementation and other allocation
315 implementations is that the free pages are not accessed unless allocated,
316 and are aggressively returned to the kernel for reuse.
317 .Bd -ragged -offset indent
318 Most allocation implementations will store a data structure containing a
319 linked list in the free chunks of memory,
320 used to tie all the free memory together.
321 That can be suboptimal,
322 as every time the free-list is traversed,
323 the otherwise unused, and likely paged out,
324 pages are faulted into primary memory.
325 On systems which are paging,
326 this can result in a factor of five increase in the number of page-faults
330 A side effect of this architecture is that many minor transgressions on
331 the interface which would traditionally not be detected are in fact
332 detected. As a result, programs that have been running happily for
333 years may suddenly start to complain loudly, when linked with this
334 allocation implementation.
336 The first and most important thing to do is to set the
339 This option forces a coredump (if possible) at the first sign of trouble,
340 rather than the normal policy of trying to continue if at all possible.
342 It is probably also a good idea to recompile the program with suitable
343 options and symbols for debugger support.
345 If the program starts to give unusual results, coredump or generally behave
346 differently without emitting any of the messages listed in the next
347 section, it is likely because it depends on the storage being filled with
348 zero bytes. Try running it with
351 if that improves the situation, this diagnosis has been confirmed.
352 If the program still misbehaves,
353 the likely problem is accessing memory outside the allocated area,
354 more likely after than before the allocated area.
356 Alternatively, if the symptoms are not easy to reproduce, setting the
358 option may help provoke the problem.
360 In truly difficult cases, the
362 option, if supported by the kernel, can provide a detailed trace of
363 all calls made to these functions.
365 Unfortunately this implementation does not provide much detail about
366 the problems it detects, the performance impact for storing such information
367 would be prohibitive.
368 There are a number of allocation implementations available on the 'Net
369 which focus on detecting and pinpointing problems by trading performance
370 for extra sanity checks and detailed diagnostics.
371 .Sh DIAGNOSTIC MESSAGES
378 detect an error or warning condition,
379 a message will be printed to file descriptor STDERR_FILENO.
380 Errors will result in the process dumping core.
383 option is set, all warnings are treated as errors.
385 The following is a brief description of possible error messages and
389 .It "(ES): mumble mumble mumble"
390 The allocation functions were compiled with
392 defined, and an error was found during the additional error checking.
393 Consult the source code for further information.
394 .It "mmap(2) failed, check limits"
395 This most likely means that the system is dangerously overloaded or that
396 the process' limits are incorrectly specified.
397 .It "freelist is destroyed"
398 The internal free-list has been corrupted.
402 option was specified and an allocation of memory failed.
405 The following is a brief description of possible warning messages and
408 .It "chunk/page is already free"
409 The process attempted to
411 memory which had already been freed.
412 .It "junk pointer, ..."
413 A pointer specified to one of the allocation functions points outside the
414 bounds of the memory of which they are aware.
415 .It "malloc() has never been called"
416 No memory has been allocated,
417 yet something is being freed or
419 .It "modified (chunk-/page-) pointer"
420 The pointer passed to
425 .It "pointer to wrong page"
431 is trying to free does not reference a possible page.
433 A process has attempted to call an allocation function recursively.
434 This is not permitted. In particular, signal handlers should not
435 attempt to allocate memory.
436 .It "unknown char in MALLOC_OPTIONS"
437 An unknown option was specified.
440 option set, this warning is still only a warning.
448 .Pa /usr/share/doc/papers/malloc.ascii.gz
459 The present allocation implementation started out as a filesystem for a
460 drum attached to a 20bit binary challenged computer which was built
461 with discrete germanium transistors. It has since graduated to
462 handle primary storage rather than secondary.
463 It first appeared in its new shape and ability in
468 function first appeared in
471 .An Poul-Henning Kamp Aq phk@FreeBSD.org
473 The messages printed in case of problems provide no detail about the
476 It can be argued that returning a
478 pointer when asked to
479 allocate zero bytes is a silly response to a silly question.