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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 $
43 .Nm malloc , calloc , realloc , free , reallocf
44 .Nd general purpose memory allocation functions
50 .Fn malloc "size_t size"
52 .Fn calloc "size_t number" "size_t size"
54 .Fn realloc "void *ptr" "size_t size"
56 .Fn reallocf "void *ptr" "size_t size"
67 The allocated space is suitably aligned (after possible pointer coercion)
68 for storage of any type of object.
69 If the space is at least
73 the returned memory will be page boundary aligned as well.
84 normally initialize the returned memory to zero bytes.
88 function allocates space for
94 The result is identical to calling
98 with the exception that the allocated memory is explicitly initialized
103 function changes the size of the previously allocated memory referenced by
108 The contents of the memory are unchanged up to the lesser of the new and
110 If the new size is larger,
111 the value of the newly allocated portion of the memory is undefined.
112 If the requested memory cannot be allocated,
115 the memory referenced by
117 is valid and unchanged.
124 function behaves identically to
126 for the specified size.
130 function call is identical to the realloc function call, except that it
131 will free the passed pointer when the requested memory cannot be allocated.
134 specific API designed to ease the problems with traditional coding styles
135 for realloc causing memory leaks in libraries.
139 function causes the allocated memory referenced by
141 to be made available for future allocations.
148 Once, when the first call is made to one of these memory allocation
149 routines, various flags will be set or reset, which affect the
150 workings of this allocation implementation.
152 The ``name'' of the file referenced by the symbolic link named
153 .Pa /etc/malloc.conf ,
154 the value of the environment variable
156 and the string pointed to by the global variable
158 will be interpreted, in that order, character by character as flags.
160 Most flags are single letters,
161 where uppercase indicates that the behavior is set, or on,
162 and lowercase means that the behavior is not set, or off.
163 .Bl -tag -width indent
165 All warnings (except for the warning about unknown
166 flags being set) become fatal.
167 The process will call
171 Each byte of new memory allocated by
176 as well as all memory returned by
181 will be initialized to 0xd0.
182 This options also sets the
185 This is intended for debugging and will impact performance negatively.
187 Pass a hint to the kernel about pages unused by the allocation functions.
188 This will help performance if the system is paging excessively. This
189 option is off by default.
195 functions to always reallocate memory even if the initial allocation was
197 This can substantially aid in compacting memory.
204 Consult the source for details on this option.
206 Attempting to allocate zero bytes will return a
210 (The default behavior is to make a minimal allocation and return a
212 This option is provided for System V compatibility.
213 This option is incompatible with the
217 Rather than return failure for any allocation function,
218 display a diagnostic message on stderr and cause the program to drop
221 This option should be set at compile time by including the following in
223 .Bd -literal -offset indent
224 extern char *malloc_options;
225 malloc_options = "X";
228 This option implicitly sets the
232 options, and then zeros out the bytes that were requested.
233 This is intended for debugging and will impact performance negatively.
235 Reduce the size of the cache by a factor of two.
236 The default cache size is 16 pages.
237 This option can be specified multiple times.
239 Double the size of the cache by a factor of two.
240 The default cache size is 16 pages.
241 This option can be specified multiple times.
248 options are intended for testing and debugging.
249 An application which changes its behavior when these options are used
252 To set a systemwide reduction of cache size, and to dump core whenever
255 .Bd -literal -offset indent
256 ln -s 'A<' /etc/malloc.conf
259 To specify in the source that a program does no return value checking
260 on calls to these functions:
261 .Bd -literal -offset indent
262 extern char *malloc_options;
263 malloc_options = "X";
266 The following environment variables affect the execution of the allocation
268 .Bl -tag -width ".Ev MALLOC_OPTIONS"
269 .It Ev MALLOC_OPTIONS
270 If the environment variable
272 is set, the characters it contains will be interpreted as flags to the
273 allocation functions.
280 functions return a pointer to the allocated memory if successful; otherwise
283 pointer is returned and
292 functions return a pointer, possibly identical to
294 to the allocated memory
295 if successful; otherwise a
297 pointer is returned, and
301 if the error was the result of an allocation failure.
304 function always leaves the original buffer intact
305 when an error occurs, whereas
307 deallocates it in this case.
311 function returns no value.
312 .Sh DEBUGGING MALLOC PROBLEMS
313 The major difference between this implementation and other allocation
314 implementations is that the free pages are not accessed unless allocated,
315 and are aggressively returned to the kernel for reuse.
316 .Bd -ragged -offset indent
317 Most allocation implementations will store a data structure containing a
318 linked list in the free chunks of memory,
319 used to tie all the free memory together.
320 That can be suboptimal,
321 as every time the free-list is traversed,
322 the otherwise unused, and likely paged out,
323 pages are faulted into primary memory.
324 On systems which are paging,
325 this can result in a factor of five increase in the number of page-faults
329 A side effect of this architecture is that many minor transgressions on
330 the interface which would traditionally not be detected are in fact
331 detected. As a result, programs that have been running happily for
332 years may suddenly start to complain loudly, when linked with this
333 allocation implementation.
335 The first and most important thing to do is to set the
338 This option forces a coredump (if possible) at the first sign of trouble,
339 rather than the normal policy of trying to continue if at all possible.
341 It is probably also a good idea to recompile the program with suitable
342 options and symbols for debugger support.
344 If the program starts to give unusual results, coredump or generally behave
345 differently without emitting any of the messages listed in the next
346 section, it is likely because it depends on the storage being filled with
347 zero bytes. Try running it with
350 if that improves the situation, this diagnosis has been confirmed.
351 If the program still misbehaves,
352 the likely problem is accessing memory outside the allocated area,
353 more likely after than before the allocated area.
355 Alternatively, if the symptoms are not easy to reproduce, setting the
357 option may help provoke the problem.
359 In truly difficult cases, the
361 option, if supported by the kernel, can provide a detailed trace of
362 all calls made to these functions.
364 Unfortunately this implementation does not provide much detail about
365 the problems it detects, the performance impact for storing such information
366 would be prohibitive.
367 There are a number of allocation implementations available on the 'Net
368 which focus on detecting and pinpointing problems by trading performance
369 for extra sanity checks and detailed diagnostics.
370 .Sh DIAGNOSTIC MESSAGES
377 detect an error or warning condition,
378 a message will be printed to file descriptor STDERR_FILENO.
379 Errors will result in the process dumping core.
382 option is set, all warnings are treated as errors.
384 The following is a brief description of possible error messages and
388 .It "(ES): mumble mumble mumble"
389 The allocation functions were compiled with
391 defined, and an error was found during the additional error checking.
392 Consult the source code for further information.
393 .It "mmap(2) failed, check limits"
394 This most likely means that the system is dangerously overloaded or that
395 the process' limits are incorrectly specified.
396 .It "freelist is destroyed"
397 The internal free-list has been corrupted.
401 option was specified and an allocation of memory failed.
404 The following is a brief description of possible warning messages and
407 .It "chunk/page is already free"
408 The process attempted to
410 memory which had already been freed.
411 .It "junk pointer, ..."
412 A pointer specified to one of the allocation functions points outside the
413 bounds of the memory of which they are aware.
414 .It "malloc() has never been called"
415 No memory has been allocated,
416 yet something is being freed or
418 .It "modified (chunk-/page-) pointer"
419 The pointer passed to
424 .It "pointer to wrong page"
430 is trying to free does not reference a possible page.
432 A process has attempted to call an allocation function recursively.
433 This is not permitted. In particular, signal handlers should not
434 attempt to allocate memory.
435 .It "unknown char in MALLOC_OPTIONS"
436 An unknown option was specified.
439 option set, this warning is still only a warning.
447 .Pa /usr/share/doc/papers/malloc.ascii.gz
458 The present allocation implementation started out as a filesystem for a
459 drum attached to a 20bit binary challenged computer which was built
460 with discrete germanium transistors. It has since graduated to
461 handle primary storage rather than secondary.
462 It first appeared in its new shape and ability in
467 function first appeared in
470 .An Poul-Henning Kamp Aq phk@FreeBSD.org
472 The messages printed in case of problems provide no detail about the
475 It can be argued that returning a
477 pointer when asked to
478 allocate zero bytes is a silly response to a silly question.