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36 .\" @(#)malloc.3 8.1 (Berkeley) 6/4/93
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"
62 .Fn \*(lp*_malloc_message\*(rp "char *p1" "char *p2" "char *p3" "char *p4"
69 The allocated space is suitably aligned (after possible pointer coercion)
70 for storage of any type of object.
71 If the space is at least
75 the returned memory will be page boundary aligned as well.
86 normally initialize the returned memory to zero bytes.
90 function allocates space for
96 The result is identical to calling
100 with the exception that the allocated memory is explicitly initialized
105 function changes the size of the previously allocated memory referenced by
110 The contents of the memory are unchanged up to the lesser of the new and
112 If the new size is larger,
113 the value of the newly allocated portion of the memory is undefined.
114 If the requested memory cannot be allocated,
117 the memory referenced by
119 is valid and unchanged.
120 If memory can be allocated, the memory referenced by
122 is freed and a pointer to the newly allocated memory is returned.
127 may move the memory allocation resulting in a different return value than
135 function behaves identically to
137 for the specified size.
141 function is identical to the
143 function, except that it
144 will free the passed pointer when the requested memory cannot be allocated.
147 specific API designed to ease the problems with traditional coding styles
148 for realloc causing memory leaks in libraries.
152 function causes the allocated memory referenced by
154 to be made available for future allocations.
161 Once, when the first call is made to one of these memory allocation
162 routines, various flags will be set or reset, which affect the
163 workings of this allocation implementation.
165 The ``name'' of the file referenced by the symbolic link named
166 .Pa /etc/malloc.conf ,
167 the value of the environment variable
169 and the string pointed to by the global variable
171 will be interpreted, in that order, character by character as flags.
173 Most flags are single letters,
174 where uppercase indicates that the behavior is set, or on,
175 and lowercase means that the behavior is not set, or off.
176 .Bl -tag -width indent
178 All warnings (except for the warning about unknown
179 flags being set) become fatal.
180 The process will call
184 Each byte of new memory allocated by
189 as well as all memory returned by
194 will be initialized to 0xd0.
195 This is intended for debugging and will impact performance negatively.
197 Pass a hint to the kernel about pages unused by the allocation functions.
198 This will help performance if the system is paging excessively.
199 This option is off by default.
205 functions to always reallocate memory even if the initial allocation was
207 This can substantially aid in compacting memory.
214 Consult the source for details on this option.
216 Attempting to allocate zero bytes will return a
220 (The default behavior is to make a minimal allocation and return a
222 This option is provided for System V compatibility.
223 This option is incompatible with the
227 Rather than return failure for any allocation function,
228 display a diagnostic message on
230 and cause the program to drop
233 This option should be set at compile time by including the following in
235 .Bd -literal -offset indent
236 _malloc_options = "X";
239 This option implicitly sets the
241 option, and then zeros out the bytes that were requested.
242 This is intended for debugging and will impact performance negatively.
244 Reduce the size of the cache by a factor of two.
245 The default cache size is 16 pages.
246 This option can be specified multiple times.
248 Double the size of the cache by a factor of two.
249 The default cache size is 16 pages.
250 This option can be specified multiple times.
257 options are intended for testing and debugging.
258 An application which changes its behavior when these options are used
265 functions return a pointer to the allocated memory if successful; otherwise
268 pointer is returned and
277 functions return a pointer, possibly identical to
279 to the allocated memory
280 if successful; otherwise a
282 pointer is returned, and
286 if the error was the result of an allocation failure.
289 function always leaves the original buffer intact
290 when an error occurs, whereas
292 deallocates it in this case.
296 function returns no value.
297 .Sh DEBUGGING MALLOC PROBLEMS
298 The major difference between this implementation and other allocation
299 implementations is that the free pages are not accessed unless allocated,
300 and are aggressively returned to the kernel for reuse.
301 .Bd -ragged -offset indent
302 Most allocation implementations will store a data structure containing a
303 linked list in the free chunks of memory,
304 used to tie all the free memory together.
305 That can be suboptimal,
306 as every time the free-list is traversed,
307 the otherwise unused, and likely paged out,
308 pages are faulted into primary memory.
309 On systems which are paging,
310 this can result in a factor of five increase in the number of page-faults
314 A side effect of this architecture is that many minor transgressions on
315 the interface which would traditionally not be detected are in fact
317 As a result, programs that have been running happily for
318 years may suddenly start to complain loudly, when linked with this
319 allocation implementation.
321 The first and most important thing to do is to set the
324 This option forces a coredump (if possible) at the first sign of trouble,
325 rather than the normal policy of trying to continue if at all possible.
327 It is probably also a good idea to recompile the program with suitable
328 options and symbols for debugger support.
330 If the program starts to give unusual results, coredump or generally behave
331 differently without emitting any of the messages listed in the next
332 section, it is likely because it depends on the storage being filled with
337 if that improves the situation, this diagnosis has been confirmed.
338 If the program still misbehaves,
339 the likely problem is accessing memory outside the allocated area,
340 more likely after than before the allocated area.
342 Alternatively, if the symptoms are not easy to reproduce, setting the
344 option may help provoke the problem.
346 In truly difficult cases, the
348 option, if supported by the kernel, can provide a detailed trace of
349 all calls made to these functions.
351 Unfortunately this implementation does not provide much detail about
352 the problems it detects, the performance impact for storing such information
353 would be prohibitive.
354 There are a number of allocation implementations available on the 'Net
355 which focus on detecting and pinpointing problems by trading performance
356 for extra sanity checks and detailed diagnostics.
357 .Sh DIAGNOSTIC MESSAGES
364 detect an error or warning condition,
365 a message will be printed to file descriptor STDERR_FILENO.
366 Errors will result in the process dumping core.
369 option is set, all warnings are treated as errors.
373 variable allows the programmer to override the function which emits
374 the text strings forming the errors and warnings if for some reason
377 file descriptor is not suitable for this.
378 Please note that doing anything which tries to allocate memory in
379 this function will assure death of the process.
381 The following is a brief description of possible error messages and
385 .It "(ES): mumble mumble mumble"
386 The allocation functions were compiled with
388 defined, and an error was found during the additional error checking.
389 Consult the source code for further information.
390 .It "mmap(2) failed, check limits"
391 This most likely means that the system is dangerously overloaded or that
392 the process' limits are incorrectly specified.
393 .It "freelist is destroyed"
394 The internal free-list has been corrupted.
398 option was specified and an allocation of memory failed.
401 The following is a brief description of possible warning messages and
404 .It "chunk/page is already free"
405 The process attempted to
407 memory which had already been freed.
408 .It "junk pointer, ..."
409 A pointer specified to one of the allocation functions points outside the
410 bounds of the memory of which they are aware.
411 .It "malloc() has never been called"
412 No memory has been allocated,
413 yet something is being freed or
415 .It "modified (chunk-/page-) pointer"
416 The pointer passed to
421 .It "pointer to wrong page"
427 is trying to free does not reference a possible page.
429 A process has attempted to call an allocation function recursively.
430 This is not permitted.
431 In particular, signal handlers should not
432 attempt to allocate memory.
433 .It "unknown char in MALLOC_OPTIONS"
434 An unknown option was specified.
437 option set, this warning is still only a warning.
440 The following environment variables affect the execution of the allocation
442 .Bl -tag -width ".Ev MALLOC_OPTIONS"
443 .It Ev MALLOC_OPTIONS
444 If the environment variable
446 is set, the characters it contains will be interpreted as flags to the
447 allocation functions.
450 To set a systemwide reduction of cache size, and to dump core whenever
453 .Bd -literal -offset indent
454 ln -s 'A<' /etc/malloc.conf
457 To specify in the source that a program does no return value checking
458 on calls to these functions:
459 .Bd -literal -offset indent
460 _malloc_options = "X";
468 .Pa /usr/share/doc/papers/malloc.ascii.gz
479 The present allocation implementation started out as a file system for a
480 drum attached to a 20bit binary challenged computer which was built
481 with discrete germanium transistors.
482 It has since graduated to
483 handle primary storage rather than secondary.
484 It first appeared in its new shape and ability in
489 function first appeared in
492 .An Poul-Henning Kamp Aq phk@FreeBSD.org
494 The messages printed in case of problems provide no detail about the
497 It can be argued that returning a
499 pointer when asked to
500 allocate zero bytes is a silly response to a silly question.