1.\" Copyright (c) 1980, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" This code is derived from software contributed to Berkeley by 5.\" the American National Standards Committee X3, on Information 6.\" Processing Systems. 7.\" 8.\" Redistribution and use in source and binary forms, with or without 9.\" modification, are permitted provided that the following conditions 10.\" are met: 11.\" 1. Redistributions of source code must retain the above copyright 12.\" notice, this list of conditions and the following disclaimer. 13.\" 2. Redistributions in binary form must reproduce the above copyright 14.\" notice, this list of conditions and the following disclaimer in the 15.\" documentation and/or other materials provided with the distribution. 16.\" 3. All advertising materials mentioning features or use of this software 17.\" must display the following acknowledgement: 18.\" This product includes software developed by the University of 19.\" California, Berkeley and its contributors. 20.\" 4. Neither the name of the University nor the names of its contributors 21.\" may be used to endorse or promote products derived from this software 22.\" without specific prior written permission. 23.\" 24.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 25.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 26.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 27.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 28.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 29.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 30.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 31.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 32.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 33.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 34.\" SUCH DAMAGE. 35.\" 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/libc/stdlib/malloc.3,v 1.5 2005/08/05 22:35:10 swildner Exp $ 39.\" 40.Dd August 27, 1996 41.Dt MALLOC 3 42.Os 43.Sh NAME 44.Nm malloc , 45.Nm calloc , 46.Nm realloc , 47.Nm free , 48.Nm reallocf 49.Nd general purpose memory allocation functions 50.Sh LIBRARY 51.Lb libc 52.Sh SYNOPSIS 53.In stdlib.h 54.Ft void * 55.Fn malloc "size_t size" 56.Ft void * 57.Fn calloc "size_t number" "size_t size" 58.Ft void * 59.Fn realloc "void *ptr" "size_t size" 60.Ft void * 61.Fn reallocf "void *ptr" "size_t size" 62.Ft void 63.Fn free "void *ptr" 64.Ft char * 65.Va malloc_options; 66.Sh DESCRIPTION 67The 68.Fn malloc 69function allocates 70.Fa size 71bytes of memory. 72The allocated space is suitably aligned (after possible pointer coercion) 73for storage of any type of object. 74If the space is at least 75.Em pagesize 76bytes in length (see 77.Xr getpagesize 3 ) , 78the returned memory will be page boundary aligned as well. 79If 80.Fn malloc 81fails, a 82.Dv NULL 83pointer is returned. 84.Pp 85Note that 86.Fn malloc 87does 88.Em NOT 89normally initialize the returned memory to zero bytes. 90.Pp 91The 92.Fn calloc 93function allocates space for 94.Fa number 95objects, 96each 97.Fa size 98bytes in length. 99The result is identical to calling 100.Fn malloc 101with an argument of 102.Dq "number * size" , 103with the exception that the allocated memory is explicitly initialized 104to zero bytes. 105.Pp 106The 107.Fn realloc 108function changes the size of the previously allocated memory referenced by 109.Fa ptr 110to 111.Fa size 112bytes. 113The contents of the memory are unchanged up to the lesser of the new and 114old sizes. 115If the new size is larger, 116the value of the newly allocated portion of the memory is undefined. 117If the requested memory cannot be allocated, 118.Dv NULL 119is returned and 120the memory referenced by 121.Fa ptr 122is valid and unchanged. 123If 124.Fa ptr 125is 126.Dv NULL , 127the 128.Fn realloc 129function behaves identically to 130.Fn malloc 131for the specified size. 132.Pp 133The 134.Fn reallocf 135function call is identical to the realloc function call, except that it 136will free the passed pointer when the requested memory cannot be allocated. 137This is a 138.Fx 139/ 140.Dx 141specific API designed to ease the problems with traditional coding styles 142for realloc causing memory leaks in libraries. 143.Pp 144The 145.Fn free 146function causes the allocated memory referenced by 147.Fa ptr 148to be made available for future allocations. 149If 150.Fa ptr 151is 152.Dv NULL , 153no action occurs. 154.Sh TUNING 155Once, when the first call is made to one of these memory allocation 156routines, various flags will be set or reset, which affect the 157workings of this allocation implementation. 158.Pp 159The ``name'' of the file referenced by the symbolic link named 160.Pa /etc/malloc.conf , 161the value of the environment variable 162.Ev MALLOC_OPTIONS , 163and the string pointed to by the global variable 164.Va malloc_options 165will be interpreted, in that order, character by character as flags. 166.Pp 167Most flags are single letters, 168where uppercase indicates that the behavior is set, or on, 169and lowercase means that the behavior is not set, or off. 170.Bl -tag -width indent 171.It A 172All warnings (except for the warning about unknown 173flags being set) become fatal. 174The process will call 175.Xr abort 3 176in these cases. 177.It J 178Each byte of new memory allocated by 179.Fn malloc , 180.Fn realloc 181or 182.Fn reallocf 183as well as all memory returned by 184.Fn free , 185.Fn realloc 186or 187.Fn reallocf 188will be initialized to 0xd0. 189This options also sets the 190.Dq R 191option. 192This is intended for debugging and will impact performance negatively. 193.It H 194Pass a hint to the kernel about pages unused by the allocation functions. 195This will help performance if the system is paging excessively. This 196option is off by default. 197.It R 198Causes the 199.Fn realloc 200and 201.Fn reallocf 202functions to always reallocate memory even if the initial allocation was 203sufficiently large. 204This can substantially aid in compacting memory. 205.It U 206Generate 207.Dq utrace 208entries for 209.Xr ktrace 1 , 210for all operations. 211Consult the source for details on this option. 212.It V 213Attempting to allocate zero bytes will return a 214.Dv NULL 215pointer instead of 216a valid pointer. 217(The default behavior is to make a minimal allocation and return a 218pointer to it.) 219This option is provided for System V compatibility. 220This option is incompatible with the 221.Dq X 222option. 223.It X 224Rather than return failure for any allocation function, 225display a diagnostic message on stderr and cause the program to drop 226core (using 227.Xr abort 3 ) . 228This option should be set at compile time by including the following in 229the source code: 230.Bd -literal -offset indent 231extern char *malloc_options; 232malloc_options = "X"; 233.Ed 234.It Z 235This option implicitly sets the 236.Dq J 237and 238.Dq R 239options, and then zeros out the bytes that were requested. 240This is intended for debugging and will impact performance negatively. 241.It < 242Reduce the size of the cache by a factor of two. 243The default cache size is 16 pages. 244This option can be specified multiple times. 245.It > 246Double the size of the cache by a factor of two. 247The default cache size is 16 pages. 248This option can be specified multiple times. 249.El 250.Pp 251The 252.Dq J 253and 254.Dq Z 255options are intended for testing and debugging. 256An application which changes its behavior when these options are used 257is flawed. 258.Sh EXAMPLES 259To set a systemwide reduction of cache size, and to dump core whenever 260a problem occurs: 261.Pp 262.Bd -literal -offset indent 263ln -s 'A<' /etc/malloc.conf 264.Ed 265.Pp 266To specify in the source that a program does no return value checking 267on calls to these functions: 268.Bd -literal -offset indent 269extern char *malloc_options; 270malloc_options = "X"; 271.Ed 272.Sh ENVIRONMENT 273The following environment variables affect the execution of the allocation 274functions: 275.Bl -tag -width ".Ev MALLOC_OPTIONS" 276.It Ev MALLOC_OPTIONS 277If the environment variable 278.Ev MALLOC_OPTIONS 279is set, the characters it contains will be interpreted as flags to the 280allocation functions. 281.El 282.Sh RETURN VALUES 283The 284.Fn malloc 285and 286.Fn calloc 287functions return a pointer to the allocated memory if successful; otherwise 288a 289.Dv NULL 290pointer is returned and 291.Va errno 292is set to 293.Er ENOMEM . 294.Pp 295The 296.Fn realloc 297and 298.Fn reallocf 299functions return a pointer, possibly identical to 300.Fa ptr , 301to the allocated memory 302if successful; otherwise a 303.Dv NULL 304pointer is returned, and 305.Va errno 306is set to 307.Er ENOMEM 308if the error was the result of an allocation failure. 309The 310.Fn realloc 311function always leaves the original buffer intact 312when an error occurs, whereas 313.Fn reallocf 314deallocates it in this case. 315.Pp 316The 317.Fn free 318function returns no value. 319.Sh DEBUGGING MALLOC PROBLEMS 320The major difference between this implementation and other allocation 321implementations is that the free pages are not accessed unless allocated, 322and are aggressively returned to the kernel for reuse. 323.Bd -ragged -offset indent 324Most allocation implementations will store a data structure containing a 325linked list in the free chunks of memory, 326used to tie all the free memory together. 327That can be suboptimal, 328as every time the free-list is traversed, 329the otherwise unused, and likely paged out, 330pages are faulted into primary memory. 331On systems which are paging, 332this can result in a factor of five increase in the number of page-faults 333done by a process. 334.Ed 335.Pp 336A side effect of this architecture is that many minor transgressions on 337the interface which would traditionally not be detected are in fact 338detected. As a result, programs that have been running happily for 339years may suddenly start to complain loudly, when linked with this 340allocation implementation. 341.Pp 342The first and most important thing to do is to set the 343.Dq A 344option. 345This option forces a coredump (if possible) at the first sign of trouble, 346rather than the normal policy of trying to continue if at all possible. 347.Pp 348It is probably also a good idea to recompile the program with suitable 349options and symbols for debugger support. 350.Pp 351If the program starts to give unusual results, coredump or generally behave 352differently without emitting any of the messages listed in the next 353section, it is likely because it depends on the storage being filled with 354zero bytes. Try running it with 355.Dq Z 356option set; 357if that improves the situation, this diagnosis has been confirmed. 358If the program still misbehaves, 359the likely problem is accessing memory outside the allocated area, 360more likely after than before the allocated area. 361.Pp 362Alternatively, if the symptoms are not easy to reproduce, setting the 363.Dq J 364option may help provoke the problem. 365.Pp 366In truly difficult cases, the 367.Dq U 368option, if supported by the kernel, can provide a detailed trace of 369all calls made to these functions. 370.Pp 371Unfortunately this implementation does not provide much detail about 372the problems it detects, the performance impact for storing such information 373would be prohibitive. 374There are a number of allocation implementations available on the 'Net 375which focus on detecting and pinpointing problems by trading performance 376for extra sanity checks and detailed diagnostics. 377.Sh DIAGNOSTIC MESSAGES 378If 379.Fn malloc , 380.Fn calloc , 381.Fn realloc 382or 383.Fn free 384detect an error or warning condition, 385a message will be printed to file descriptor STDERR_FILENO. 386Errors will result in the process dumping core. 387If the 388.Dq A 389option is set, all warnings are treated as errors. 390.Pp 391The following is a brief description of possible error messages and 392their meanings: 393.Pp 394.Bl -diag 395.It "(ES): mumble mumble mumble" 396The allocation functions were compiled with 397.Dq EXTRA_SANITY 398defined, and an error was found during the additional error checking. 399Consult the source code for further information. 400.It "mmap(2) failed, check limits" 401This most likely means that the system is dangerously overloaded or that 402the process' limits are incorrectly specified. 403.It "freelist is destroyed" 404The internal free-list has been corrupted. 405.It "out of memory" 406The 407.Dq X 408option was specified and an allocation of memory failed. 409.El 410.Pp 411The following is a brief description of possible warning messages and 412their meanings: 413.Bl -diag 414.It "chunk/page is already free" 415The process attempted to 416.Fn free 417memory which had already been freed. 418.It "junk pointer, ..." 419A pointer specified to one of the allocation functions points outside the 420bounds of the memory of which they are aware. 421.It "malloc() has never been called" 422No memory has been allocated, 423yet something is being freed or 424realloc'ed. 425.It "modified (chunk-/page-) pointer" 426The pointer passed to 427.Fn free 428or 429.Fn realloc 430has been modified. 431.It "pointer to wrong page" 432The pointer that 433.Fn free , 434.Fn realloc , 435or 436.Fn reallocf 437is trying to free does not reference a possible page. 438.It "recursive call" 439A process has attempted to call an allocation function recursively. 440This is not permitted. In particular, signal handlers should not 441attempt to allocate memory. 442.It "unknown char in MALLOC_OPTIONS" 443An unknown option was specified. 444Even with the 445.Dq A 446option set, this warning is still only a warning. 447.El 448.Sh SEE ALSO 449.Xr brk 2 , 450.Xr mmap 2 , 451.Xr alloca 3 , 452.Xr getpagesize 3 , 453.Xr memory 3 454.Pa /usr/share/doc/papers/malloc.ascii.gz 455.Sh STANDARDS 456The 457.Fn malloc , 458.Fn calloc , 459.Fn realloc 460and 461.Fn free 462functions conform to 463.St -isoC . 464.Sh HISTORY 465The present allocation implementation started out as a filesystem for a 466drum attached to a 20bit binary challenged computer which was built 467with discrete germanium transistors. It has since graduated to 468handle primary storage rather than secondary. 469It first appeared in its new shape and ability in 470.Fx 2.2 . 471.Pp 472The 473.Fn reallocf 474function first appeared in 475.Fx 3.0 . 476.Sh AUTHORS 477.An Poul-Henning Kamp Aq phk@FreeBSD.org 478.Sh BUGS 479The messages printed in case of problems provide no detail about the 480actual values. 481.Pp 482It can be argued that returning a 483.Dv NULL 484pointer when asked to 485allocate zero bytes is a silly response to a silly question. 486