diff options
Diffstat (limited to 'lib/libc/stdlib/malloc.3')
| -rw-r--r-- | lib/libc/stdlib/malloc.3 | 443 |
1 files changed, 443 insertions, 0 deletions
diff --git a/lib/libc/stdlib/malloc.3 b/lib/libc/stdlib/malloc.3 new file mode 100644 index 0000000..ba11ca2 --- /dev/null +++ b/lib/libc/stdlib/malloc.3 @@ -0,0 +1,443 @@ +.\" Copyright (c) 1980, 1991, 1993 +.\" The Regents of the University of California. All rights reserved. +.\" +.\" This code is derived from software contributed to Berkeley by +.\" the American National Standards Committee X3, on Information +.\" Processing Systems. +.\" +.\" Redistribution and use in source and binary forms, with or without +.\" modification, are permitted provided that the following conditions +.\" are met: +.\" 1. Redistributions of source code must retain the above copyright +.\" notice, this list of conditions and the following disclaimer. +.\" 2. Redistributions in binary form must reproduce the above copyright +.\" notice, this list of conditions and the following disclaimer in the +.\" documentation and/or other materials provided with the distribution. +.\" 3. Neither the name of the University nor the names of its contributors +.\" may be used to endorse or promote products derived from this software +.\" without specific prior written permission. +.\" +.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND +.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE +.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE +.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE +.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL +.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS +.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) +.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT +.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY +.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF +.\" SUCH DAMAGE. +.\" +.\" @(#)malloc.3 8.1 (Berkeley) 6/4/93 +.\" $FreeBSD$ +.\" +.Dd January 12, 2006 +.Dt MALLOC 3 +.Os +.Sh NAME +.Nm malloc , calloc , realloc , free , reallocf +.Nd general purpose memory allocation functions +.Sh LIBRARY +.Lb libc +.Sh SYNOPSIS +.In stdlib.h +.Ft void * +.Fn malloc "size_t size" +.Ft void * +.Fn calloc "size_t number" "size_t size" +.Ft void * +.Fn realloc "void *ptr" "size_t size" +.Ft void * +.Fn reallocf "void *ptr" "size_t size" +.Ft void +.Fn free "void *ptr" +.Ft const char * +.Va _malloc_options ; +.Ft void +.Fo \*(lp*_malloc_message\*(rp +.Fa "const char *p1" "const char *p2" "const char *p3" "const char *p4" +.Fc +.Sh DESCRIPTION +The +.Fn malloc +function allocates +.Fa size +bytes of uninitialized memory. +The allocated space is suitably aligned (after possible pointer coercion) +for storage of any type of object. +.Pp +The +.Fn calloc +function allocates space for +.Fa number +objects, +each +.Fa size +bytes in length. +The result is identical to calling +.Fn malloc +with an argument of +.Dq "number * size" , +with the exception that the allocated memory is explicitly initialized +to zero bytes. +.Pp +The +.Fn realloc +function changes the size of the previously allocated memory referenced by +.Fa ptr +to +.Fa size +bytes. +The contents of the memory are unchanged up to the lesser of the new and +old sizes. +If the new size is larger, +the value of the newly allocated portion of the memory is undefined. +Upon success, the memory referenced by +.Fa ptr +is freed and a pointer to the newly allocated memory is returned. +Note that +.Fn realloc +and +.Fn reallocf +may move the memory allocation, resulting in a different return value than +.Fa ptr . +If +.Fa ptr +is +.Dv NULL , +the +.Fn realloc +function behaves identically to +.Fn malloc +for the specified size. +.Pp +The +.Fn reallocf +function is identical to the +.Fn realloc +function, except that it +will free the passed pointer when the requested memory cannot be allocated. +This is a +.Fx +specific API designed to ease the problems with traditional coding styles +for realloc causing memory leaks in libraries. +.Pp +The +.Fn free +function causes the allocated memory referenced by +.Fa ptr +to be made available for future allocations. +If +.Fa ptr +is +.Dv NULL , +no action occurs. +.Sh TUNING +Once, when the first call is made to one of these memory allocation +routines, various flags will be set or reset, which affect the +workings of this allocation implementation. +.Pp +The ``name'' of the file referenced by the symbolic link named +.Pa /etc/malloc.conf , +the value of the environment variable +.Ev MALLOC_OPTIONS , +and the string pointed to by the global variable +.Va _malloc_options +will be interpreted, in that order, character by character as flags. +.Pp +Most flags are single letters, +where uppercase indicates that the behavior is set, or on, +and lowercase means that the behavior is not set, or off. +.Bl -tag -width indent +.It A +All warnings (except for the warning about unknown +flags being set) become fatal. +The process will call +.Xr abort 3 +in these cases. +.It C +Increase/decrease the size of the cache by a factor of two. +The default cache size is 256 objects for each arena. +This option can be specified multiple times. +.It J +Each byte of new memory allocated by +.Fn malloc , +.Fn realloc +or +.Fn reallocf +will be initialized to 0xa5. +All memory returned by +.Fn free , +.Fn realloc +or +.Fn reallocf +will be initialized to 0x5a. +This is intended for debugging and will impact performance negatively. +.It K +Increase/decrease the virtual memory chunk size by a factor of two. +The default chunk size is 16 MB. +This option can be specified multiple times. +.It N +Increase/decrease the number of arenas by a factor of two. +The default number of arenas is twice the number of CPUs, or one if there is a +single CPU. +This option can be specified multiple times. +.It P +Various statistics are printed at program exit via an +.Xr atexit 3 +function. +This has the potential to cause deadlock for a multi-threaded process that exits +while one or more threads are executing in the memory allocation functions. +Therefore, this option should only be used with care; it is primarily intended +as a performance tuning aid during application development. +.It Q +Increase/decrease the size of the allocation quantum by a factor of two. +The default quantum is the minimum allowed by the architecture (typically 8 or +16 bytes). +This option can be specified multiple times. +.It U +Generate +.Dq utrace +entries for +.Xr ktrace 1 , +for all operations. +Consult the source for details on this option. +.It V +Attempting to allocate zero bytes will return a +.Dv NULL +pointer instead of +a valid pointer. +(The default behavior is to make a minimal allocation and return a +pointer to it.) +This option is provided for System V compatibility. +This option is incompatible with the +.Dq X +option. +.It X +Rather than return failure for any allocation function, +display a diagnostic message on +.Dv stderr +and cause the program to drop +core (using +.Xr abort 3 ) . +This option should be set at compile time by including the following in +the source code: +.Bd -literal -offset indent +_malloc_options = "X"; +.Ed +.It Z +Each byte of new memory allocated by +.Fn malloc , +.Fn realloc +or +.Fn reallocf +will be initialized to 0x0. +Note that this initialization only happens once for each byte, so +.Fn realloc +and +.Fn reallocf +calls do not zero memory that was previously allocated. +This is intended for debugging and will impact performance negatively. +.El +.Pp +The +.Dq J +and +.Dq Z +options are intended for testing and debugging. +An application which changes its behavior when these options are used +is flawed. +.Sh RETURN VALUES +The +.Fn malloc +and +.Fn calloc +functions return a pointer to the allocated memory if successful; otherwise +a +.Dv NULL +pointer is returned and +.Va errno +is set to +.Er ENOMEM . +.Pp +The +.Fn realloc +and +.Fn reallocf +functions return a pointer, possibly identical to +.Fa ptr , +to the allocated memory +if successful; otherwise a +.Dv NULL +pointer is returned, and +.Va errno +is set to +.Er ENOMEM +if the error was the result of an allocation failure. +The +.Fn realloc +function always leaves the original buffer intact +when an error occurs, whereas +.Fn reallocf +deallocates it in this case. +.Pp +The +.Fn free +function returns no value. +.Sh IMPLEMENTATION NOTES +This allocator uses multiple arenas in order to reduce lock contention for +threaded programs on multi-processor systems. +This works well with regard to threading scalability, but incurs some costs. +There is a small fixed per-arena overhead, and additionally, arenas manage +memory completely independently of each other, which means a small fixed +increase in overall memory fragmentation. +These overheads are not generally an issue, given the number of arenas normally +used. +Note that using substantially more arenas than the default is not likely to +improve performance, mainly due to reduced cache performance. +However, it may make sense to reduce the number of arenas if an application +does not make much use of the allocation functions. +.Pp +This allocator uses a novel approach to object caching. +For objects below a size threshold (use the +.Dq P +option to discover the threshold), full deallocation and attempted coalescence +with adjacent memory regions are delayed. +This is so that if the application requests an allocation of that size soon +thereafter, the request can be met much more quickly. +Most applications heavily use a small number of object sizes, so this caching +has the potential to have a large positive performance impact. +However, the effectiveness of the cache depends on the cache being large enough +to absorb typical fluctuations in the number of allocated objects. +If an application routinely fluctuates by thousands of objects, then it may +make sense to increase the size of the cache. +Conversely, if an application's memory usage fluctuates very little, it may +make sense to reduce the size of the cache, so that unused regions can be +coalesced sooner. +.Pp +This allocator is very aggressive about tightly packing objects in memory, even +for objects much larger than the system page size. +For programs that allocate objects larger than half the system page size, this +has the potential to reduce memory footprint in comparison to other allocators. +However, it has some side effects that are important to keep in mind. +First, even multi-page objects can start at non-page-aligned addresses, since +the implementation only guarantees quantum alignment. +Second, this tight packing of objects can cause objects to share L1 cache +lines, which can be a performance issue for multi-threaded applications. +There are two ways to approach these issues. +First, +.Fn posix_memalign +provides the ability to align allocations as needed. +By aligning an allocation to at least the L1 cache line size, and padding the +allocation request by one cache line unit, the programmer can rest assured that +no cache line sharing will occur for the object. +Second, the +.Dq Q +option can be used to force all allocations to be aligned with the L1 cache +lines. +This approach should be used with care though, because although easy to +implement, it means that all allocations must be at least as large as the +quantum, which can cause severe internal fragmentation if the application +allocates many small objects. +.Sh DEBUGGING MALLOC PROBLEMS +The first thing to do is to set the +.Dq A +option. +This option forces a coredump (if possible) at the first sign of trouble, +rather than the normal policy of trying to continue if at all possible. +.Pp +It is probably also a good idea to recompile the program with suitable +options and symbols for debugger support. +.Pp +If the program starts to give unusual results, coredump or generally behave +differently without emitting any of the messages mentioned in the next +section, it is likely because it depends on the storage being filled with +zero bytes. +Try running it with the +.Dq Z +option set; +if that improves the situation, this diagnosis has been confirmed. +If the program still misbehaves, +the likely problem is accessing memory outside the allocated area. +.Pp +Alternatively, if the symptoms are not easy to reproduce, setting the +.Dq J +option may help provoke the problem. +.Pp +In truly difficult cases, the +.Dq U +option, if supported by the kernel, can provide a detailed trace of +all calls made to these functions. +.Pp +Unfortunately this implementation does not provide much detail about +the problems it detects; the performance impact for storing such information +would be prohibitive. +There are a number of allocation implementations available on the Internet +which focus on detecting and pinpointing problems by trading performance for +extra sanity checks and detailed diagnostics. +.Sh DIAGNOSTIC MESSAGES +If any of the memory allocation/deallocation functions detect an error or +warning condition, a message will be printed to file descriptor STDERR_FILENO. +Errors will result in the process dumping core. +If the +.Dq A +option is set, all warnings are treated as errors. +.Pp +The +.Va _malloc_message +variable allows the programmer to override the function which emits +the text strings forming the errors and warnings if for some reason +the +.Dv stderr +file descriptor is not suitable for this. +Please note that doing anything which tries to allocate memory in +this function is likely to result in a crash or deadlock. +.Pp +All messages are prefixed by: +.Bl -diag +.It <progname>: (malloc) +.El +.Sh ENVIRONMENT +The following environment variables affect the execution of the allocation +functions: +.Bl -tag -width ".Ev MALLOC_OPTIONS" +.It Ev MALLOC_OPTIONS +If the environment variable +.Ev MALLOC_OPTIONS +is set, the characters it contains will be interpreted as flags to the +allocation functions. +.El +.Sh EXAMPLES +To dump core whenever a problem occurs: +.Pp +.Bd -literal -offset indent +ln -s 'A' /etc/malloc.conf +.Ed +.Pp +To specify in the source that a program does no return value checking +on calls to these functions: +.Bd -literal -offset indent +_malloc_options = "X"; +.Ed +.Sh SEE ALSO +.Xr mmap 2 , +.Xr alloca 3 , +.Xr atexit 3 , +.Xr getpagesize 3 , +.Xr memory 3 , +.Xr posix_memalign 3 +.Sh STANDARDS +The +.Fn malloc , +.Fn calloc , +.Fn realloc +and +.Fn free +functions conform to +.St -isoC . +.Sh HISTORY +The +.Fn reallocf +function first appeared in +.Fx 3.0 . |
