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@(#)malloc.3 8.1 (Berkeley) 6/4/93
$FreeBSD: head/lib/libc/stdlib/malloc.3 173969 2007-11-27 03:18:26Z jasone $
.Dd October 1, 2007 .Dt MALLOC 3 .Os .Sh NAME .Nm malloc , calloc , realloc , free , reallocf , malloc_usable_size .Nd general purpose memory allocation functions .Sh LIBRARY .Lb libc .Sh SYNOPSIS n 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 n malloc_np.h .Ft size_t .Fn malloc_usable_size "const void *ptr" .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.
p 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.
p 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.
p 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.
p 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.
p The .Fn malloc_usable_size function returns the usable size of the allocation pointed to by .Fa ptr . The return value may be larger than the size that was requested during allocation. The .Fn malloc_usable_size function is not a mechanism for in-place .Fn realloc ; rather it is provided solely as a tool for introspection purposes. Any discrepancy between the requested allocation size and the size reported by .Fn malloc_usable_size should not be depended on, since such behavior is entirely implementation-dependent. .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 allocator implementation.
p The .Dq name of the file referenced by the symbolic link named
a /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.
p 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. l -tag -width indent t A All warnings (except for the warning about unknown flags being set) become fatal. The process will call .Xr abort 3 in these cases. t B Increase/decrease the per-arena lock contention threshold at which a thread is randomly re-assigned to an arena. This dynamic load balancing tends to push threads away from highly contended arenas, which avoids worst case contention scenarios in which threads disproportionately utilize arenas. However, due to the highly dynamic load that applications may place on the allocator, it is impossible for the allocator to know in advance how sensitive it should be to contention over arenas. Therefore, some applications may benefit from increasing or decreasing this threshold parameter. This option is not available for some configurations (non-PIC). This option can be specified multiple times. t H Use .Xr madvise 2 when pages within a chunk are no longer in use, but the chunk as a whole cannot yet be deallocated. This is primarily of use when swapping is a real possibility, due to the high overhead of the .Fn madvise system call. t 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. t K Increase/decrease the virtual memory chunk size by a factor of two. The default chunk size is 1 MB. This option can be specified multiple times. t L Increase/decrease the per-arena number of slots for lazy deallocation. Lazy deallocation can decrease lock contention, especially for programs that use the producer/consumer model. The default is 256 slots per arena (so .Ev MALLOC_OPTIONS=lllllllll will disable lazy deallocation), but note that due to algorithmic details, the cache is typically flushed well before it completely fills. This option has no impact unless there are multiple CPUs, and lazy deallocation does not activate unless the program uses multiple threads. This option is not available for some configurations (non-PIC). This option can be specified multiple times. t N Increase/decrease the number of arenas by a factor of two. The default number of arenas is four times the number of CPUs, or one if there is a single CPU. This option can be specified multiple times. t 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. t 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. t S Increase/decrease the size of the maximum size class that is a multiple of the quantum by a factor of two. Above this size, power-of-two spacing is used for size classes. The default value is 512 bytes. This option can be specified multiple times. t U Generate .Dq utrace entries for .Xr ktrace 1 , for all operations. Consult the source for details on this option. t 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. t 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: d -literal -offset indent _malloc_options = "X"; .Ed t Z Each byte of new memory allocated by .Fn malloc , .Fn realloc or .Fn reallocf will be initialized to 0. 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
p 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 IMPLEMENTATION NOTES Traditionally, allocators have used .Xr sbrk 2 to obtain memory, but this implementation uses .Xr mmap 2 , and only uses .Xr sbrk 2 under limited circumstances, and only for 32-bit architectures. As a result, the .Ar datasize resource limit has little practical effect for typical applications. The .Ar vmemoryuse resource limit, however, can be used to bound the total virtual memory used by a process, as described in .Xr limits 1 .
p 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.
p Memory is conceptually broken into equal-sized chunks, where the chunk size is a power of two that is greater than the page size. Chunks are always aligned to multiples of the chunk size. This alignment makes it possible to find metadata for user objects very quickly.
p User objects are broken into three categories according to size: small, large, and huge. Small objects are no larger than one half of a page. Large objects are smaller than the chunk size. Huge objects are a multiple of the chunk size. Small and large objects are managed by arenas; huge objects are managed separately in a single data structure that is shared by all threads. Huge objects are used by applications infrequently enough that this single data structure is not a scalability issue.
p Each chunk that is managed by an arena tracks its contents in a page map as runs of contiguous pages (unused, backing a set of small objects, or backing one large object). The combination of chunk alignment and chunk page maps makes it possible to determine all metadata regarding small and large allocations in constant time.
p Small objects are managed in groups by page runs. Each run maintains a bitmap that tracks which regions are in use. Allocation requests that are no more than half the quantum (see the .Dq Q option) are rounded up to the nearest power of two (typically 2, 4, or 8). Allocation requests that are more than half the quantum, but no more than the maximum quantum-multiple size class (see the .Dq S option) are rounded up to the nearest multiple of the quantum. Allocation requests that are larger than the maximum quantum-multiple size class, but no larger than one half of a page, are rounded up to the nearest power of two. Allocation requests that are larger than half of a page, but small enough to fit in an arena-managed chunk (see the .Dq K option), are rounded up to the nearest run size. Allocation requests that are too large to fit in an arena-managed chunk are rounded up to the nearest multiple of the chunk size.
p Allocations are packed tightly together, which can be an issue for multi-threaded applications. If you need to assure that allocations do not suffer from cache line sharing, round your allocation requests up to the nearest multiple of the cache line size. .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.
p It is probably also a good idea to recompile the program with suitable options and symbols for debugger support.
p 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.
p Alternatively, if the symptoms are not easy to reproduce, setting the .Dq J option may help provoke the problem.
p 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.
p 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 allocator 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 .Dv STDERR_FILENO . Errors will result in the process dumping core. If the .Dq A option is set, all warnings are treated as errors.
p 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.
p All messages are prefixed by .Dq Ao Ar progname Ac Ns Li : (malloc) . .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 .
p 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.
p The .Fn free function returns no value.
p The .Fn malloc_usable_size function returns the usable size of the allocation pointed to by .Fa ptr . .Sh ENVIRONMENT The following environment variables affect the execution of the allocation functions: l -tag -width ".Ev MALLOC_OPTIONS" t 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:
p d -literal -offset indent ln -s 'A' /etc/malloc.conf .Ed
p To specify in the source that a program does no return value checking on calls to these functions: d -literal -offset indent _malloc_options = "X"; .Ed .Sh SEE ALSO .Xr limits 1 , .Xr madvise 2 , .Xr mmap 2 , .Xr sbrk 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 .
p The .Fn malloc_usable_size function first appeared in .Fx 7.0 .