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Diffstat (limited to 'lib/libc/stdlib/malloc.3')
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diff --git a/lib/libc/stdlib/malloc.3 b/lib/libc/stdlib/malloc.3 new file mode 100644 index 0000000..764bad1 --- /dev/null +++ b/lib/libc/stdlib/malloc.3 @@ -0,0 +1,448 @@ +.\" 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. All advertising materials mentioning features or use of this software +.\" must display the following acknowledgement: +.\" This product includes software developed by the University of +.\" California, Berkeley and its contributors. +.\" 4. 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 August 27, 1996 +.Dt MALLOC 3 +.Os FreeBSD 2 +.Sh NAME +.Nm malloc, calloc, realloc, free, reallocf +.Nd general purpose memory allocation functions +.Sh SYNOPSIS +.Fd #include <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 char * +.Va malloc_options; +.Sh DESCRIPTION +The +.Fn malloc +function allocates +.Fa size +bytes of memory. +The allocated space is suitably aligned (after possible pointer coercion) +for storage of any type of object. +If the space is at least +.Em pagesize +bytes in length (see +.Xr getpagesize 3 ), +the returned memory will be page boundary aligned as well. +If +.Fn malloc +fails, a NULL pointer is returned. +.Pp +Note that +.Fn malloc +does +.Em NOT +normally initialize the returned memory to zero bytes. +.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. +If the requested memory cannot be allocated, NULL is returned and +the memory referenced by +.Fa ptr +is valid and unchanged. +If +.Fa ptr +is NULL, the +.Fn realloc +function behaves identically to +.Fn malloc +for the specified size. +.Pp +The +.Fn reallocf +function call is identical to the realloc function call, except that it +will free the passed pointer when the requested memory cannot be allocated. +This is a FreeBSD +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 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), and failure to allocate memory become fatal. +The process will call +.Xr abort 3 +in these cases. +.It J +Each byte of new memory allocated by +.Fn malloc , +.Fn realloc +or +.Fn reallocf +as well as all memory returned by +.Fn free , +.Fn realloc +or +.Fn reallocf +will be initialized to 0xd0. +This options also sets the +.Dq R +option. +This is intended for debugging and will impact performance negatively. +.It H +Pass a hint to the kernel about pages unused by the allocation functions. +This will help performance if the system is paging excessively. This +option is off by default. +.It R +Causes the +.Fn realloc +and +.Fn reallocf +functions to always reallocate memory even if the initial allocation was +sufficiently large. +This can substantially aid in compacting memory. +.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 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 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 +extern char *malloc_options; +malloc_options = "X"; +.Ed +.It Z +This option implicitly sets the +.Dq J +and +.Dq R +options, and then zeros out the bytes that were requested. +This is intended for debugging and will impact performance negatively. +.It < +Reduce the size of the cache by a factor of two. +The default cache size is 16 pages. +This option can be specified multiple times. +.It > +Double the size of the cache by a factor of two. +The default cache size is 16 pages. +This option can be specified multiple times. +.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 EXAMPLES +To set a systemwide reduction of cache size, and 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 +extern char *malloc_options; +malloc_options = "X"; +.Ed +.Sh ENVIRONMENT +The following environment variables affect the execution of the allocation +functions: +.Bl -tag -width MMM +.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. +.Sh RETURN VALUES +The +.Fn malloc +and +.Fn calloc +functions return a pointer to the allocated memory if successful; otherwise +a NULL pointer is returned. +.Pp +The +.Fn realloc +and +.Fn reallocf +functions return a pointer, possibly identical to +.Fa ptr , +to the allocated memory +if successful; otherwise a NULL pointer is returned, in which case the +memory referenced by +.Fa ptr +is still available and intact. +.Pp +The +.Fn free +function returns no value. +.Sh DEBUGGING MALLOC PROBLEMS +.Pp +The major difference between this implementation and other allocation +implementations is that the free pages are not accessed unless allocated, +and are aggressively returned to the kernel for reuse. +.Bd -filled -offset indent +Most allocation implementations will store a data structure containing a +linked list in the free chunks of memory, +used to tie all the free memory together. +That can be suboptimal, +as every time the free-list is traversed, +the otherwise unused, and likely paged out, +pages are faulted into primary memory. +On systems which are paging, +this can result in a factor of five increase in the number of page-faults +done by a process. +.Ed +.Pp +A side effect of this architecture is that many minor transgressions on +the interface which would traditionally not be detected are in fact +detected. As a result, programs that have been running happily for +years may suddenly start to complain loudly, when linked with this +allocation implementation. +.Pp +The first and most important 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 listed in the next +section, it is likely because it depends on the storage being filled with +zero bytes. Try running it with +.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, +more likely after than before 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 'Net +which focus on detecting and pinpointing problems by trading performance +for extra sanity checks and detailed diagnostics. +.Sh DIAGNOSTIC MESSAGES +If +.Fn malloc , +.Fn calloc , +.Fn realloc +or +.Fn free +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 following is a brief description of possible error messages and +their meanings: +.Pp +.Bl -tag -width indent +.It "(ES): mumble mumble mumble +The allocation functions were compiled with +.Dq EXTRA_SANITY +defined, and an error was found during the additional error checking. +Consult the source code for further information. +.It "allocation failed +If the +.Dq A +option is specified it is a fatal error for an allocation function to fail. +.It "mmap(2) failed, check limits +This most likely means that the system is dangerously overloaded or that +the process' limits are incorrectly specified. +.It "freelist is destroyed +The internal free-list has been corrupted. +.El +.Pp +.Bl -tag -width indent +The following is a brief description of possible warning messages and +their meanings: +.Pp +.It "chunk/page is already free +The process attempted to +.Fn free +memory which had already been freed. +.It "junk pointer ... +A pointer specified to one of the allocation functions points outside the +bounds of the memory of which they are aware. +.It "malloc() has never been called +No memory has been allocated, +yet something is being freed or +realloc'ed. +.It "modified (chunk-/page-) pointer +The pointer passed to +.Fn free +or +.Fn realloc +has been modified. +.It "pointer to wrong page +The pointer that +.Fn malloc +or +.Fn calloc +is trying to free does not reference a possible page. +.It "recursive call +A process has attempted to call an allocation function recursively. +This is not permitted. In particular, signal handlers should not +attempt to allocate memory. +.It "out of memory +The +.Dq X +option was specified and an allocation of memory failed. +.It "unknown char in MALLOC_OPTIONS +An unknown option was specified. +Even with the +.Dq A +option set, this warning is still only a warning. +.Sh SEE ALSO +.Xr brk 2 , +.Xr alloca 3 , +.Xr getpagesize 3 , +.Xr memory 3 +.Pa /usr/share/doc/papers/malloc.ascii.gz +.Sh STANDARDS +The +.Fn malloc , +.Fn calloc , +.Fn realloc +and +.Fn free +functions conform to +.St -ansiC . +.Sh BUGS +The messages printed in case of problems provide no detail about the +actual values. +.Pp +It can be argued that returning a null pointer when asked to +allocate zero bytes is a silly response to a silly question. +.Pp +This implementation was authored by Poul-Henning Kamp. +Please report any problems to him at +.Li <phk@FreeBSD.org> . +.Sh HISTORY +The present allocation implementation started out as a filesystem for a +drum attached to a 20bit binary challenged computer which was built +with discrete germanium transistors. It has since graduated to +handle primary storage rather than secondary. +It first appeared in its new shape and ability in +.Fx 2.2 . |
