Libc-583.tar.gz

[apple/libc.git] / gen / malloc.3

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.Dd Aug 13, 2008
.Dt MALLOC 3
.Os
.Sh NAME
.Nm calloc ,
.Nm free ,
.Nm malloc ,
.Nm realloc ,
.Nm reallocf ,
.Nm valloc
.Nd memory allocation
.Sh SYNOPSIS
.In stdlib.h
.Ft void *
.Fo calloc
.Fa "size_t count"
.Fa "size_t size"
.Fc
.Ft void
.Fo free
.Fa "void *ptr"
.Fc
.Ft void *
.Fo malloc
.Fa "size_t size"
.Fc
.Ft void *
.Fo realloc
.Fa "void *ptr"
.Fa "size_t size"
.Fc
.Ft void *
.Fo reallocf
.Fa "void *ptr"
.Fa "size_t size"
.Fc
.Ft void *
.Fo valloc
.Fa "size_t size"
.Fc
.Sh DESCRIPTION
The
.Fn malloc ,
.Fn calloc ,
.Fn valloc ,
.Fn realloc ,
and
.Fn reallocf
functions allocate memory.
The allocated memory is aligned such that it can be used for any data type,
including AltiVec- and SSE-related types.
The
.Fn free
function frees allocations that were created via the preceding allocation
functions.
.Pp
The
.Fn malloc
function allocates
.Fa size
bytes of memory and returns a pointer to the allocated memory.
.Pp
The
.Fn calloc
function contiguously allocates enough space for
.Fa count
objects that are
.Fa size
bytes of memory each and returns a pointer to the allocated memory.
The allocated memory is filled with bytes of value zero.
.Pp
The
.Fn valloc
function allocates
.Fa size
bytes of memory and returns a pointer to the allocated memory.
The allocated memory is aligned on a page boundary.
.Pp
The
.Fn realloc
function tries to change the size of the allocation pointed to by
.Fa ptr
to
.Fa size ,
and returns
.Fa ptr .
If there is not enough room to enlarge the memory allocation pointed to by
.Fa ptr ,
.Fn realloc
creates a new allocation, copies as much of the old data pointed to by
.Fa ptr
as will fit to the new allocation, frees the old allocation, and returns a
pointer to the allocated memory.
If
.Fa ptr
is 
.Dv NULL ,
.Fn realloc
is identical to a call to 
.Fn malloc
for 
.Fa size
bytes.
If
.Fa size
is zero and 
.Fa ptr
is not 
.Dv NULL ,
a new, minimum sized object is allocated and the original object is freed.
.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 deallocates the memory allocation pointed to by
.Fa ptr .  If
.Fa ptr 
is a NULL pointer, no operation is performed.
.Sh RETURN VALUES
If successful,
.Fn calloc ,
.Fn malloc ,
.Fn realloc ,
.Fn reallocf ,
and
.Fn valloc
functions return a pointer to allocated memory.
If there is an error, they return a
.Dv NULL
pointer and set
.Va errno
to
.Er ENOMEM .
.Pp
For
.Fn realloc ,
the input pointer is still valid if reallocation failed.
For
.Fn reallocf ,
the input pointer will have been freed if reallocation failed.
.Pp
The
.Fn free
function does not return a value.
.Sh DEBUGGING ALLOCATION ERRORS
A number of facilities are provided to aid in debugging allocation errors in
applications.
These facilities are primarily controlled via environment variables.
The recognized environment variables and their meanings are documented below.
.Sh ENVIRONMENT
The following environment variables change the behavior of the
allocation-related functions.
.Bl -tag -width ".Ev MallocStackLoggingNoCompact"
.It Ev MallocLogFile <f>
Create/append messages to the given file path
.Fa <f>
instead of writing to the standard error.
.It Ev MallocGuardEdges
If set, add a guard page before and after each large block.
.It Ev MallocDoNotProtectPrelude
If set, do not add a guard page before large blocks,
even if the
.Ev MallocGuardEdges
environment variable is set.
.It Ev MallocDoNotProtectPostlude
If set, do not add a guard page after large blocks,
even if the
.Ev MallocGuardEdges
environment variable is set.
.It Ev MallocStackLogging
If set, record all stacks, so that tools like
.Nm leaks
can be used.
.It Ev MallocStackLoggingNoCompact
If set, record all stacks in a manner that is compatible with the
.Nm malloc_history
program.
.It Ev MallocStackLoggingDirectory
If set, records stack logs to the directory specified instead of saving them to the default location (/tmp).
.It Ev MallocScribble
If set, fill memory that has been allocated with 0xaa bytes.
This increases the likelihood that a program making assumptions about the contents of
freshly allocated memory will fail.
Also if set, fill memory that has been deallocated with 0x55 bytes.
This increases the likelihood that a program will fail due to accessing memory
that is no longer allocated.
.It Ev MallocCheckHeapStart <s>
If set, specifies the number of allocations
.Fa <s>
to wait before begining periodic heap checks every
.Fa <n>
as specified by 
.Ev MallocCheckHeapEach .
If
.Ev MallocCheckHeapStart
is set but 
.Ev MallocCheckHeapEach
is not specified, the default check repetition is 1000.
.It Ev MallocCheckHeapEach <n>
If set, run a consistency check on the heap every
.Fa <n>
operations.
.Ev MallocCheckHeapEach
is only meaningful if
.Ev MallocCheckHeapStart
is also set.
.It Ev MallocCheckHeapSleep <t>
Sets the number of seconds to sleep (waiting for a debugger to attach) when
.Ev MallocCheckHeapStart
is set and a heap corruption is detected.
The default is 100 seconds.
Setting this to zero means not to sleep at all.
Setting this to a negative number means to sleep (for the positive number of
seconds) only the very first time a heap corruption is detected.
.It Ev MallocCheckHeapAbort <b>
When
.Ev MallocCheckHeapStart
is set and this is set to a non-zero value, causes
.Xr abort 3
to be called if a heap corruption is detected, instead of any sleeping.
.It Ev MallocErrorAbort
If set, causes
.Xr abort 3
to be called if an error was encountered in
.Xr malloc 3
or 
.Xr free 3
, such as a calling
.Xr free 3
on a pointer previously freed.
.It Ev MallocCorruptionAbort
Similar to
.Ev
MallocErrorAbort 
but will not abort in out of memory conditions, making it more useful to catch
only those errors which will cause memory corruption.
MallocCorruptionAbort is always set on 64-bit processes.
.It Ev MallocHelp
If set, print a list of environment variables that are paid heed to by the
allocation-related functions, along with short descriptions.
The list should correspond to this documentation.
.El
.Sh DIAGNOSTIC MESSAGES
.Sh SEE ALSO
.Xr leaks 1 ,
.Xr malloc_history 1 ,
.Xr abort 3 ,
.Xr malloc_size 3 ,
.Xr malloc_zone_malloc 3 ,
.Xr posix_memalign 3 ,
.Xr libgmalloc 3