Libc-320.tar.gz

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

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.Dd November 21, 2002
.Dt MALLOC 3
.Os
.Sh NAME
.Nm malloc , calloc , valloc , realloc , free , malloc_size , malloc_good_size
.Nd memory allocation
.Sh SYNOPSIS
.In stdlib.h
.Ft void *
.Fn malloc "size_t size"
.Ft void *
.Fn calloc "size_t count" "size_t size"
.Ft void *
.Fn valloc "size_t size"
.Ft void *
.Fn realloc "void *ptr" "size_t size"
.Ft void
.Fn free "void *ptr"
.Ft size_t
.Fn malloc_size "void *ptr"
.Ft size_t
.Fn malloc_good_size "size_t size"
.Sh DESCRIPTION
The
.Fn malloc ,
.Fn calloc ,
.Fn valloc ,
and
.Fn realloc
functions allocate memory.
The allocated memory is aligned such that it can be used for any data type,
including AltiVec-related types.
The
.Fn free
function frees allocations that were created via the preceding allocation
functions.
The
.Fn malloc_size
and
.Fn malloc_good_size
functions provide information related to the amount of padding space at the end
of allocations.
.Pp
The
.Fn malloc
function allocates
.Fa size
bytes of memory and returns a pointer to the allocated memory.
.Fn malloc
returns a
.Dv NULL
pointer if there is an error.
.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.
.Fn calloc
returns a
.Dv NULL
pointer if there is an error.
.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.
.Fn valloc
returns a
.Dv NULL
pointer if there is an error.
.Pp
The
.Fn realloc
function tries to change the size of the allocation pointed to by
.Fa ptr
to
.Fa size ,
and return
.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.
.Fn realloc
returns a
.Dv NULL
pointer if there is an error, and the allocation pointed to by
.Fa ptr
is still valid.
.Pp
The
.Fn free
function deallocates the memory allocation pointed to by
.Fa ptr .
.Pp
The
.Fn malloc_size
function
returns the size of the memory block that backs the allocation pointed to by
.Fa ptr .
The memory block size is always at least as large as the allocation it backs,
and may be larger.
.Pp
The
.Fn malloc_good_size
function rounds
.Fa size
up to a value that the allocator implementation can allocate without adding any
padding and returns that rounded up value.
.Sh RETURN VALUES
If successful, the
.Fn malloc ,
.Fn calloc ,
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
If successful, the
.Fn realloc
function returns a pointer to allocated memory.
If there is an error, it returns a
.Dv NULL
pointer and sets
.Va errno
to
.Er ENOMEM .
.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 MallocScribble
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 MallocBadFreeAbort <b>
If set to a non-zero value, causes
.Xr abort 3
to be called if the pointer passed to
.Xr free 3
was previously freed, or is otherwise illegal.
.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
.Pa /Developer/Documentation/ReleaseNotes/MallocOptions.html