libc/stdlib/malloc.3

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.\"	$OpenBSD: malloc.3,v 1.62 2009/02/13 23:36:17 jmc Exp $
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.Dd $Mdocdate: February 13 2009 $
.Dt MALLOC 3
.Os
.Sh NAME
.Nm malloc ,
.Nm calloc ,
.Nm realloc ,
.Nm free ,
.Nm cfree
.Nd memory allocation and deallocation
.Sh SYNOPSIS
.Fd #include <stdlib.h>
.Ft void *
.Fn malloc "size_t size"
.Ft void *
.Fn calloc "size_t nmemb" "size_t size"
.Ft void *
.Fn realloc "void *ptr" "size_t size"
.Ft void
.Fn free "void *ptr"
.Ft void
.Fn cfree "void *ptr"
.Ft char *
.Va malloc_options ;
.Sh DESCRIPTION
The
.Fn malloc
function allocates uninitialized space for an object whose
size is specified by
.Fa size .
The
.Fn malloc
function maintains multiple lists of free blocks according to size, allocating
space from the appropriate list.
.Pp
The allocated space is
suitably aligned (after possible pointer
coercion) for storage of any type of object.
If the space is of
.Em pagesize
or larger, the memory returned will be page-aligned.
.Pp
Allocation of a zero size object returns a pointer to a zero size object.
This zero size object is access protected, so any access to it will
generate an exception (SIGSEGV).
Many zero-sized objects can be placed consecutively in shared
protected pages.
The minimum size of the protection on each object is suitably aligned and
sized as previously stated, but the protection may extend further depending
on where in a protected zone the object lands.
.Pp
When using
.Fn malloc
be careful to avoid the following idiom:
.Bd -literal -offset indent
if ((p = malloc(num * size)) == NULL)
	err(1, "malloc");
.Ed
.Pp
The multiplication may lead to an integer overflow.
To avoid this,
.Fn calloc
is recommended.
.Pp
If
.Fn malloc
must be used, be sure to test for overflow:
.Bd -literal -offset indent
if (size && num > SIZE_MAX / size) {
	errno = ENOMEM;
	err(1, "overflow");
}
.Ed
.Pp
The
.Fn calloc
function allocates space for an array of
.Fa nmemb
objects, each of whose size is
.Fa size .
The space is initialized to zero.
The use of
.Fn calloc
is strongly encouraged when allocating multiple sized objects
in order to avoid possible integer overflows.
.Pp
The
.Fn free
function causes the space pointed to by
.Fa ptr
to be either placed on a list of free pages to make it available for future
allocation or, if required, to be returned to the kernel using
.Xr munmap 2 .
If
.Fa ptr
is a null pointer, no action occurs.
.Pp
A
.Fn cfree
function is also provided for compatibility with old systems and other
.Nm malloc
libraries; it is simply an alias for
.Fn free .
.Pp
The
.Fn realloc
function changes the size of the object pointed to by
.Fa ptr
to
.Fa size
bytes and returns a pointer to the (possibly moved) object.
The contents of the object 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 object is indeterminate and uninitialized.
If
.Fa ptr
is a null pointer, the
.Fn realloc
function behaves like the
.Fn malloc
function for the specified size.
If the space cannot be allocated, the object
pointed to by
.Fa ptr
is unchanged.
If
.Fa size
is zero and
.Fa ptr
is not a null pointer, the object it points to is freed and a new zero size
object is returned.
.Pp
When using
.Fn realloc
be careful to avoid the following idiom:
.Bd -literal -offset indent
size += 50;
if ((p = realloc(p, size)) == NULL)
	return (NULL);
.Ed
.Pp
Do not adjust the variable describing how much memory has been allocated
until the allocation has been successful.
This can cause aberrant program behavior if the incorrect size value is used.
In most cases, the above sample will also result in a leak of memory.
As stated earlier, a return value of
.Dv NULL
indicates that the old object still remains allocated.
Better code looks like this:
.Bd -literal -offset indent
newsize = size + 50;
if ((newp = realloc(p, newsize)) == NULL) {
	free(p);
	p = NULL;
	size = 0;
	return (NULL);
}
p = newp;
size = newsize;
.Ed
.Pp
As with
.Fn malloc
it is important to ensure the new size value will not overflow;
i.e. avoid allocations like the following:
.Bd -literal -offset indent
if ((newp = realloc(p, num * size)) == NULL) {
	...
.Ed
.Pp
Malloc will first look for a symbolic link called
.Pa /etc/malloc.conf
and next check the environment for a variable called
.Ev MALLOC_OPTIONS
and finally for the global variable
.Va malloc_options
and scan them for flags in that order.
Flags are single letters, uppercase means on, lowercase means off.
.Bl -tag -width indent
.It Cm A
.Dq Abort .
.Fn malloc
will coredump the process, rather than tolerate internal
inconsistencies or incorrect usage.
This is the default and a very handy debugging aid,
since the core file represents the time of failure,
rather than when the bogus pointer was used.
.It Cm D
.Dq Dump .
.Fn malloc
will dump statistics in a file called
.Pa malloc.out
at exit.
This option requires the library to have been compiled with -DMALLOC_STATS in
order to have any effect.
.It Cm F
.Dq Freeguard .
Enable use after free protection.
Unused pages on the freelist are read and write protected to
cause a segmentation fault upon access.
This will also switch off the delayed freeing of chunks,
reducing random behaviour but detecting double
.Fn free
calls as early as possible.
.It Cm G
.Dq Guard .
Enable guard pages.
Each page size or larger allocation is followed by a guard page that will
cause a segmentation fault upon any access.
.It Cm H
.Dq Hint .
Pass a hint to the kernel about pages we don't use.
If the machine is paging a lot this may help a bit.
.It Cm J
.Dq Junk .
Fill some junk into the area allocated.
Currently junk is bytes of 0xd0 when allocating; this is pronounced
.Dq Duh .
\&:-)
Freed chunks are filled with 0xdf.
.It Cm P
.Dq Move allocations within a page.
Allocations larger than half a page but smaller than a page
are aligned to the end of a page to catch buffer overruns in more
cases.
This is the default.
.It Cm R
.Dq realloc .
Always reallocate when
.Fn realloc
is called, even if the initial allocation was big enough.
This can substantially aid in compacting memory.
.\".Pp
.\".It Cm U
.\".Dq utrace .
.\"Generate entries for
.\".Xr ktrace 1
.\"for all operations.
.\"Consult the source for this one.
.It Cm X
.Dq xmalloc .
Rather than return failure,
.Xr abort 3
the program with a diagnostic message on stderr.
It is the intention that this option be set at compile time by
including in the source:
.Bd -literal -offset indent
extern char *malloc_options;
malloc_options = "X";
.Ed
.Pp
Note that this will cause code that is supposed to handle
out-of-memory conditions gracefully to abort instead.
.It Cm Z
.Dq Zero .
Fill some junk into the area allocated (see
.Cm J ) ,
except for the exact length the user asked for, which is zeroed.
.It Cm <
.Dq Half the cache size .
Decrease the size of the free page cache by a factor of two.
.It Cm >
.Dq Double the cache size .
Increase the size of the free page cache by a factor of two.
.El
.Pp
So to set a systemwide reduction of cache size and use guard pages:
.Dl # ln -s 'G\*(Lt' /etc/malloc.conf
.Pp
The flags are mostly for testing and debugging.
If a program changes behavior if any of these options (except
.Cm X )
are used,
it is buggy.
.Pp
The default number of free pages cached is 64.
.Sh RETURN VALUES
The
.Fn malloc
and
.Fn calloc
functions return a pointer to the allocated space if successful; otherwise,
a null pointer is returned and
.Va errno
is set to
.Er ENOMEM .
.Pp
The
.Fn free
and
.Fn cfree
functions return no value.
.Pp
The
.Fn realloc
function returns a pointer to the (possibly moved) allocated space
if successful; otherwise, a null pointer is returned and
.Va errno
is set to
.Er ENOMEM .
.Sh ENVIRONMENT
.Bl -tag -width Ev
.It Ev MALLOC_OPTIONS
See above.
.El
.Sh FILES
.Bl -tag -width "/etc/malloc.conf"
.It Pa /etc/malloc.conf
symbolic link to filename containing option flags
.El
.Sh DIAGNOSTICS
If
.Fn malloc ,
.Fn calloc ,
.Fn realloc ,
or
.Fn free
detect an error condition,
a message will be printed to file descriptor
2 (not using stdio).
Errors will result in the process being aborted,
unless the
.Cm a
option has been specified.
.Pp
Here is a brief description of the error messages and what they mean:
.Bl -tag -width Ds
.It Dq out of memory
If the
.Cm X
option is specified it is an error for
.Fn malloc ,
.Fn calloc ,
or
.Fn realloc
to return
.Dv NULL .
.It Dq malloc init mmap failed
This is a rather weird condition that is most likely to indicate a
seriously overloaded system or a ulimit restriction.
.It Dq bogus pointer (double free?)
An attempt to
.Fn free
or
.Fn realloc
an unallocated pointer was made.
.It Dq chunk is already free
There was an attempt to free a chunk that had already been freed.
.It Dq modified chunk-pointer
The pointer passed to
.Fn free
or
.Fn realloc
has been modified.
.It Dq recursive call
An attempt was made to call recursively into these functions, i.e., from a
signal handler.
This behavior is not supported.
In particular, signal handlers should
.Em not
use any of the
.Fn malloc
functions nor utilize any other functions which may call
.Fn malloc
(e.g.,
.Xr stdio 3
routines).
.It Dq unknown char in MALLOC_OPTIONS
We found something we didn't understand.
.It Dq malloc cache overflow/underflow
The internal malloc page cache has been corrupted.
.It Dq malloc free slot lost
The internal malloc page cache has been corrupted.
.It Dq guard size
An inconsistent guard size was detected.
.It any other error
.Fn malloc
detected an internal error;
consult sources and/or wizards.
.El
.Sh SEE ALSO
.Xr brk 2 ,
.Xr mmap 2 ,
.Xr munmap 2 ,
.Xr alloca 3 ,
.Xr getpagesize 3
.Sh STANDARDS
The
.Fn malloc
function conforms to
.St -ansiC .
.Sh HISTORY
The present implementation of
.Fn malloc
started out as a filesystem on a drum
attached to a 20-bit binary challenged computer built with discrete germanium
transistors, and it has since graduated to handle primary storage rather than
secondary.
.Pp
The main difference from other
.Fn malloc
implementations are believed to be that
the free pages are not accessed until allocated.
Most
.Fn malloc
implementations will store a data structure containing a,
possibly double-, linked list in the free chunks of memory, used to tie
all the free memory together.
That is a quite suboptimal thing to do.
Every time the free-list is traversed, all the otherwise unused, and very
likely paged out, pages get faulted into primary memory, just to see what
lies after them in the list.
.Pp
On systems which are paging, this can increase the page-faults
of a process by a factor of five.