libc/stdlib/malloc.3

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.\"	$OpenBSD: malloc.3,v 1.41 2005/07/26 04:20:23 jaredy Exp $
.\"
.Dd August 27, 1996
.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
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 all bits zero.
.Pp
The
.Fn free
function causes the space pointed to by
.Fa ptr
to be deallocated, that is, at least made available for further allocation,
but if possible, it will passed back to the kernel with
.Xr sbrk 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
one must 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 one knows 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
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 failure.
This is a very handy debugging aid, since the core file will represent the
time of failure, rather than when the null pointer was accessed.
.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.
.It Cm G
.Dq Guard .
Enable guard pages and chunk randomization.
Each page size or larger allocation is followed by a guard page that will
cause a segmentation fault upon any access.
Smaller than page size chunks are returned in a random order.
.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; this is pronounced
.Dq Duh .
\&:-)
.It Cm N
Do not output warning messages when encountering possible corruption
or bad pointers.
.It Cm P
.Dq Pointer Protection .
Pointer sized allocations are aligned to the end of a page to catch
sizeof(ptr) errors where sizeof(*ptr) is meant.
.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
.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 .
Reduce the size of the cache by a factor of two.
.It Cm >
.Dq Double the cache size .
Double the size of the cache by a factor of two.
.El
.Pp
So to set a systemwide reduction of cache size and coredumps on problems
one would:
.Li ln -s 'A<' /etc/malloc.conf
.Pp
The
.Cm J
and
.Cm Z
flags are mostly for testing and debugging.
If a program changes behavior if either of these options are used,
it is buggy.
.Pp
The default cache size is 16 pages.
.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 or warning condition,
a message will be printed to file descriptor
2 (not using stdio).
Errors will always result in the process being
.Xr abort 3 'ed.
If the
.Cm A
option has been specified, warnings will also
.Xr abort 3
the process.
.Pp
Here is a brief description of the error messages and what they mean:
.Bl -tag -width Ds
.It Dq (ES): mumble mumble mumble
.Fn malloc
has been compiled with
.Dv \&-DEXTRA_SANITY
and something looks fishy in there.
Consult sources and/or wizards.
.It Dq allocation failed
If the
.Cm A
option is specified it is an error for
.Fn malloc ,
.Fn calloc ,
or
.Fn realloc
to return
.Dv NULL .
.It Dq mmap(2) failed, check limits.
This is a rather weird condition that is most likely to indicate a
seriously overloaded system or a
.Xr ulimit 1
restriction.
.It Dq freelist is destroyed.
.Fn malloc Ns 's
internal freelist has been stomped on.
.El
.Pp
Here is a brief description of the warning messages and what they mean:
.Bl -tag -width Ds
.It Dq chunk/page is already free.
There was an attempt to free a chunk that had already been freed.
.It Dq junk pointer, too high to make sense.
The pointer doesn't make sense.
It's above the area of memory that
.Fn malloc
knows something about.
This could be a pointer from some
.Xr mmap 2 'ed
memory.
.It Dq junk pointer, too low to make sense.
The pointer doesn't make sense.
It's below the area of memory that
.Fn malloc
knows something about.
This pointer probably came from your data or bss segments.
.It Dq malloc() has never been called.
Nothing has ever been allocated, yet something is being freed or
realloc'ed.
.It Dq modified (chunk-/page-) pointer.
The pointer passed to
.Fn free
or
.Fn realloc
has been modified.
.It Dq pointer to wrong page.
The pointer that
.Fn malloc
is trying to free is not pointing to
a sensible page.
.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.
.El
.Sh SEE ALSO
.Xr brk 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.