libc/stdlib/malloc.3

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.\"	$OpenBSD: malloc.3,v 1.85 2014/10/30 21:47:47 deraadt Exp $
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.Dd $Mdocdate: October 30 2014 $
.Dt MALLOC 3
.Os
.Sh NAME
.Nm malloc ,
.Nm calloc ,
.Nm reallocarray ,
.Nm realloc ,
.Nm free ,
.Nm cfree
.Nd memory allocation and deallocation
.Sh SYNOPSIS
.In stdlib.h
.Ft void *
.Fn malloc "size_t size"
.Ft void *
.Fn calloc "size_t nmemb" "size_t size"
.Ft void *
.Fn reallocarray "void *ptr" "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 * Ns
.Va malloc_options ;
.Sh DESCRIPTION
The
.Fn malloc
function allocates uninitialized space for an object of
the specified
.Fa size .
.Fn malloc
maintains multiple lists of free blocks according to size, allocating
space from the appropriate list.
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
The
.Fn calloc
function allocates space for an array of
.Fa nmemb
objects, each of the specified
.Fa size .
The space is initialized to zero.
.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 the space cannot be allocated, the object
pointed to by
.Fa ptr
is unchanged.
If
.Fa ptr
is
.Dv NULL ,
.Fn realloc
behaves like
.Fn malloc
and allocates a new object.
.Pp
The
.Fn reallocarray
function is similar to
.Fn realloc
except it operates on
.Fa nmemb
members of size
.Fa size
and checks for integer overflow in the calculation
.Fa nmemb
*
.Fa size .
.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
.Dv NULL
pointer, no action occurs.
If
.Fa ptr
was previously freed by
.Fn free
.Fn realloc ,
or
.Fn reallocarray ,
the behavior is undefined and the double free is a security concern.
.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 .
.Sh RETURN VALUES
Upon successful completion, the functions
.Fn malloc ,
.Fn calloc ,
.Fn realloc ,
and
.Fn reallocarray
return a pointer to the allocated space; otherwise, a
.Dv NULL
pointer is returned and
.Va errno
is set to
.Er ENOMEM .
.Pp
If
.Fa size
or
.Fa nmemb
is equal to 0, a unique pointer to an access protected,
zero sized object is returned.
Access via this pointer will generate a
.Dv SIGSEGV
exception.
.Pp
If multiplying
.Fa nmemb
and
.Fa size
results in integer overflow,
.Fn calloc
and
.Fn reallocarray
return
.Dv NULL
and set
.Va errno
to
.Er ENOMEM .
.Pp
The
.Fn free
and
.Fn cfree
functions return no value.
.Sh IDIOMS
Consider
.Fn calloc
or the extension
.Fn reallocarray
when there is multiplication in the
.Fa size
argument of
.Fn malloc
or
.Fn realloc .
For example, avoid this common idiom as it may lead to integer overflow:
.Bd -literal -offset indent
if ((p = malloc(num * size)) == NULL)
	err(1, "malloc");
.Ed
.Pp
A drop-in replacement is the
.Ox
extension
.Fn reallocarray :
.Bd -literal -offset indent
if ((p = reallocarray(NULL, num, size)) == NULL)
	err(1, "reallocarray");
.Ed
.Pp
Alternatively,
.Fn calloc
may be used at the cost of initialization overhead.
.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
Instead, use
.Fn reallocarray :
.Bd -literal -offset indent
if ((newp = reallocarray(p, num, size)) == NULL) {
	...
.Ed
.Pp
Calling
.Fn realloc
with a
.Dv NULL
.Fa ptr
is equivalent to calling
.Fn malloc .
Instead of this idiom:
.Bd -literal -offset indent
if (p == NULL)
	newp = malloc(newsize);
else
	newp = realloc(p, newsize);
.Ed
.Pp
Use the following:
.Bd -literal -offset indent
newp = realloc(p, newsize);
.Ed
.Sh ENVIRONMENT
.Bl -tag -width Ev
.It Ev MALLOC_OPTIONS
See below.
.El
.Sh FILES
.Bl -tag -width "/etc/malloc.conf"
.It Pa /etc/malloc.conf
symbolic link to filename containing option flags
.El
.Sh EXAMPLES
If
.Fn malloc
must be used with multiplication, be sure to test for overflow:
.Bd -literal -offset indent
size_t num, size;
\&...

/* Check for size_t overflow */
if (size && num > SIZE_MAX / size)
	errc(1, EOVERFLOW, "overflow");

if ((p = malloc(size * num)) == NULL)
	err(1, "malloc");
.Ed
.Pp
The above test is not sufficient in all cases.
For example, multiplying ints requires a different set of checks:
.Bd -literal -offset indent
int num, size;
\&...

/* Avoid invalid requests */
if (size < 0 || num < 0)
	errc(1, EOVERFLOW, "overflow");

/* Check for signed int overflow */
if (size && num > INT_MAX / size)
	errc(1, EOVERFLOW, "overflow");

if ((p = malloc(size * num)) == NULL)
	err(1, "malloc");
.Ed
.Pp
Assuming the implementation checks for integer overflow as
.Ox
does, it is much easier to use
.Fn calloc
or
.Fn reallocarray .
.Pp
The above examples could be simplified to:
.Bd -literal -offset indent
if ((p = reallocarray(NULL, num, size)) == NULL)
	err(1, "reallocarray");
.Ed
.Pp
or at the cost of initialization:
.Bd -literal -offset indent
if ((p = calloc(num, size)) == NULL)
	err(1, "calloc");
.Ed
.Sh DIAGNOSTICS
If
.Fn malloc ,
.Fn calloc ,
.Fn realloc ,
.Fn reallocarray ,
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 ,
.Fn realloc ,
or
.Fn reallocarray
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 ,
.Fn realloc ,
or
.Fn reallocarray
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 ,
.Fn realloc ,
or
.Fn reallocarray
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 MALLOC_OPTIONS
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 to the file
.Pa ./malloc.out ,
if it already exists,
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 detection.
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.
This option is intended for debugging rather than improved security
(use the
.Cm U
option for security).
.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 j
.Dq Don't Junk .
By default, small chunks are always junked, and the first part of pages
is junked after free.
This option ensures that no junking is performed.
.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 S
Enable all options suitable for security auditing.
.It Cm U
.Dq Free unmap .
Enable use after free protection for larger allocations.
Unused pages on the freelist are read and write protected to
cause a segmentation fault upon access.
.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 <
.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 the cache to a quarter of the
default size and use guard pages:
.Dl # ln -s 'G<<' /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 SEE ALSO
.Xr brk 2 ,
.Xr mmap 2 ,
.Xr munmap 2 ,
.Xr alloca 3 ,
.Xr getpagesize 3 ,
.Xr posix_memalign 3 ,
.Xr sysconf 3
.Sh STANDARDS
The
.Fn malloc ,
.Fn calloc ,
.Fn realloc ,
and
.Fn free
functions conform to
.St -ansiC .
.Pp
If
.Fa size
or
.Fa nmemb
are 0, the return value is implementation defined;
other conforming implementations may return
.Dv NULL
in this case.
.Pp
The standard does not require
.Fn calloc
to check for integer overflow,
but most modern implementations provide this check.
.Pp
The
.Ev MALLOC_OPTIONS
environment variable, the file
.Pa /etc/malloc.conf ,
and the
.Sx DIAGNOSTICS
output are extensions to the standard.
.Sh HISTORY
A
.Fn free
internal kernel function and a predecessor to
.Fn malloc ,
.Fn alloc ,
first appeared in
.At v1 .
C library functions
.Fn alloc
and
.Fn free
appeared in
.At v6 .
The functions
.Fn malloc ,
.Fn calloc ,
and
.Fn realloc
first appeared in
.At v7 .
.Pp
A new implementation by Chris Kingsley was introduced in
.Bx 4.2 ,
followed by a complete rewrite by Poul-Henning Kamp which appeared in
.Fx 2.2
and was included in
.Ox 2.0 .
These implementations were all
.Xr sbrk 2
based.
In
.Ox 3.8 ,
Thierry Deval rewrote
.Nm
to use the
.Xr mmap 2
system call,
making the page addresses returned by
.Nm
random.
A rewrite by Otto Moerbeek introducing a new central data structure and more
randomization appeared in
.Ox 4.4 .
.Pp
The
.Fn reallocarray
function appeared in
.Ox 5.6 .
.Pp
The
.Fn cfree
function appeared in SunOS 4.x.
.Sh CAVEATS
When using
.Fn malloc ,
be wary of signed integer and
.Vt size_t
overflow especially when there is multiplication in the
.Fa size
argument.
.Pp
Signed integer overflow will cause undefined behavior which compilers
typically handle by wrapping back around to negative numbers.
Depending on the input, this can result in allocating more or less
memory than intended.
.Pp
An unsigned overflow has defined behavior which will wrap back around and
return less memory than intended.
.Pp
A signed or unsigned integer overflow is a
.Em security
risk if less memory is returned than intended.
Subsequent code may corrupt the heap by writing beyond the memory that was
allocated.
An attacker may be able to leverage this heap corruption to execute arbitrary
code.
.Pp
Consider using
.Fn calloc
or
.Fn reallocarray
instead of using multiplication in
.Fn malloc
and
.Fn realloc
to avoid these problems on
.Ox .