1 files changed, 389 insertions, 30 deletions

diff --git a/src/lib/libc/stdlib/malloc.3 b/src/lib/libc/stdlib/malloc.3
index 3bbf2bf65e..75b8c69eb7 100644
--- a/src/lib/libc/stdlib/malloc.3
+++ b/src/lib/libc/stdlib/malloc.3
@@ -1,5 +1,6 @@
-.\" Copyright (c) 1980, 1991 Regents of the University of California.
-.\" All rights reserved.
+.\"
+.\" Copyright (c) 1980, 1991, 1993
+.\"     The Regents of the University of California.  All rights reserved.
 .\"
 .\" This code is derived from software contributed to Berkeley by
 .\" the American National Standards Committee X3, on Information
@@ -13,11 +14,7 @@
 .\" 2. Redistributions in binary form must reproduce the above copyright
 .\"    notice, this list of conditions and the following disclaimer in the
 .\"    documentation and/or other materials provided with the distribution.
-.\" 3. All advertising materials mentioning features or use of this software
-.\"    must display the following acknowledgement:
-.\"     This product includes software developed by the University of
-.\"     California, Berkeley and its contributors.
-.\" 4. Neither the name of the University nor the names of its contributors
+.\" 3. Neither the name of the University nor the names of its contributors
 .\"    may be used to endorse or promote products derived from this software
 .\"    without specific prior written permission.
 .\"
@@ -33,19 +30,32 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.\"     from: @(#)malloc.3      6.7 (Berkeley) 6/29/91
-.\"     $Id: malloc.3,v 1.1.1.1 1995/10/18 08:42:18 deraadt Exp $
+.\"     $OpenBSD: malloc.3,v 1.62 2009/02/13 23:36:17 jmc Exp $
 .\"
-.Dd June 29, 1991
+.Dd $Mdocdate: February 13 2009 $
 .Dt MALLOC 3
-.Os BSD 4
+.Os
 .Sh NAME
-.Nm malloc
-.Nd general memory allocation function
+.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
@@ -59,33 +69,382 @@ 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
+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
-function returns
-a pointer to the allocated space if successful; otherwise
-a null pointer is returned.
+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 getpagesize 2 ,
-.Xr free 3 ,
-.Xr calloc 3 ,
+.Xr mmap 2 ,
+.Xr munmap 2 ,
 .Xr alloca 3 ,
-.Xr realloc 3 ,
-.Xr memory 3
+.Xr getpagesize 3
 .Sh STANDARDS
 The
 .Fn malloc
 function conforms to
 .St -ansiC .
-.Sh BUGS
-The current implementation of
-.Xr malloc
-does not always fail gracefully when system
-memory limits are approached.
-It may fail to allocate memory when larger free blocks could be broken
-up, or when limits are exceeded because the size is rounded up.
-It is optimized for sizes that are powers of two.
+.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.