Wed Jan 17 11:04:58 1996 Roland McGrath <roland@churchy.gnu.ai.mit.edu>

[glibc.git] / manual / memory.texi

@comment !!! describe mmap et al (here?)
@c !!! doc brk/sbrk

@node Memory Allocation, Character Handling, Error Reporting, Top
@chapter Memory Allocation
@cindex memory allocation
@cindex storage allocation

The GNU system provides several methods for allocating memory space
under explicit program control.  They vary in generality and in
efficiency.

@iftex
@itemize @bullet
@item
The @code{malloc} facility allows fully general dynamic allocation.
@xref{Unconstrained Allocation}.

@item
Obstacks are another facility, less general than @code{malloc} but more
efficient and convenient for stacklike allocation.  @xref{Obstacks}.

@item
The function @code{alloca} lets you allocate storage dynamically that
will be freed automatically.  @xref{Variable Size Automatic}.
@end itemize
@end iftex

@menu
* Memory Concepts::             An introduction to concepts and terminology.
* Dynamic Allocation and C::    How to get different kinds of allocation in C.
* Unconstrained Allocation::    The @code{malloc} facility allows fully general
                                 dynamic allocation.
* Obstacks::                    Obstacks are less general than malloc
                                 but more efficient and convenient.
* Variable Size Automatic::     Allocation of variable-sized blocks
                                 of automatic storage that are freed when the
                                 calling function returns.
* Relocating Allocator::        Waste less memory, if you can tolerate
                                 automatic relocation of the blocks you get.
* Memory Warnings::             Getting warnings when memory is nearly full.
@end menu

@node Memory Concepts
@section Dynamic Memory Allocation Concepts
@cindex dynamic allocation
@cindex static allocation
@cindex automatic allocation

@dfn{Dynamic memory allocation} is a technique in which programs
determine as they are running where to store some information.  You need
dynamic allocation when the number of memory blocks you need, or how
long you continue to need them, depends on the data you are working on.

For example, you may need a block to store a line read from an input file;
since there is no limit to how long a line can be, you must allocate the
storage dynamically and make it dynamically larger as you read more of the
line.

Or, you may need a block for each record or each definition in the input
data; since you can't know in advance how many there will be, you must
allocate a new block for each record or definition as you read it.

When you use dynamic allocation, the allocation of a block of memory is an
action that the program requests explicitly.  You call a function or macro
when you want to allocate space, and specify the size with an argument.  If
you want to free the space, you do so by calling another function or macro.
You can do these things whenever you want, as often as you want.

@node Dynamic Allocation and C
@section Dynamic Allocation and C

The C language supports two kinds of memory allocation through the variables
in C programs:

@itemize @bullet
@item
@dfn{Static allocation} is what happens when you declare a static or
global variable.  Each static or global variable defines one block of
space, of a fixed size.  The space is allocated once, when your program
is started, and is never freed.

@item
@dfn{Automatic allocation} happens when you declare an automatic
variable, such as a function argument or a local variable.  The space
for an automatic variable is allocated when the compound statement
containing the declaration is entered, and is freed when that
compound statement is exited.

In GNU C, the length of the automatic storage can be an expression
that varies.  In other C implementations, it must be a constant.
@end itemize

Dynamic allocation is not supported by C variables; there is no storage
class ``dynamic'', and there can never be a C variable whose value is
stored in dynamically allocated space.  The only way to refer to
dynamically allocated space is through a pointer.  Because it is less
convenient, and because the actual process of dynamic allocation
requires more computation time, programmers generally use dynamic
allocation only when neither static nor automatic allocation will serve.

For example, if you want to allocate dynamically some space to hold a
@code{struct foobar}, you cannot declare a variable of type @code{struct
foobar} whose contents are the dynamically allocated space.  But you can
declare a variable of pointer type @code{struct foobar *} and assign it the
address of the space.  Then you can use the operators @samp{*} and
@samp{->} on this pointer variable to refer to the contents of the space:

@smallexample
@{
  struct foobar *ptr
     = (struct foobar *) malloc (sizeof (struct foobar));
  ptr->name = x;
  ptr->next = current_foobar;
  current_foobar = ptr;
@}
@end smallexample

@node Unconstrained Allocation
@section Unconstrained Allocation
@cindex unconstrained storage allocation
@cindex @code{malloc} function
@cindex heap, dynamic allocation from

The most general dynamic allocation facility is @code{malloc}.  It
allows you to allocate blocks of memory of any size at any time, make
them bigger or smaller at any time, and free the blocks individually at
any time (or never).

@menu
* Basic Allocation::            Simple use of @code{malloc}.
* Malloc Examples::             Examples of @code{malloc}.  @code{xmalloc}.
* Freeing after Malloc::        Use @code{free} to free a block you
                                 got with @code{malloc}.
* Changing Block Size::         Use @code{realloc} to make a block
                                 bigger or smaller.
* Allocating Cleared Space::    Use @code{calloc} to allocate a
                                 block and clear it.
* Efficiency and Malloc::       Efficiency considerations in use of
                                 these functions.
* Aligned Memory Blocks::       Allocating specially aligned memory:
                                 @code{memalign} and @code{valloc}.
* Heap Consistency Checking::   Automatic checking for errors.
* Hooks for Malloc::            You can use these hooks for debugging
                                 programs that use @code{malloc}.
* Statistics of Malloc::        Getting information about how much
                                 memory your program is using.
* Summary of Malloc::           Summary of @code{malloc} and related functions.
@end menu

@node Basic Allocation
@subsection Basic Storage Allocation
@cindex allocation of memory with @code{malloc}

To allocate a block of memory, call @code{malloc}.  The prototype for
this function is in @file{stdlib.h}.
@pindex stdlib.h

@comment malloc.h stdlib.h
@comment ANSI
@deftypefun {void *} malloc (size_t @var{size})
This function returns a pointer to a newly allocated block @var{size}
bytes long, or a null pointer if the block could not be allocated.
@end deftypefun

The contents of the block are undefined; you must initialize it yourself
(or use @code{calloc} instead; @pxref{Allocating Cleared Space}).
Normally you would cast the value as a pointer to the kind of object
that you want to store in the block.  Here we show an example of doing
so, and of initializing the space with zeros using the library function
@code{memset} (@pxref{Copying and Concatenation}):

@smallexample
struct foo *ptr;
@dots{}
ptr = (struct foo *) malloc (sizeof (struct foo));
if (ptr == 0) abort ();
memset (ptr, 0, sizeof (struct foo));
@end smallexample

You can store the result of @code{malloc} into any pointer variable
without a cast, because ANSI C automatically converts the type
@code{void *} to another type of pointer when necessary.  But the cast
is necessary in contexts other than assignment operators or if you might
want your code to run in traditional C.

Remember that when allocating space for a string, the argument to
@code{malloc} must be one plus the length of the string.  This is
because a string is terminated with a null character that doesn't count
in the ``length'' of the string but does need space.  For example:

@smallexample
char *ptr;
@dots{}
ptr = (char *) malloc (length + 1);
@end smallexample

@noindent
@xref{Representation of Strings}, for more information about this.

@node Malloc Examples
@subsection Examples of @code{malloc}

If no more space is available, @code{malloc} returns a null pointer.
You should check the value of @emph{every} call to @code{malloc}.  It is
useful to write a subroutine that calls @code{malloc} and reports an
error if the value is a null pointer, returning only if the value is
nonzero.  This function is conventionally called @code{xmalloc}.  Here
it is:

@smallexample
void *
xmalloc (size_t size)
@{
  register void *value = malloc (size);
  if (value == 0)
    fatal ("virtual memory exhausted");
  return value;
@}
@end smallexample

Here is a real example of using @code{malloc} (by way of @code{xmalloc}).
The function @code{savestring} will copy a sequence of characters into
a newly allocated null-terminated string:

@smallexample
@group
char *
savestring (const char *ptr, size_t len)
@{
  register char *value = (char *) xmalloc (len + 1);
  memcpy (value, ptr, len);
  value[len] = '\0';
  return value;
@}
@end group
@end smallexample

The block that @code{malloc} gives you is guaranteed to be aligned so
that it can hold any type of data.  In the GNU system, the address is
always a multiple of eight; if the size of block is 16 or more, then the
address is always a multiple of 16.  Only rarely is any higher boundary
(such as a page boundary) necessary; for those cases, use
@code{memalign} or @code{valloc} (@pxref{Aligned Memory Blocks}).

Note that the memory located after the end of the block is likely to be
in use for something else; perhaps a block already allocated by another
call to @code{malloc}.  If you attempt to treat the block as longer than
you asked for it to be, you are liable to destroy the data that
@code{malloc} uses to keep track of its blocks, or you may destroy the
contents of another block.  If you have already allocated a block and
discover you want it to be bigger, use @code{realloc} (@pxref{Changing
Block Size}).

@node Freeing after Malloc
@subsection Freeing Memory Allocated with @code{malloc}
@cindex freeing memory allocated with @code{malloc}
@cindex heap, freeing memory from

When you no longer need a block that you got with @code{malloc}, use the
function @code{free} to make the block available to be allocated again.
The prototype for this function is in @file{stdlib.h}.
@pindex stdlib.h

@comment malloc.h stdlib.h
@comment ANSI
@deftypefun void free (void *@var{ptr})
The @code{free} function deallocates the block of storage pointed at
by @var{ptr}.
@end deftypefun

@comment stdlib.h
@comment Sun
@deftypefun void cfree (void *@var{ptr})
This function does the same thing as @code{free}.  It's provided for
backward compatibility with SunOS; you should use @code{free} instead.
@end deftypefun

Freeing a block alters the contents of the block.  @strong{Do not expect to
find any data (such as a pointer to the next block in a chain of blocks) in
the block after freeing it.}  Copy whatever you need out of the block before
freeing it!  Here is an example of the proper way to free all the blocks in
a chain, and the strings that they point to:

@smallexample
struct chain
  @{
    struct chain *next;
    char *name;
  @}

void
free_chain (struct chain *chain)
@{
  while (chain != 0)
    @{
      struct chain *next = chain->next;
      free (chain->name);
      free (chain);
      chain = next;
    @}
@}
@end smallexample

Occasionally, @code{free} can actually return memory to the operating
system and make the process smaller.  Usually, all it can do is allow a
later call to @code{malloc} to reuse the space.  In the meantime, the
space remains in your program as part of a free-list used internally by
@code{malloc}.

There is no point in freeing blocks at the end of a program, because all
of the program's space is given back to the system when the process
terminates.

@node Changing Block Size
@subsection Changing the Size of a Block
@cindex changing the size of a block (@code{malloc})

Often you do not know for certain how big a block you will ultimately need
at the time you must begin to use the block.  For example, the block might
be a buffer that you use to hold a line being read from a file; no matter
how long you make the buffer initially, you may encounter a line that is
longer.

You can make the block longer by calling @code{realloc}.  This function
is declared in @file{stdlib.h}.
@pindex stdlib.h

@comment malloc.h stdlib.h
@comment ANSI
@deftypefun {void *} realloc (void *@var{ptr}, size_t @var{newsize})
The @code{realloc} function changes the size of the block whose address is
@var{ptr} to be @var{newsize}.

Since the space after the end of the block may be in use, @code{realloc}
may find it necessary to copy the block to a new address where more free
space is available.  The value of @code{realloc} is the new address of the
block.  If the block needs to be moved, @code{realloc} copies the old
contents.

If you pass a null pointer for @var{ptr}, @code{realloc} behaves just
like @samp{malloc (@var{newsize})}.  This can be convenient, but beware
that older implementations (before ANSI C) may not support this
behavior, and will probably crash when @code{realloc} is passed a null
pointer.
@end deftypefun

Like @code{malloc}, @code{realloc} may return a null pointer if no
memory space is available to make the block bigger.  When this happens,
the original block is untouched; it has not been modified or relocated.

In most cases it makes no difference what happens to the original block
when @code{realloc} fails, because the application program cannot continue
when it is out of memory, and the only thing to do is to give a fatal error
message.  Often it is convenient to write and use a subroutine,
conventionally called @code{xrealloc}, that takes care of the error message
as @code{xmalloc} does for @code{malloc}:

@smallexample
void *
xrealloc (void *ptr, size_t size)
@{
  register void *value = realloc (ptr, size);
  if (value == 0)
    fatal ("Virtual memory exhausted");
  return value;
@}
@end smallexample

You can also use @code{realloc} to make a block smaller.  The reason you
would do this is to avoid tying up a lot of memory space when only a little
is needed.  Making a block smaller sometimes necessitates copying it, so it
can fail if no other space is available.

If the new size you specify is the same as the old size, @code{realloc}
is guaranteed to change nothing and return the same address that you gave.

@node Allocating Cleared Space
@subsection Allocating Cleared Space

The function @code{calloc} allocates memory and clears it to zero.  It
is declared in @file{stdlib.h}.
@pindex stdlib.h

@comment malloc.h stdlib.h
@comment ANSI
@deftypefun {void *} calloc (size_t @var{count}, size_t @var{eltsize})
This function allocates a block long enough to contain a vector of
@var{count} elements, each of size @var{eltsize}.  Its contents are
cleared to zero before @code{calloc} returns.
@end deftypefun

You could define @code{calloc} as follows:

@smallexample
void *
calloc (size_t count, size_t eltsize)
@{
  size_t size = count * eltsize;
  void *value = malloc (size);
  if (value != 0)
    memset (value, 0, size);
  return value;
@}
@end smallexample

@node Efficiency and Malloc
@subsection Efficiency Considerations for @code{malloc}
@cindex efficiency and @code{malloc}

To make the best use of @code{malloc}, it helps to know that the GNU
version of @code{malloc} always dispenses small amounts of memory in
blocks whose sizes are powers of two.  It keeps separate pools for each
power of two.  This holds for sizes up to a page size.  Therefore, if
you are free to choose the size of a small block in order to make
@code{malloc} more efficient, make it a power of two.
@c !!! xref getpagesize

Once a page is split up for a particular block size, it can't be reused
for another size unless all the blocks in it are freed.  In many
programs, this is unlikely to happen.  Thus, you can sometimes make a
program use memory more efficiently by using blocks of the same size for
many different purposes.

When you ask for memory blocks of a page or larger, @code{malloc} uses a
different strategy; it rounds the size up to a multiple of a page, and
it can coalesce and split blocks as needed.

The reason for the two strategies is that it is important to allocate
and free small blocks as fast as possible, but speed is less important
for a large block since the program normally spends a fair amount of
time using it.  Also, large blocks are normally fewer in number.
Therefore, for large blocks, it makes sense to use a method which takes
more time to minimize the wasted space.

@node Aligned Memory Blocks
@subsection Allocating Aligned Memory Blocks

@cindex page boundary
@cindex alignment (with @code{malloc})
@pindex stdlib.h
The address of a block returned by @code{malloc} or @code{realloc} in
the GNU system is always a multiple of eight.  If you need a block whose
address is a multiple of a higher power of two than that, use
@code{memalign} or @code{valloc}.  These functions are declared in
@file{stdlib.h}.

With the GNU library, you can use @code{free} to free the blocks that
@code{memalign} and @code{valloc} return.  That does not work in BSD,
however---BSD does not provide any way to free such blocks.

@comment malloc.h stdlib.h
@comment BSD
@deftypefun {void *} memalign (size_t @var{boundary}, size_t @var{size})
The @code{memalign} function allocates a block of @var{size} bytes whose
address is a multiple of @var{boundary}.  The @var{boundary} must be a
power of two!  The function @code{memalign} works by calling
@code{malloc} to allocate a somewhat larger block, and then returning an
address within the block that is on the specified boundary.
@end deftypefun

@comment malloc.h stdlib.h
@comment BSD
@deftypefun {void *} valloc (size_t @var{size})
Using @code{valloc} is like using @code{memalign} and passing the page size
as the value of the second argument.  It is implemented like this:

@smallexample
void *
valloc (size_t size)
@{
  return memalign (getpagesize (), size);
@}
@end smallexample
@c !!! xref getpagesize
@end deftypefun

@node Heap Consistency Checking
@subsection Heap Consistency Checking

@cindex heap consistency checking
@cindex consistency checking, of heap

You can ask @code{malloc} to check the consistency of dynamic storage by
using the @code{mcheck} function.  This function is a GNU extension,
declared in @file{malloc.h}.
@pindex malloc.h

@comment malloc.h
@comment GNU
@deftypefun int mcheck (void (*@var{abortfn}) (enum mcheck_status @var{status}))
Calling @code{mcheck} tells @code{malloc} to perform occasional
consistency checks.  These will catch things such as writing
past the end of a block that was allocated with @code{malloc}.

The @var{abortfn} argument is the function to call when an inconsistency
is found.  If you supply a null pointer, then @code{mcheck} uses a
default function which prints a message and calls @code{abort}
(@pxref{Aborting a Program}).  The function you supply is called with
one argument, which says what sort of inconsistency was detected; its
type is described below.

It is too late to begin allocation checking once you have allocated
anything with @code{malloc}.  So @code{mcheck} does nothing in that
case.  The function returns @code{-1} if you call it too late, and
@code{0} otherwise (when it is successful).

The easiest way to arrange to call @code{mcheck} early enough is to use
the option @samp{-lmcheck} when you link your program; then you don't
need to modify your program source at all.
@end deftypefun

@deftypefun {enum mcheck_status} mprobe (void *@var{pointer})
The @code{mprobe} function lets you explicitly check for inconsistencies
in a particular allocated block.  You must have already called
@code{mcheck} at the beginning of the program, to do its occasional
checks; calling @code{mprobe} requests an additional consistency check
to be done at the time of the call.

The argument @var{pointer} must be a pointer returned by @code{malloc}
or @code{realloc}.  @code{mprobe} returns a value that says what
inconsistency, if any, was found.  The values are described below.
@end deftypefun

@deftp {Data Type} {enum mcheck_status}
This enumerated type describes what kind of inconsistency was detected
in an allocated block, if any.  Here are the possible values:

@table @code
@item MCHECK_DISABLED
@code{mcheck} was not called before the first allocation.
No consistency checking can be done.
@item MCHECK_OK
No inconsistency detected.
@item MCHECK_HEAD
The data immediately before the block was modified.
This commonly happens when an array index or pointer
is decremented too far.
@item MCHECK_TAIL
The data immediately after the block was modified.
This commonly happens when an array index or pointer
is incremented too far.
@item MCHECK_FREE
The block was already freed.
@end table
@end deftp

@node Hooks for Malloc
@subsection Storage Allocation Hooks
@cindex allocation hooks, for @code{malloc}

The GNU C library lets you modify the behavior of @code{malloc},
@code{realloc}, and @code{free} by specifying appropriate hook
functions.  You can use these hooks to help you debug programs that use
dynamic storage allocation, for example.

The hook variables are declared in @file{malloc.h}.
@pindex malloc.h

@comment malloc.h
@comment GNU
@defvar __malloc_hook
The value of this variable is a pointer to function that @code{malloc}
uses whenever it is called.  You should define this function to look
like @code{malloc}; that is, like:

@smallexample
void *@var{function} (size_t @var{size})
@end smallexample
@end defvar

@comment malloc.h
@comment GNU
@defvar __realloc_hook
The value of this variable is a pointer to function that @code{realloc}
uses whenever it is called.  You should define this function to look
like @code{realloc}; that is, like:

@smallexample
void *@var{function} (void *@var{ptr}, size_t @var{size})
@end smallexample
@end defvar

@comment malloc.h
@comment GNU
@defvar __free_hook
The value of this variable is a pointer to function that @code{free}
uses whenever it is called.  You should define this function to look
like @code{free}; that is, like:

@smallexample
void @var{function} (void *@var{ptr})
@end smallexample
@end defvar

You must make sure that the function you install as a hook for one of
these functions does not call that function recursively without restoring
the old value of the hook first!  Otherwise, your program will get stuck
in an infinite recursion.

Here is an example showing how to use @code{__malloc_hook} properly.  It
installs a function that prints out information every time @code{malloc}
is called.

@smallexample
static void *(*old_malloc_hook) (size_t);
static void *
my_malloc_hook (size_t size)
@{
  void *result;
  __malloc_hook = old_malloc_hook;
  result = malloc (size);
  /* @r{@code{printf} might call @code{malloc}, so protect it too.} */
  printf ("malloc (%u) returns %p\n", (unsigned int) size, result);
  __malloc_hook = my_malloc_hook;
  return result;
@}

main ()
@{
  ...
  old_malloc_hook = __malloc_hook;
  __malloc_hook = my_malloc_hook;
  ...
@}
@end smallexample

The @code{mcheck} function (@pxref{Heap Consistency Checking}) works by
installing such hooks.

@c __morecore, __after_morecore_hook are undocumented
@c It's not clear whether to document them.

@node Statistics of Malloc
@subsection Statistics for Storage Allocation with @code{malloc}

@cindex allocation statistics
You can get information about dynamic storage allocation by calling the
@code{mstats} function.  This function and its associated data type are
declared in @file{malloc.h}; they are a GNU extension.
@pindex malloc.h

@comment malloc.h
@comment GNU
@deftp {Data Type} {struct mstats}
This structure type is used to return information about the dynamic
storage allocator.  It contains the following members:

@table @code
@item size_t bytes_total
This is the total size of memory managed by @code{malloc}, in bytes.

@item size_t chunks_used
This is the number of chunks in use.  (The storage allocator internally
gets chunks of memory from the operating system, and then carves them up
to satisfy individual @code{malloc} requests; see @ref{Efficiency and
Malloc}.)

@item size_t bytes_used
This is the number of bytes in use.

@item size_t chunks_free
This is the number of chunks which are free -- that is, that have been
allocated by the operating system to your program, but which are not
now being used.

@item size_t bytes_free
This is the number of bytes which are free.
@end table
@end deftp

@comment malloc.h
@comment GNU
@deftypefun {struct mstats} mstats (void)
This function returns information about the current dynamic memory usage
in a structure of type @code{struct mstats}.
@end deftypefun

@node Summary of Malloc
@subsection Summary of @code{malloc}-Related Functions

Here is a summary of the functions that work with @code{malloc}:

@table @code
@item void *malloc (size_t @var{size})
Allocate a block of @var{size} bytes.  @xref{Basic Allocation}.

@item void free (void *@var{addr})
Free a block previously allocated by @code{malloc}.  @xref{Freeing after
Malloc}.

@item void *realloc (void *@var{addr}, size_t @var{size})
Make a block previously allocated by @code{malloc} larger or smaller,
possibly by copying it to a new location.  @xref{Changing Block Size}.

@item void *calloc (size_t @var{count}, size_t @var{eltsize})
Allocate a block of @var{count} * @var{eltsize} bytes using
@code{malloc}, and set its contents to zero.  @xref{Allocating Cleared
Space}.

@item void *valloc (size_t @var{size})
Allocate a block of @var{size} bytes, starting on a page boundary.
@xref{Aligned Memory Blocks}.

@item void *memalign (size_t @var{size}, size_t @var{boundary})
Allocate a block of @var{size} bytes, starting on an address that is a
multiple of @var{boundary}.  @xref{Aligned Memory Blocks}.

@item int mcheck (void (*@var{abortfn}) (void))
Tell @code{malloc} to perform occasional consistency checks on
dynamically allocated memory, and to call @var{abortfn} when an
inconsistency is found.  @xref{Heap Consistency Checking}.

@item void *(*__malloc_hook) (size_t @var{size})
A pointer to a function that @code{malloc} uses whenever it is called.

@item void *(*__realloc_hook) (void *@var{ptr}, size_t @var{size})
A pointer to a function that @code{realloc} uses whenever it is called.

@item void (*__free_hook) (void *@var{ptr})
A pointer to a function that @code{free} uses whenever it is called.

@item struct mstats mstats (void)
Return information about the current dynamic memory usage.
@xref{Statistics of Malloc}.
@end table

@node Obstacks
@section Obstacks
@cindex obstacks

An @dfn{obstack} is a pool of memory containing a stack of objects.  You
can create any number of separate obstacks, and then allocate objects in
specified obstacks.  Within each obstack, the last object allocated must
always be the first one freed, but distinct obstacks are independent of
each other.

Aside from this one constraint of order of freeing, obstacks are totally
general: an obstack can contain any number of objects of any size.  They
are implemented with macros, so allocation is usually very fast as long as
the objects are usually small.  And the only space overhead per object is
the padding needed to start each object on a suitable boundary.

@menu
* Creating Obstacks::           How to declare an obstack in your program.
* Preparing for Obstacks::      Preparations needed before you can
                                 use obstacks.
* Allocation in an Obstack::    Allocating objects in an obstack.
* Freeing Obstack Objects::     Freeing objects in an obstack.
* Obstack Functions::           The obstack functions are both
                                 functions and macros.
* Growing Objects::             Making an object bigger by stages.
* Extra Fast Growing::          Extra-high-efficiency (though more
                                 complicated) growing objects.
* Status of an Obstack::        Inquiries about the status of an obstack.
* Obstacks Data Alignment::     Controlling alignment of objects in obstacks.
* Obstack Chunks::              How obstacks obtain and release chunks;
                                 efficiency considerations.
* Summary of Obstacks::         
@end menu

@node Creating Obstacks
@subsection Creating Obstacks

The utilities for manipulating obstacks are declared in the header
file @file{obstack.h}.
@pindex obstack.h

@comment obstack.h
@comment GNU
@deftp {Data Type} {struct obstack}
An obstack is represented by a data structure of type @code{struct
obstack}.  This structure has a small fixed size; it records the status
of the obstack and how to find the space in which objects are allocated.
It does not contain any of the objects themselves.  You should not try
to access the contents of the structure directly; use only the functions
described in this chapter.
@end deftp

You can declare variables of type @code{struct obstack} and use them as
obstacks, or you can allocate obstacks dynamically like any other kind
of object.  Dynamic allocation of obstacks allows your program to have a
variable number of different stacks.  (You can even allocate an
obstack structure in another obstack, but this is rarely useful.)

All the functions that work with obstacks require you to specify which
obstack to use.  You do this with a pointer of type @code{struct obstack
*}.  In the following, we often say ``an obstack'' when strictly
speaking the object at hand is such a pointer.

The objects in the obstack are packed into large blocks called
@dfn{chunks}.  The @code{struct obstack} structure points to a chain of
the chunks currently in use.

The obstack library obtains a new chunk whenever you allocate an object
that won't fit in the previous chunk.  Since the obstack library manages
chunks automatically, you don't need to pay much attention to them, but
you do need to supply a function which the obstack library should use to
get a chunk.  Usually you supply a function which uses @code{malloc}
directly or indirectly.  You must also supply a function to free a chunk.
These matters are described in the following section.

@node Preparing for Obstacks
@subsection Preparing for Using Obstacks

Each source file in which you plan to use the obstack functions
must include the header file @file{obstack.h}, like this:

@smallexample
#include <obstack.h>
@end smallexample

@findex obstack_chunk_alloc
@findex obstack_chunk_free
Also, if the source file uses the macro @code{obstack_init}, it must
declare or define two functions or macros that will be called by the
obstack library.  One, @code{obstack_chunk_alloc}, is used to allocate
the chunks of memory into which objects are packed.  The other,
@code{obstack_chunk_free}, is used to return chunks when the objects in
them are freed.  These macros should appear before any use of obstacks
in the source file.

Usually these are defined to use @code{malloc} via the intermediary
@code{xmalloc} (@pxref{Unconstrained Allocation}).  This is done with
the following pair of macro definitions:

@smallexample
#define obstack_chunk_alloc xmalloc
#define obstack_chunk_free free
@end smallexample

@noindent
Though the storage you get using obstacks really comes from @code{malloc},
using obstacks is faster because @code{malloc} is called less often, for
larger blocks of memory.  @xref{Obstack Chunks}, for full details.

At run time, before the program can use a @code{struct obstack} object
as an obstack, it must initialize the obstack by calling
@code{obstack_init}.

@comment obstack.h
@comment GNU
@deftypefun int obstack_init (struct obstack *@var{obstack-ptr})
Initialize obstack @var{obstack-ptr} for allocation of objects.  This
function calls the obstack's @code{obstack_chunk_alloc} function.  It
returns 0 if @code{obstack_chunk_alloc} returns a null pointer, meaning
that it is out of memory.  Otherwise, it returns 1.  If you supply an
@code{obstack_chunk_alloc} function that calls @code{exit}
(@pxref{Program Termination}) or @code{longjmp} (@pxref{Non-Local
Exits}) when out of memory, you can safely ignore the value that
@code{obstack_init} returns.
@end deftypefun

Here are two examples of how to allocate the space for an obstack and
initialize it.  First, an obstack that is a static variable:

@smallexample
static struct obstack myobstack;
@dots{}
obstack_init (&myobstack);
@end smallexample

@noindent
Second, an obstack that is itself dynamically allocated:

@smallexample
struct obstack *myobstack_ptr
  = (struct obstack *) xmalloc (sizeof (struct obstack));

obstack_init (myobstack_ptr);
@end smallexample

@node Allocation in an Obstack
@subsection Allocation in an Obstack
@cindex allocation (obstacks)

The most direct way to allocate an object in an obstack is with
@code{obstack_alloc}, which is invoked almost like @code{malloc}.

@comment obstack.h
@comment GNU
@deftypefun {void *} obstack_alloc (struct obstack *@var{obstack-ptr}, int @var{size})
This allocates an uninitialized block of @var{size} bytes in an obstack
and returns its address.  Here @var{obstack-ptr} specifies which obstack
to allocate the block in; it is the address of the @code{struct obstack}
object which represents the obstack.  Each obstack function or macro
requires you to specify an @var{obstack-ptr} as the first argument.

This function calls the obstack's @code{obstack_chunk_alloc} function if
it needs to allocate a new chunk of memory; it returns a null pointer if
@code{obstack_chunk_alloc} returns one.  In that case, it has not
changed the amount of memory allocated in the obstack.  If you supply an
@code{obstack_chunk_alloc} function that calls @code{exit}
(@pxref{Program Termination}) or @code{longjmp} (@pxref{Non-Local
Exits}) when out of memory, then @code{obstack_alloc} will never return
a null pointer.
@end deftypefun

For example, here is a function that allocates a copy of a string @var{str}
in a specific obstack, which is in the variable @code{string_obstack}:

@smallexample
struct obstack string_obstack;

char *
copystring (char *string)
@{
  char *s = (char *) obstack_alloc (&string_obstack,
                                    strlen (string) + 1);
  memcpy (s, string, strlen (string));
  return s;
@}
@end smallexample

To allocate a block with specified contents, use the function
@code{obstack_copy}, declared like this:

@comment obstack.h
@comment GNU
@deftypefun {void *} obstack_copy (struct obstack *@var{obstack-ptr}, void *@var{address}, int @var{size})
This allocates a block and initializes it by copying @var{size}
bytes of data starting at @var{address}.  It can return a null pointer
under the same conditions as @code{obstack_alloc}.
@end deftypefun

@comment obstack.h
@comment GNU
@deftypefun {void *} obstack_copy0 (struct obstack *@var{obstack-ptr}, void *@var{address}, int @var{size})
Like @code{obstack_copy}, but appends an extra byte containing a null
character.  This extra byte is not counted in the argument @var{size}.
@end deftypefun

The @code{obstack_copy0} function is convenient for copying a sequence
of characters into an obstack as a null-terminated string.  Here is an
example of its use:

@smallexample
char *
obstack_savestring (char *addr, int size)
@{
  return obstack_copy0 (&myobstack, addr, size);
@}
@end smallexample

@noindent
Contrast this with the previous example of @code{savestring} using
@code{malloc} (@pxref{Basic Allocation}).

@node Freeing Obstack Objects
@subsection Freeing Objects in an Obstack
@cindex freeing (obstacks)

To free an object allocated in an obstack, use the function
@code{obstack_free}.  Since the obstack is a stack of objects, freeing
one object automatically frees all other objects allocated more recently
in the same obstack.

@comment obstack.h
@comment GNU
@deftypefun void obstack_free (struct obstack *@var{obstack-ptr}, void *@var{object})
If @var{object} is a null pointer, everything allocated in the obstack
is freed.  Otherwise, @var{object} must be the address of an object
allocated in the obstack.  Then @var{object} is freed, along with
everything allocated in @var{obstack} since @var{object}.
@end deftypefun

Note that if @var{object} is a null pointer, the result is an
uninitialized obstack.  To free all storage in an obstack but leave it
valid for further allocation, call @code{obstack_free} with the address
of the first object allocated on the obstack:

@smallexample
obstack_free (obstack_ptr, first_object_allocated_ptr);
@end smallexample

Recall that the objects in an obstack are grouped into chunks.  When all
the objects in a chunk become free, the obstack library automatically
frees the chunk (@pxref{Preparing for Obstacks}).  Then other
obstacks, or non-obstack allocation, can reuse the space of the chunk.

@node Obstack Functions
@subsection Obstack Functions and Macros
@cindex macros

The interfaces for using obstacks may be defined either as functions or
as macros, depending on the compiler.  The obstack facility works with
all C compilers, including both ANSI C and traditional C, but there are
precautions you must take if you plan to use compilers other than GNU C.

If you are using an old-fashioned non-ANSI C compiler, all the obstack
``functions'' are actually defined only as macros.  You can call these
macros like functions, but you cannot use them in any other way (for
example, you cannot take their address).

Calling the macros requires a special precaution: namely, the first
operand (the obstack pointer) may not contain any side effects, because
it may be computed more than once.  For example, if you write this:

@smallexample
obstack_alloc (get_obstack (), 4);
@end smallexample

@noindent
you will find that @code{get_obstack} may be called several times.
If you use @code{*obstack_list_ptr++} as the obstack pointer argument,
you will get very strange results since the incrementation may occur
several times.

In ANSI C, each function has both a macro definition and a function
definition.  The function definition is used if you take the address of the
function without calling it.  An ordinary call uses the macro definition by
default, but you can request the function definition instead by writing the
function name in parentheses, as shown here:

@smallexample
char *x;
void *(*funcp) ();
/* @r{Use the macro}.  */
x = (char *) obstack_alloc (obptr, size);
/* @r{Call the function}.  */
x = (char *) (obstack_alloc) (obptr, size);
/* @r{Take the address of the function}.  */
funcp = obstack_alloc;
@end smallexample

@noindent
This is the same situation that exists in ANSI C for the standard library
functions.  @xref{Macro Definitions}.

@strong{Warning:} When you do use the macros, you must observe the
precaution of avoiding side effects in the first operand, even in ANSI
C.

If you use the GNU C compiler, this precaution is not necessary, because
various language extensions in GNU C permit defining the macros so as to
compute each argument only once.

@node Growing Objects
@subsection Growing Objects
@cindex growing objects (in obstacks)
@cindex changing the size of a block (obstacks)

Because storage in obstack chunks is used sequentially, it is possible to
build up an object step by step, adding one or more bytes at a time to the
end of the object.  With this technique, you do not need to know how much
data you will put in the object until you come to the end of it.  We call
this the technique of @dfn{growing objects}.  The special functions
for adding data to the growing object are described in this section.

You don't need to do anything special when you start to grow an object.
Using one of the functions to add data to the object automatically
starts it.  However, it is necessary to say explicitly when the object is
finished.  This is done with the function @code{obstack_finish}.

The actual address of the object thus built up is not known until the
object is finished.  Until then, it always remains possible that you will
add so much data that the object must be copied into a new chunk.

While the obstack is in use for a growing object, you cannot use it for
ordinary allocation of another object.  If you try to do so, the space
already added to the growing object will become part of the other object.

@comment obstack.h
@comment GNU
@deftypefun void obstack_blank (struct obstack *@var{obstack-ptr}, int @var{size})
The most basic function for adding to a growing object is
@code{obstack_blank}, which adds space without initializing it.
@end deftypefun

@comment obstack.h
@comment GNU
@deftypefun void obstack_grow (struct obstack *@var{obstack-ptr}, void *@var{data}, int @var{size})
To add a block of initialized space, use @code{obstack_grow}, which is
the growing-object analogue of @code{obstack_copy}.  It adds @var{size}
bytes of data to the growing object, copying the contents from
@var{data}.
@end deftypefun

@comment obstack.h
@comment GNU
@deftypefun void obstack_grow0 (struct obstack *@var{obstack-ptr}, void *@var{data}, int @var{size})
This is the growing-object analogue of @code{obstack_copy0}.  It adds
@var{size} bytes copied from @var{data}, followed by an additional null
character.
@end deftypefun

@comment obstack.h
@comment GNU
@deftypefun void obstack_1grow (struct obstack *@var{obstack-ptr}, char @var{c})
To add one character at a time, use the function @code{obstack_1grow}.
It adds a single byte containing @var{c} to the growing object.
@end deftypefun

@comment obstack.h
@comment GNU
@deftypefun {void *} obstack_finish (struct obstack *@var{obstack-ptr})
When you are finished growing the object, use the function
@code{obstack_finish} to close it off and return its final address.

Once you have finished the object, the obstack is available for ordinary
allocation or for growing another object.

This function can return a null pointer under the same conditions as
@code{obstack_alloc} (@pxref{Allocation in an Obstack}).
@end deftypefun

When you build an object by growing it, you will probably need to know
afterward how long it became.  You need not keep track of this as you grow
the object, because you can find out the length from the obstack just
before finishing the object with the function @code{obstack_object_size},
declared as follows:

@comment obstack.h
@comment GNU
@deftypefun int obstack_object_size (struct obstack *@var{obstack-ptr})
This function returns the current size of the growing object, in bytes.
Remember to call this function @emph{before} finishing the object.
After it is finished, @code{obstack_object_size} will return zero.
@end deftypefun

If you have started growing an object and wish to cancel it, you should
finish it and then free it, like this:

@smallexample
obstack_free (obstack_ptr, obstack_finish (obstack_ptr));
@end smallexample

@noindent
This has no effect if no object was growing.

@cindex shrinking objects
You can use @code{obstack_blank} with a negative size argument to make
the current object smaller.  Just don't try to shrink it beyond zero
length---there's no telling what will happen if you do that.

@node Extra Fast Growing
@subsection Extra Fast Growing Objects
@cindex efficiency and obstacks

The usual functions for growing objects incur overhead for checking
whether there is room for the new growth in the current chunk.  If you
are frequently constructing objects in small steps of growth, this
overhead can be significant.

You can reduce the overhead by using special ``fast growth''
functions that grow the object without checking.  In order to have a
robust program, you must do the checking yourself.  If you do this checking
in the simplest way each time you are about to add data to the object, you
have not saved anything, because that is what the ordinary growth
functions do.  But if you can arrange to check less often, or check
more efficiently, then you make the program faster.

The function @code{obstack_room} returns the amount of room available
in the current chunk.  It is declared as follows:

@comment obstack.h
@comment GNU
@deftypefun int obstack_room (struct obstack *@var{obstack-ptr})
This returns the number of bytes that can be added safely to the current
growing object (or to an object about to be started) in obstack
@var{obstack} using the fast growth functions.
@end deftypefun

While you know there is room, you can use these fast growth functions
for adding data to a growing object:

@comment obstack.h
@comment GNU
@deftypefun void obstack_1grow_fast (struct obstack *@var{obstack-ptr}, char @var{c})
The function @code{obstack_1grow_fast} adds one byte containing the
character @var{c} to the growing object in obstack @var{obstack-ptr}.
@end deftypefun

@comment obstack.h
@comment GNU
@deftypefun void obstack_blank_fast (struct obstack *@var{obstack-ptr}, int @var{size})
The function @code{obstack_blank_fast} adds @var{size} bytes to the
growing object in obstack @var{obstack-ptr} without initializing them.
@end deftypefun

When you check for space using @code{obstack_room} and there is not
enough room for what you want to add, the fast growth functions
are not safe.  In this case, simply use the corresponding ordinary
growth function instead.  Very soon this will copy the object to a
new chunk; then there will be lots of room available again. 

So, each time you use an ordinary growth function, check afterward for
sufficient space using @code{obstack_room}.  Once the object is copied
to a new chunk, there will be plenty of space again, so the program will
start using the fast growth functions again.

Here is an example:

@smallexample
@group
void
add_string (struct obstack *obstack, const char *ptr, int len)
@{
  while (len > 0)
    @{
      int room = obstack_room (obstack);
      if (room == 0)
        @{
          /* @r{Not enough room. Add one character slowly,}
             @r{which may copy to a new chunk and make room.}  */
          obstack_1grow (obstack, *ptr++);
          len--;
        @}
      else 
        @{
          if (room > len)
            room = len;
          /* @r{Add fast as much as we have room for.} */
          len -= room;
          while (room-- > 0)
            obstack_1grow_fast (obstack, *ptr++);
        @}
    @}
@}
@end group
@end smallexample

@node Status of an Obstack
@subsection Status of an Obstack
@cindex obstack status
@cindex status of obstack

Here are functions that provide information on the current status of
allocation in an obstack.  You can use them to learn about an object while
still growing it.

@comment obstack.h
@comment GNU
@deftypefun {void *} obstack_base (struct obstack *@var{obstack-ptr})
This function returns the tentative address of the beginning of the
currently growing object in @var{obstack-ptr}.  If you finish the object
immediately, it will have that address.  If you make it larger first, it
may outgrow the current chunk---then its address will change!

If no object is growing, this value says where the next object you
allocate will start (once again assuming it fits in the current
chunk).
@end deftypefun

@comment obstack.h
@comment GNU
@deftypefun {void *} obstack_next_free (struct obstack *@var{obstack-ptr})
This function returns the address of the first free byte in the current
chunk of obstack @var{obstack-ptr}.  This is the end of the currently
growing object.  If no object is growing, @code{obstack_next_free}
returns the same value as @code{obstack_base}.
@end deftypefun

@comment obstack.h
@comment GNU
@deftypefun int obstack_object_size (struct obstack *@var{obstack-ptr})
This function returns the size in bytes of the currently growing object.
This is equivalent to

@smallexample
obstack_next_free (@var{obstack-ptr}) - obstack_base (@var{obstack-ptr})
@end smallexample
@end deftypefun

@node Obstacks Data Alignment
@subsection Alignment of Data in Obstacks
@cindex alignment (in obstacks)

Each obstack has an @dfn{alignment boundary}; each object allocated in
the obstack automatically starts on an address that is a multiple of the
specified boundary.  By default, this boundary is 4 bytes.

To access an obstack's alignment boundary, use the macro
@code{obstack_alignment_mask}, whose function prototype looks like
this:

@comment obstack.h
@comment GNU
@deftypefn Macro int obstack_alignment_mask (struct obstack *@var{obstack-ptr})
The value is a bit mask; a bit that is 1 indicates that the corresponding
bit in the address of an object should be 0.  The mask value should be one
less than a power of 2; the effect is that all object addresses are
multiples of that power of 2.  The default value of the mask is 3, so that
addresses are multiples of 4.  A mask value of 0 means an object can start
on any multiple of 1 (that is, no alignment is required).

The expansion of the macro @code{obstack_alignment_mask} is an lvalue,
so you can alter the mask by assignment.  For example, this statement:

@smallexample
obstack_alignment_mask (obstack_ptr) = 0;
@end smallexample

@noindent
has the effect of turning off alignment processing in the specified obstack.
@end deftypefn

Note that a change in alignment mask does not take effect until
@emph{after} the next time an object is allocated or finished in the
obstack.  If you are not growing an object, you can make the new
alignment mask take effect immediately by calling @code{obstack_finish}.
This will finish a zero-length object and then do proper alignment for
the next object.

@node Obstack Chunks
@subsection Obstack Chunks
@cindex efficiency of chunks
@cindex chunks

Obstacks work by allocating space for themselves in large chunks, and
then parceling out space in the chunks to satisfy your requests.  Chunks
are normally 4096 bytes long unless you specify a different chunk size.
The chunk size includes 8 bytes of overhead that are not actually used
for storing objects.  Regardless of the specified size, longer chunks
will be allocated when necessary for long objects.

The obstack library allocates chunks by calling the function
@code{obstack_chunk_alloc}, which you must define.  When a chunk is no
longer needed because you have freed all the objects in it, the obstack
library frees the chunk by calling @code{obstack_chunk_free}, which you
must also define.

These two must be defined (as macros) or declared (as functions) in each
source file that uses @code{obstack_init} (@pxref{Creating Obstacks}).
Most often they are defined as macros like this:

@smallexample
#define obstack_chunk_alloc xmalloc
#define obstack_chunk_free free
@end smallexample

Note that these are simple macros (no arguments).  Macro definitions with
arguments will not work!  It is necessary that @code{obstack_chunk_alloc}
or @code{obstack_chunk_free}, alone, expand into a function name if it is
not itself a function name.

If you allocate chunks with @code{malloc}, the chunk size should be a
power of 2.  The default chunk size, 4096, was chosen because it is long
enough to satisfy many typical requests on the obstack yet short enough
not to waste too much memory in the portion of the last chunk not yet used.

@comment obstack.h
@comment GNU
@deftypefn Macro int obstack_chunk_size (struct obstack *@var{obstack-ptr})
This returns the chunk size of the given obstack.
@end deftypefn

Since this macro expands to an lvalue, you can specify a new chunk size by
assigning it a new value.  Doing so does not affect the chunks already
allocated, but will change the size of chunks allocated for that particular
obstack in the future.  It is unlikely to be useful to make the chunk size
smaller, but making it larger might improve efficiency if you are
allocating many objects whose size is comparable to the chunk size.  Here
is how to do so cleanly:

@smallexample
if (obstack_chunk_size (obstack_ptr) < @var{new-chunk-size})
  obstack_chunk_size (obstack_ptr) = @var{new-chunk-size};
@end smallexample

@node Summary of Obstacks
@subsection Summary of Obstack Functions

Here is a summary of all the functions associated with obstacks.  Each
takes the address of an obstack (@code{struct obstack *}) as its first
argument.

@table @code
@item void obstack_init (struct obstack *@var{obstack-ptr})
Initialize use of an obstack.  @xref{Creating Obstacks}.

@item void *obstack_alloc (struct obstack *@var{obstack-ptr}, int @var{size})
Allocate an object of @var{size} uninitialized bytes.
@xref{Allocation in an Obstack}.

@item void *obstack_copy (struct obstack *@var{obstack-ptr}, void *@var{address}, int @var{size})
Allocate an object of @var{size} bytes, with contents copied from
@var{address}.  @xref{Allocation in an Obstack}.

@item void *obstack_copy0 (struct obstack *@var{obstack-ptr}, void *@var{address}, int @var{size})
Allocate an object of @var{size}+1 bytes, with @var{size} of them copied
from @var{address}, followed by a null character at the end.
@xref{Allocation in an Obstack}.

@item void obstack_free (struct obstack *@var{obstack-ptr}, void *@var{object})
Free @var{object} (and everything allocated in the specified obstack
more recently than @var{object}).  @xref{Freeing Obstack Objects}.

@item void obstack_blank (struct obstack *@var{obstack-ptr}, int @var{size})
Add @var{size} uninitialized bytes to a growing object.
@xref{Growing Objects}.

@item void obstack_grow (struct obstack *@var{obstack-ptr}, void *@var{address}, int @var{size})
Add @var{size} bytes, copied from @var{address}, to a growing object.
@xref{Growing Objects}.

@item void obstack_grow0 (struct obstack *@var{obstack-ptr}, void *@var{address}, int @var{size})
Add @var{size} bytes, copied from @var{address}, to a growing object,
and then add another byte containing a null character.  @xref{Growing
Objects}.

@item void obstack_1grow (struct obstack *@var{obstack-ptr}, char @var{data-char})
Add one byte containing @var{data-char} to a growing object.
@xref{Growing Objects}.

@item void *obstack_finish (struct obstack *@var{obstack-ptr})
Finalize the object that is growing and return its permanent address.
@xref{Growing Objects}.

@item int obstack_object_size (struct obstack *@var{obstack-ptr})
Get the current size of the currently growing object.  @xref{Growing
Objects}.

@item void obstack_blank_fast (struct obstack *@var{obstack-ptr}, int @var{size})
Add @var{size} uninitialized bytes to a growing object without checking
that there is enough room.  @xref{Extra Fast Growing}.

@item void obstack_1grow_fast (struct obstack *@var{obstack-ptr}, char @var{data-char})
Add one byte containing @var{data-char} to a growing object without
checking that there is enough room.  @xref{Extra Fast Growing}.

@item int obstack_room (struct obstack *@var{obstack-ptr})
Get the amount of room now available for growing the current object.
@xref{Extra Fast Growing}.

@item int obstack_alignment_mask (struct obstack *@var{obstack-ptr})
The mask used for aligning the beginning of an object.  This is an
lvalue.  @xref{Obstacks Data Alignment}.

@item int obstack_chunk_size (struct obstack *@var{obstack-ptr})
The size for allocating chunks.  This is an lvalue.  @xref{Obstack Chunks}.

@item void *obstack_base (struct obstack *@var{obstack-ptr})
Tentative starting address of the currently growing object.
@xref{Status of an Obstack}.

@item void *obstack_next_free (struct obstack *@var{obstack-ptr})
Address just after the end of the currently growing object.
@xref{Status of an Obstack}.
@end table

@node Variable Size Automatic
@section Automatic Storage with Variable Size
@cindex automatic freeing
@cindex @code{alloca} function
@cindex automatic storage with variable size

The function @code{alloca} supports a kind of half-dynamic allocation in
which blocks are allocated dynamically but freed automatically.

Allocating a block with @code{alloca} is an explicit action; you can
allocate as many blocks as you wish, and compute the size at run time.  But
all the blocks are freed when you exit the function that @code{alloca} was
called from, just as if they were automatic variables declared in that
function.  There is no way to free the space explicitly.

The prototype for @code{alloca} is in @file{stdlib.h}.  This function is
a BSD extension.
@pindex stdlib.h

@comment stdlib.h
@comment GNU, BSD
@deftypefun {void *} alloca (size_t @var{size});
The return value of @code{alloca} is the address of a block of @var{size}
bytes of storage, allocated in the stack frame of the calling function.
@end deftypefun

Do not use @code{alloca} inside the arguments of a function call---you
will get unpredictable results, because the stack space for the
@code{alloca} would appear on the stack in the middle of the space for
the function arguments.  An example of what to avoid is @code{foo (x,
alloca (4), y)}.
@c This might get fixed in future versions of GCC, but that won't make
@c it safe with compilers generally.

@menu
* Alloca Example::              Example of using @code{alloca}.
* Advantages of Alloca::        Reasons to use @code{alloca}.
* Disadvantages of Alloca::     Reasons to avoid @code{alloca}.
* GNU C Variable-Size Arrays::  Only in GNU C, here is an alternative
                                 method of allocating dynamically and
                                 freeing automatically.
@end menu

@node Alloca Example
@subsection @code{alloca} Example

As an example of use of @code{alloca}, here is a function that opens a file
name made from concatenating two argument strings, and returns a file
descriptor or minus one signifying failure:

@smallexample
int
open2 (char *str1, char *str2, int flags, int mode)
@{
  char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1);
  strcpy (name, str1);
  strcat (name, str2);
  return open (name, flags, mode);
@}
@end smallexample

@noindent
Here is how you would get the same results with @code{malloc} and
@code{free}:

@smallexample
int
open2 (char *str1, char *str2, int flags, int mode)
@{
  char *name = (char *) malloc (strlen (str1) + strlen (str2) + 1);
  int desc;
  if (name == 0)
    fatal ("virtual memory exceeded");
  strcpy (name, str1);
  strcat (name, str2);
  desc = open (name, flags, mode);
  free (name);
  return desc;
@}
@end smallexample

As you can see, it is simpler with @code{alloca}.  But @code{alloca} has
other, more important advantages, and some disadvantages.

@node Advantages of Alloca
@subsection Advantages of @code{alloca}

Here are the reasons why @code{alloca} may be preferable to @code{malloc}:

@itemize @bullet
@item
Using @code{alloca} wastes very little space and is very fast.  (It is
open-coded by the GNU C compiler.)

@item
Since @code{alloca} does not have separate pools for different sizes of
block, space used for any size block can be reused for any other size.
@code{alloca} does not cause storage fragmentation.

@item
@cindex longjmp
Nonlocal exits done with @code{longjmp} (@pxref{Non-Local Exits})
automatically free the space allocated with @code{alloca} when they exit
through the function that called @code{alloca}.  This is the most
important reason to use @code{alloca}.

To illustrate this, suppose you have a function
@code{open_or_report_error} which returns a descriptor, like
@code{open}, if it succeeds, but does not return to its caller if it
fails.  If the file cannot be opened, it prints an error message and
jumps out to the command level of your program using @code{longjmp}.
Let's change @code{open2} (@pxref{Alloca Example}) to use this
subroutine:@refill

@smallexample
int
open2 (char *str1, char *str2, int flags, int mode)
@{
  char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1);
  strcpy (name, str1);
  strcat (name, str2);
  return open_or_report_error (name, flags, mode);
@}
@end smallexample

@noindent
Because of the way @code{alloca} works, the storage it allocates is
freed even when an error occurs, with no special effort required.

By contrast, the previous definition of @code{open2} (which uses
@code{malloc} and @code{free}) would develop a storage leak if it were
changed in this way.  Even if you are willing to make more changes to
fix it, there is no easy way to do so.
@end itemize

@node Disadvantages of Alloca
@subsection Disadvantages of @code{alloca}

@cindex @code{alloca} disadvantages
@cindex disadvantages of @code{alloca}
These are the disadvantages of @code{alloca} in comparison with
@code{malloc}:

@itemize @bullet
@item
If you try to allocate more storage than the machine can provide, you
don't get a clean error message.  Instead you get a fatal signal like
the one you would get from an infinite recursion; probably a
segmentation violation (@pxref{Program Error Signals}).

@item
Some non-GNU systems fail to support @code{alloca}, so it is less
portable.  However, a slower emulation of @code{alloca} written in C
is available for use on systems with this deficiency.
@end itemize

@node GNU C Variable-Size Arrays
@subsection GNU C Variable-Size Arrays
@cindex variable-sized arrays

In GNU C, you can replace most uses of @code{alloca} with an array of
variable size.  Here is how @code{open2} would look then:

@smallexample
int open2 (char *str1, char *str2, int flags, int mode)
@{
  char name[strlen (str1) + strlen (str2) + 1];
  strcpy (name, str1);
  strcat (name, str2);
  return open (name, flags, mode);
@}
@end smallexample

But @code{alloca} is not always equivalent to a variable-sized array, for
several reasons:

@itemize @bullet
@item
A variable size array's space is freed at the end of the scope of the
name of the array.  The space allocated with @code{alloca}
remains until the end of the function.

@item
It is possible to use @code{alloca} within a loop, allocating an
additional block on each iteration.  This is impossible with
variable-sized arrays.
@end itemize

@strong{Note:} If you mix use of @code{alloca} and variable-sized arrays
within one function, exiting a scope in which a variable-sized array was
declared frees all blocks allocated with @code{alloca} during the
execution of that scope.


@node Relocating Allocator
@section Relocating Allocator

@cindex relocating memory allocator
Any system of dynamic memory allocation has overhead: the amount of
space it uses is more than the amount the program asks for.  The
@dfn{relocating memory allocator} achieves very low overhead by moving
blocks in memory as necessary, on its own initiative.

@menu
* Relocator Concepts::          How to understand relocating allocation.
* Using Relocator::             Functions for relocating allocation.
@end menu

@node Relocator Concepts
@subsection Concepts of Relocating Allocation

@ifinfo
The @dfn{relocating memory allocator} achieves very low overhead by
moving blocks in memory as necessary, on its own initiative.
@end ifinfo

When you allocate a block with @code{malloc}, the address of the block
never changes unless you use @code{realloc} to change its size.  Thus,
you can safely store the address in various places, temporarily or
permanently, as you like.  This is not safe when you use the relocating
memory allocator, because any and all relocatable blocks can move
whenever you allocate memory in any fashion.  Even calling @code{malloc}
or @code{realloc} can move the relocatable blocks.

@cindex handle
For each relocatable block, you must make a @dfn{handle}---a pointer
object in memory, designated to store the address of that block.  The
relocating allocator knows where each block's handle is, and updates the
address stored there whenever it moves the block, so that the handle
always points to the block.  Each time you access the contents of the
block, you should fetch its address anew from the handle.

To call any of the relocating allocator functions from a signal handler
is almost certainly incorrect, because the signal could happen at any
time and relocate all the blocks.  The only way to make this safe is to
block the signal around any access to the contents of any relocatable
block---not a convenient mode of operation.  @xref{Nonreentrancy}.

@node Using Relocator
@subsection Allocating and Freeing Relocatable Blocks

@pindex malloc.h
In the descriptions below, @var{handleptr} designates the address of the
handle.  All the functions are declared in @file{malloc.h}; all are GNU
extensions.

@comment malloc.h
@comment GNU
@deftypefun {void *} r_alloc (void **@var{handleptr}, size_t @var{size})
This function allocates a relocatable block of size @var{size}.  It
stores the block's address in @code{*@var{handleptr}} and returns
a non-null pointer to indicate success.

If @code{r_alloc} can't get the space needed, it stores a null pointer
in @code{*@var{handleptr}}, and returns a null pointer.
@end deftypefun

@comment malloc.h
@comment GNU
@deftypefun void r_alloc_free (void **@var{handleptr})
This function is the way to free a relocatable block.  It frees the
block that @code{*@var{handleptr}} points to, and stores a null pointer
in @code{*@var{handleptr}} to show it doesn't point to an allocated
block any more.
@end deftypefun

@comment malloc.h
@comment GNU
@deftypefun {void *} r_re_alloc (void **@var{handleptr}, size_t @var{size})
The function @code{r_re_alloc} adjusts the size of the block that
@code{*@var{handleptr}} points to, making it @var{size} bytes long.  It
stores the address of the resized block in @code{*@var{handleptr}} and
returns a non-null pointer to indicate success.

If enough memory is not available, this function returns a null pointer
and does not modify @code{*@var{handleptr}}.
@end deftypefun

@node Memory Warnings
@section Memory Usage Warnings
@cindex memory usage warnings
@cindex warnings of memory almost full

@pindex malloc.c
You can ask for warnings as the program approaches running out of memory
space, by calling @code{memory_warnings}.  This tells @code{malloc} to
check memory usage every time it asks for more memory from the operating
system.  This is a GNU extension declared in @file{malloc.h}.

@comment malloc.h
@comment GNU
@deftypefun void memory_warnings (void *@var{start}, void (*@var{warn-func}) (const char *))
Call this function to request warnings for nearing exhaustion of virtual
memory.

The argument @var{start} says where data space begins, in memory.  The
allocator compares this against the last address used and against the
limit of data space, to determine the fraction of available memory in
use.  If you supply zero for @var{start}, then a default value is used
which is right in most circumstances.

For @var{warn-func}, supply a function that @code{malloc} can call to
warn you.  It is called with a string (a warning message) as argument.
Normally it ought to display the string for the user to read.
@end deftypefun

The warnings come when memory becomes 75% full, when it becomes 85%
full, and when it becomes 95% full.  Above 95% you get another warning
each time memory usage increases.