774 lines
30 KiB
Text
774 lines
30 KiB
Text
@node Obstacks
|
|
@subsection 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 really 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
|
|
@subsubsection 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 macros
|
|
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 macros 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
|
|
@subsubsection Preparing for Using Obstacks
|
|
|
|
Each source file in which you plan to use obstacks
|
|
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 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, , , libc, The GNU C Library Reference Manual}). 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 memory 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} or one of its variants, @code{obstack_begin},
|
|
@code{obstack_specify_allocation}, or
|
|
@code{obstack_specify_allocation_with_arg}.
|
|
|
|
@comment obstack.h
|
|
@comment GNU
|
|
@deftypefun int obstack_init (struct obstack *@var{obstack-ptr})
|
|
Initialize obstack @var{obstack-ptr} for allocation of objects. This
|
|
macro calls the obstack's @code{obstack_chunk_alloc} function. If
|
|
allocation of memory fails, the function pointed to by
|
|
@code{obstack_alloc_failed_handler} is called. The @code{obstack_init}
|
|
macro always returns 1 (Compatibility notice: Former versions of
|
|
obstack returned 0 if allocation failed).
|
|
@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
|
|
|
|
@comment obstack.h
|
|
@comment GNU
|
|
@deftypefun int obstack_begin (struct obstack *@var{obstack-ptr}, size_t chunk_size)
|
|
Like @code{obstack_init}, but specify chunks to be at least
|
|
@var{chunk_size} bytes in size.
|
|
@end deftypefun
|
|
|
|
@comment obstack.h
|
|
@comment GNU
|
|
@deftypefun int obstack_specify_allocation (struct obstack *@var{obstack-ptr}, size_t chunk_size, size_t alignment, void *(*chunkfun) (size_t), void (*freefun) (void *))
|
|
Like @code{obstack_init}, specifying chunk size, chunk
|
|
alignment, and memory allocation functions. A @var{chunk_size} or
|
|
@var{alignment} of zero results in the default size or alignment
|
|
respectively being used.
|
|
@end deftypefun
|
|
|
|
@comment obstack.h
|
|
@comment GNU
|
|
@deftypefun int obstack_specify_allocation_with_arg (struct obstack *@var{obstack-ptr}, size_t chunk_size, size_t alignment, void *(*chunkfun) (void *, size_t), void (*freefun) (void *, void *), void *arg)
|
|
Like @code{obstack_specify_allocation}, but specifying memory
|
|
allocation functions that take an extra first argument, @var{arg}.
|
|
@end deftypefun
|
|
|
|
@comment obstack.h
|
|
@comment GNU
|
|
@defvar obstack_alloc_failed_handler
|
|
The value of this variable is a pointer to a function that
|
|
@code{obstack} uses when @code{obstack_chunk_alloc} fails to allocate
|
|
memory. The default action is to print a message and abort.
|
|
You should supply a function that either calls @code{exit}
|
|
(@pxref{Program Termination, , , libc, The GNU C Library Reference Manual}) or @code{longjmp} (@pxref{Non-Local
|
|
Exits, , , libc, The GNU C Library Reference Manual}) and doesn't return.
|
|
|
|
@smallexample
|
|
void my_obstack_alloc_failed (void)
|
|
@dots{}
|
|
obstack_alloc_failed_handler = &my_obstack_alloc_failed;
|
|
@end smallexample
|
|
|
|
@end defvar
|
|
|
|
@node Allocation in an Obstack
|
|
@subsubsection 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}, size_t @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 macro
|
|
requires you to specify an @var{obstack-ptr} as the first argument.
|
|
|
|
This macro calls the obstack's @code{obstack_chunk_alloc} function if
|
|
it needs to allocate a new chunk of memory; it calls
|
|
@code{obstack_alloc_failed_handler} if allocation of memory by
|
|
@code{obstack_chunk_alloc} failed.
|
|
@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)
|
|
@{
|
|
size_t len = strlen (string) + 1;
|
|
char *s = (char *) obstack_alloc (&string_obstack, len);
|
|
memcpy (s, string, len);
|
|
return s;
|
|
@}
|
|
@end smallexample
|
|
|
|
To allocate a block with specified contents, use the macro @code{obstack_copy}.
|
|
|
|
@comment obstack.h
|
|
@comment GNU
|
|
@deftypefun {void *} obstack_copy (struct obstack *@var{obstack-ptr}, void *@var{address}, size_t @var{size})
|
|
This allocates a block and initializes it by copying @var{size}
|
|
bytes of data starting at @var{address}. It calls
|
|
@code{obstack_alloc_failed_handler} if allocation of memory by
|
|
@code{obstack_chunk_alloc} failed.
|
|
@end deftypefun
|
|
|
|
@comment obstack.h
|
|
@comment GNU
|
|
@deftypefun {void *} obstack_copy0 (struct obstack *@var{obstack-ptr}, void *@var{address}, size_t @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} macro 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, size_t 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, , , libc, The GNU C Library Reference Manual}).
|
|
|
|
@node Freeing Obstack Objects
|
|
@subsubsection Freeing Objects in an Obstack
|
|
@cindex freeing (obstacks)
|
|
|
|
To free an object allocated in an obstack, use the macro
|
|
@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 memory 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
|
|
@subsubsection Obstack Functions and Macros
|
|
@cindex macros
|
|
|
|
The interfaces for using obstacks are shown here as functions to
|
|
specify the return type and argument types, but they are really
|
|
defined as macros. This means that the arguments don't actually have
|
|
types, but they generally behave as if they have the types shown.
|
|
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.
|
|
|
|
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.
|
|
|
|
Note that arguments other than the first will only be evaluated once,
|
|
even when not using GNU C.
|
|
|
|
@code{obstack.h} does declare a number of functions,
|
|
@code{_obstack_begin}, @code{_obstack_begin_1},
|
|
@code{_obstack_newchunk}, @code{_obstack_free}, and
|
|
@code{_obstack_memory_used}. You should not call these directly.
|
|
|
|
@node Growing Objects
|
|
@subsubsection Growing Objects
|
|
@cindex growing objects (in obstacks)
|
|
@cindex changing the size of a block (obstacks)
|
|
|
|
Because memory 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 macros
|
|
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 macros 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 @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}, size_t @var{size})
|
|
The most basic macro 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}, size_t @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}, size_t @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 @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_ptr_grow (struct obstack *@var{obstack-ptr}, void *@var{data})
|
|
Adding the value of a pointer one can use
|
|
@code{obstack_ptr_grow}. It adds @code{sizeof (void *)} bytes
|
|
containing the value of @var{data}.
|
|
@end deftypefun
|
|
|
|
@comment obstack.h
|
|
@comment GNU
|
|
@deftypefun void obstack_int_grow (struct obstack *@var{obstack-ptr}, int @var{data})
|
|
A single value of type @code{int} can be added by using
|
|
@code{obstack_int_grow}. It adds @code{sizeof (int)} bytes to
|
|
the growing object and initializes them with the value of @var{data}.
|
|
@end deftypefun
|
|
|
|
@comment obstack.h
|
|
@comment GNU
|
|
@deftypefun {void *} obstack_finish (struct obstack *@var{obstack-ptr})
|
|
When you are finished growing the object, use
|
|
@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.
|
|
@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
|
|
with @code{obstack_object_size}, before finishing the object.
|
|
|
|
@comment obstack.h
|
|
@comment GNU
|
|
@deftypefun size_t obstack_object_size (struct obstack *@var{obstack-ptr})
|
|
This macro returns the current size of the growing object, in bytes.
|
|
Remember to call @code{obstack_object_size} @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.
|
|
|
|
@node Extra Fast Growing
|
|
@subsubsection Extra Fast Growing Objects
|
|
@cindex efficiency and obstacks
|
|
|
|
The usual macros 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''
|
|
macros 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
|
|
macros do. But if you can arrange to check less often, or check
|
|
more efficiently, then you make the program faster.
|
|
|
|
@code{obstack_room} returns the amount of room available
|
|
in the current chunk.
|
|
|
|
@comment obstack.h
|
|
@comment GNU
|
|
@deftypefun size_t 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 macros.
|
|
@end deftypefun
|
|
|
|
While you know there is room, you can use these fast growth macros
|
|
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})
|
|
@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_ptr_grow_fast (struct obstack *@var{obstack-ptr}, void *@var{data})
|
|
@code{obstack_ptr_grow_fast} adds @code{sizeof (void *)}
|
|
bytes containing the value of @var{data} to the growing object in
|
|
obstack @var{obstack-ptr}.
|
|
@end deftypefun
|
|
|
|
@comment obstack.h
|
|
@comment GNU
|
|
@deftypefun void obstack_int_grow_fast (struct obstack *@var{obstack-ptr}, int @var{data})
|
|
@code{obstack_int_grow_fast} adds @code{sizeof (int)} bytes
|
|
containing the value of @var{data} 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}, size_t @var{size})
|
|
@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 macros
|
|
are not safe. In this case, simply use the corresponding ordinary
|
|
growth macro 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 macro, 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 macros again.
|
|
|
|
Here is an example:
|
|
|
|
@smallexample
|
|
@group
|
|
void
|
|
add_string (struct obstack *obstack, const char *ptr, size_t len)
|
|
@{
|
|
while (len > 0)
|
|
@{
|
|
size_t 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
|
|
|
|
@cindex shrinking objects
|
|
You can use @code{obstack_blank_fast} 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. Earlier versions of obstacks allowed you to use
|
|
@code{obstack_blank} to shrink objects. This will no longer work.
|
|
|
|
@node Status of an Obstack
|
|
@subsubsection Status of an Obstack
|
|
@cindex obstack status
|
|
@cindex status of obstack
|
|
|
|
Here are macros 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 macro 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 macro 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 size_t obstack_object_size (struct obstack *@var{obstack-ptr})
|
|
This macro returns the size in bytes of the currently growing object.
|
|
This is equivalent to
|
|
|
|
@smallexample
|
|
((size_t) (obstack_next_free (@var{obstack-ptr}) - obstack_base (@var{obstack-ptr})))
|
|
@end smallexample
|
|
@end deftypefun
|
|
|
|
@node Obstacks Data Alignment
|
|
@subsubsection 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 aligned so that
|
|
the object can hold any type of data.
|
|
|
|
To access an obstack's alignment boundary, use the macro
|
|
@code{obstack_alignment_mask}.
|
|
|
|
@comment obstack.h
|
|
@comment GNU
|
|
@deftypefn Macro size_t 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 a value
|
|
that allows aligned objects to hold any type of data: for example, if
|
|
its value is 3, any type of data can be stored at locations whose
|
|
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
|
|
@subsubsection 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 malloc
|
|
#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 size_t 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
|
|
@subsubsection Summary of Obstack Macros
|
|
|
|
Here is a summary of all the macros associated with obstacks. Each
|
|
takes the address of an obstack (@code{struct obstack *}) as its first
|
|
argument.
|
|
|
|
@table @code
|
|
@item int obstack_init (struct obstack *@var{obstack-ptr})
|
|
Initialize use of an obstack. @xref{Creating Obstacks}.
|
|
|
|
@item int obstack_begin (struct obstack *@var{obstack-ptr}, size_t chunk_size)
|
|
Initialize use of an obstack, with an initial chunk of
|
|
@var{chunk_size} bytes.
|
|
|
|
@item int obstack_specify_allocation (struct obstack *@var{obstack-ptr}, size_t chunk_size, size_t alignment, void *(*chunkfun) (size_t), void (*freefun) (void *))
|
|
Initialize use of an obstack, specifying intial chunk size, chunk
|
|
alignment, and memory allocation functions.
|
|
|
|
@item int obstack_specify_allocation_with_arg (struct obstack *@var{obstack-ptr}, size_t chunk_size, size_t alignment, void *(*chunkfun) (void *, size_t), void (*freefun) (void *, void *), void *arg)
|
|
Like @code{obstack_specify_allocation}, but specifying memory
|
|
allocation functions that take an extra first argument, @var{arg}.
|
|
|
|
@item void *obstack_alloc (struct obstack *@var{obstack-ptr}, size_t @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}, size_t @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}, size_t @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}, size_t @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}, size_t @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}, size_t @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 size_t 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}, size_t @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 size_t obstack_room (struct obstack *@var{obstack-ptr})
|
|
Get the amount of room now available for growing the current object.
|
|
@xref{Extra Fast Growing}.
|
|
|
|
@item size_t 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 size_t 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
|
|
|