| @node Obstacks,Licenses,Functions,Top |
| @chapter 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 |
| @section 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 |
| @section 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, , , 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}. |
| |
| @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. If |
| allocation of memory fails, the function pointed to by |
| @code{obstack_alloc_failed_handler} is called. The @code{obstack_init} |
| function 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 |
| @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 |
| @section 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 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 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 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}, 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, , , libc, The GNU C Library Reference Manual}). |
| |
| @node Freeing Obstack Objects |
| @section 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 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 |
| @section 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 @w{ISO 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 @w{non-ISO 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 @w{ISO 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 @w{ISO C} for the standard library |
| functions. @xref{Macro Definitions, , , libc, The GNU C Library Reference Manual}. |
| |
| @strong{Warning:} When you do use the macros, you must observe the |
| precaution of avoiding side effects in the first operand, even in @w{ISO 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 |
| @section 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 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_ptr_grow (struct obstack *@var{obstack-ptr}, void *@var{data}) |
| Adding the value of a pointer one can use the function |
| @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 the |
| @code{obstack_int_grow} function. 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 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 |
| @section 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_ptr_grow_fast (struct obstack *@var{obstack-ptr}, void *@var{data}) |
| The function @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}) |
| The function @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}, 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 |
| @section 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 |
| @section 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 |
| @section 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 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 |
| @section 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 |
| |