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The GNU system provides several methods for allocating memory space under explicit program control. They vary in generality and in efficiency.
malloc facility allows fully general dynamic allocation.
See section Unconstrained Allocation.
malloc but more
efficient and convenient for stacklike allocation. See section Obstacks.
alloca lets you allocate storage dynamically that
will be freed automatically. See section Automatic Storage with Variable Size.
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.
The C language supports two kinds of memory allocation through the variables in C programs:
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
struct foobar, you cannot declare a variable of type struct
foobar whose contents are the dynamically allocated space. But you can
declare a variable of pointer type struct foobar * and assign it the
address of the space. Then you can use the operators `*' and
`->' on this pointer variable to refer to the contents of the space:
{
struct foobar *ptr
= (struct foobar *) malloc (sizeof (struct foobar));
ptr->name = x;
ptr->next = current_foobar;
current_foobar = ptr;
}
The most general dynamic allocation facility is 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).
To allocate a block of memory, call malloc. The prototype for
this function is in `stdlib.h'.
The contents of the block are undefined; you must initialize it yourself
(or use calloc instead; see section 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
memset (see section Copying and Concatenation):
struct foo *ptr; ... ptr = (struct foo *) malloc (sizeof (struct foo)); if (ptr == 0) abort (); memset (ptr, 0, sizeof (struct foo));
You can store the result of malloc into any pointer variable
without a cast, because ISO C automatically converts the type
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
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:
char *ptr; ... ptr = (char *) malloc (length + 1);
See section Representation of Strings, for more information about this.
malloc
If no more space is available, malloc returns a null pointer.
You should check the value of every call to malloc. It is
useful to write a subroutine that calls malloc and reports an
error if the value is a null pointer, returning only if the value is
nonzero. This function is conventionally called xmalloc. Here
it is:
void *
xmalloc (size_t size)
{
register void *value = malloc (size);
if (value == 0)
fatal ("virtual memory exhausted");
return value;
}
Here is a real example of using malloc (by way of xmalloc).
The function savestring will copy a sequence of characters into
a newly allocated null-terminated string:
char *
savestring (const char *ptr, size_t len)
{
register char *value = (char *) xmalloc (len + 1);
memcpy (value, ptr, len);
value[len] = '\0';
return value;
}
The block that 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 on most systems, and a multiple of 16 on
64-bit systems. Only rarely is any higher boundary (such as a page
boundary) necessary; for those cases, use memalign or
valloc (see section Allocating 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 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
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 realloc (see section Changing the Size of a Block).
malloc
When you no longer need a block that you got with malloc, use the
function free to make the block available to be allocated again.
The prototype for this function is in `stdlib.h'.
free function deallocates the block of storage pointed at
by ptr.
free. It's provided for
backward compatibility with SunOS; you should use free instead.
Freeing a block alters the contents of the block. 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:
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;
}
}
Occasionally, 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 malloc to reuse the space. In the meantime, the
space remains in your program as part of a free-list used internally by
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.
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 realloc. This function
is declared in `stdlib.h'.
realloc function changes the size of the block whose address is
ptr to be newsize.
Since the space after the end of the block may be in use, realloc
may find it necessary to copy the block to a new address where more free
space is available. The value of realloc is the new address of the
block. If the block needs to be moved, realloc copies the old
contents.
If you pass a null pointer for ptr, realloc behaves just
like `malloc (newsize)'. This can be convenient, but beware
that older implementations (before ISO C) may not support this
behavior, and will probably crash when realloc is passed a null
pointer.
Like malloc, 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 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 xrealloc, that takes care of the error message
as xmalloc does for malloc:
void *
xrealloc (void *ptr, size_t size)
{
register void *value = realloc (ptr, size);
if (value == 0)
fatal ("Virtual memory exhausted");
return value;
}
You can also use realloc to make a block smaller. The reason you
is needed.
In several allocation implementations, 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, realloc
is guaranteed to change nothing and return the same address that you gave.
The function calloc allocates memory and clears it to zero. It
is declared in `stdlib.h'.
calloc returns.
You could define calloc as follows:
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;
}
But in general, it is not guaranteed that calloc calls
malloc internally. Therefore, if an application provides its own
malloc/realloc/free outside the C library, it
should always define calloc, too.
malloc
As apposed to other versions, the malloc in GNU libc does not
round up block sizes to powers of two, neither for large nor for small
sizes. Neighboring chunks can be coalesced on a free no matter
what their size is. This makes the implementation suitable for all
kinds of allocation patterns without generally incurring high memory
waste through fragmentation.
Very large blocks (much larger than a page) are allocated with
mmap (anonymous or via /dev/zero) by this implementation.
This has the great advantage that these chunks are returned to the
system immediately when they are freed. Therefore, it cannot happen
that a large chunk becomes "locked" in between smaller ones and even
after calling free wastes memory. The size threshold for
mmap to be used can be adjusted with mallopt. The use of
mmap can also be disabled completely.
The address of a block returned by malloc or realloc in
the GNU system is always a multiple of eight (or sixteen on 64-bit
systems). If you need a block whose address is a multiple of a higher
power of two than that, use memalign or valloc. These
functions are declared in `stdlib.h'.
With the GNU library, you can use free to free the blocks that
memalign and valloc return. That does not work in BSD,
however--BSD does not provide any way to free such blocks.
memalign function allocates a block of size bytes whose
address is a multiple of boundary. The boundary must be a
power of two! The function memalign works by allocating a
somewhat larger block, and then returning an address within the block
that is on the specified boundary.
valloc is like using memalign and passing the page size
as the value of the second argument. It is implemented like this:
void *
valloc (size_t size)
{
return memalign (getpagesize (), size);
}
You can adjust some parameters for dynamic memory allocation with the
mallopt function. This function is the general SVID/XPG
interface, defined in `malloc.h'.
mallopt, the param argument specifies the
parameter to be set, and value the new value to be set. Possible
choices for param, as defined in `malloc.h', are:
M_TRIM_THRESHOLD
sbrk to be called with a negative argument in
order to return memory to the system.
M_TOP_PAD
sbrk is required. It also specifies the
number of bytes to retain when shrinking the heap by calling sbrk
with a negative argument. This provides the necessary hysteresis in
heap size such that excessive amounts of system calls can be avoided.
M_MMAP_THRESHOLD
mmap system call. This way it is guaranteed
that the memory for these chunks can be returned to the system on
free.
M_MMAP_MAX
mmap. Setting this
to zero disables all use of mmap.
You can ask malloc to check the consistency of dynamic storage by
using the mcheck function. This function is a GNU extension,
declared in `malloc.h'.
mcheck tells malloc to perform occasional
consistency checks. These will catch things such as writing
past the end of a block that was allocated with malloc.
The abortfn argument is the function to call when an inconsistency
is found. If you supply a null pointer, then mcheck uses a
default function which prints a message and calls abort
(see section 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 malloc. So mcheck does nothing in that
case. The function returns -1 if you call it too late, and
0 otherwise (when it is successful).
The easiest way to arrange to call mcheck early enough is to use
the option `-lmcheck' when you link your program; then you don't
need to modify your program source at all.
mprobe function lets you explicitly check for inconsistencies
in a particular allocated block. You must have already called
mcheck at the beginning of the program, to do its occasional
checks; calling mprobe requests an additional consistency check
to be done at the time of the call.
The argument pointer must be a pointer returned by malloc
or realloc. mprobe returns a value that says what
inconsistency, if any, was found. The values are described below.
MCHECK_DISABLED
mcheck was not called before the first allocation.
No consistency checking can be done.
MCHECK_OK
MCHECK_HEAD
MCHECK_TAIL
MCHECK_FREE
The GNU C library lets you modify the behavior of malloc,
realloc, and 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 `malloc.h'.
malloc
uses whenever it is called. You should define this function to look
like malloc; that is, like:
void *function (size_t size)
realloc
uses whenever it is called. You should define this function to look
like realloc; that is, like:
void *function (void *ptr, size_t size)
free
uses whenever it is called. You should define this function to look
like free; that is, like:
void function (void *ptr)
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 __malloc_hook properly. It
installs a function that prints out information every time malloc
is called.
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);
/* printf might call 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;
...
}
The mcheck function (see section Heap Consistency Checking) works by
installing such hooks.
malloc
You can get information about dynamic storage allocation by calling the
mallinfo function. This function and its associated data type
are declared in `malloc.h'; they are an extension of the standard
SVID/XPG version.
int arena
sbrk by
malloc, in bytes.
int ordblks
malloc requests; see
section Efficiency Considerations for malloc.)
int smblks
int hblks
mmap.
int hblkhd
mmap, in bytes.
int usmblks
int fsmblks
int uordblks
malloc.
int fordblks
int keepcost
struct mallinfo.
malloc-Related Functions
Here is a summary of the functions that work with malloc:
void *malloc (size_t size)
void free (void *addr)
malloc. See section Freeing Memory Allocated with malloc.
void *realloc (void *addr, size_t size)
malloc larger or smaller,
possibly by copying it to a new location. See section Changing the Size of a Block.
void *calloc (size_t count, size_t eltsize)
malloc, and set its contents to zero. See section Allocating Cleared Space.
void *valloc (size_t size)
void *memalign (size_t size, size_t boundary)
int mallopt (int param, int value)
int mcheck (void (*abortfn) (void))
malloc to perform occasional consistency checks on
dynamically allocated memory, and to call abortfn when an
inconsistency is found. See section Heap Consistency Checking.
void *(*__malloc_hook) (size_t size)
malloc uses whenever it is called.
void *(*__realloc_hook) (void *ptr, size_t size)
realloc uses whenever it is called.
void (*__free_hook) (void *ptr)
free uses whenever it is called.
struct mallinfo mallinfo (void)
malloc.
An 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.
The utilities for manipulating obstacks are declared in the header file `obstack.h'.
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.
You can declare variables of type 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 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
chunks. The 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 malloc
directly or indirectly. You must also supply a function to free a chunk.
These matters are described in the following section.
Each source file in which you plan to use the obstack functions must include the header file `obstack.h', like this:
#include <obstack.h>
Also, if the source file uses the macro obstack_init, it must
declare or define two functions or macros that will be called by the
obstack library. One, obstack_chunk_alloc, is used to allocate
the chunks of memory into which objects are packed. The other,
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 malloc via the intermediary
xmalloc (see section Unconstrained Allocation). This is done with
the following pair of macro definitions:
#define obstack_chunk_alloc xmalloc #define obstack_chunk_free free
Though the storage you get using obstacks really comes from malloc,
using obstacks is faster because malloc is called less often, for
larger blocks of memory. See section Obstack Chunks, for full details.
At run time, before the program can use a struct obstack object
as an obstack, it must initialize the obstack by calling
obstack_init.
obstack_chunk_alloc function. It
returns 0 if obstack_chunk_alloc returns a null pointer, meaning
that it is out of memory. Otherwise, it returns 1. If you supply an
obstack_chunk_alloc function that calls exit
(see section Program Termination) or longjmp (see section Non-Local Exits) when out of memory, you can safely ignore the value that
obstack_init returns.
Here are two examples of how to allocate the space for an obstack and initialize it. First, an obstack that is a static variable:
static struct obstack myobstack; ... obstack_init (&myobstack);
Second, an obstack that is itself dynamically allocated:
struct obstack *myobstack_ptr = (struct obstack *) xmalloc (sizeof (struct obstack)); obstack_init (myobstack_ptr);
The most direct way to allocate an object in an obstack is with
obstack_alloc, which is invoked almost like malloc.
struct obstack
object which represents the obstack. Each obstack function or macro
requires you to specify an obstack-ptr as the first argument.
This function calls the obstack's obstack_chunk_alloc function if
it needs to allocate a new chunk of memory; it returns a null pointer if
obstack_chunk_alloc returns one. In that case, it has not
changed the amount of memory allocated in the obstack. If you supply an
obstack_chunk_alloc function that calls exit
(see section Program Termination) or longjmp (see section Non-Local Exits) when out of memory, then obstack_alloc will never return
a null pointer.
For example, here is a function that allocates a copy of a string str
in a specific obstack, which is in the variable string_obstack:
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;
}
To allocate a block with specified contents, use the function
obstack_copy, declared like this:
obstack_alloc.
obstack_copy, but appends an extra byte containing a null
character. This extra byte is not counted in the argument size.
The 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:
char *
obstack_savestring (char *addr, int size)
{
return obstack_copy0 (&myobstack, addr, size);
}
Contrast this with the previous example of savestring using
malloc (see section Basic Storage Allocation).
To free an object allocated in an obstack, use the function
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.
Note that if 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 obstack_free with the address
of the first object allocated on the obstack:
obstack_free (obstack_ptr, first_object_allocated_ptr);
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 (see section Preparing for Using Obstacks). Then other obstacks, or non-obstack allocation, can reuse the space of the chunk.
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 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 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:
obstack_alloc (get_obstack (), 4);
you will find that get_obstack may be called several times.
If you use *obstack_list_ptr++ as the obstack pointer argument,
you will get very strange results since the incrementation may occur
several times.
In 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:
char *x; void *(*funcp) (); /* Use the macro. */ x = (char *) obstack_alloc (obptr, size); /* Call the function. */ x = (char *) (obstack_alloc) (obptr, size); /* Take the address of the function. */ funcp = obstack_alloc;
This is the same situation that exists in ISO C for the standard library functions. See section Macro Definitions of Functions.
Warning: When you do use the macros, you must observe the precaution of avoiding side effects in the first operand, even in 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.
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 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 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.
obstack_blank, which adds space without initializing it.
obstack_grow, which is
the growing-object analogue of obstack_copy. It adds size
bytes of data to the growing object, copying the contents from
data.
obstack_copy0. It adds
size bytes copied from data, followed by an additional null
character.
obstack_1grow.
It adds a single byte containing c to the growing object.
obstack_ptr_grow. It adds sizeof (void *) bytes
containing the value of data.
int can be added by using the
obstack_int_grow function. It adds sizeof (int) bytes to
the growing object and initializes them with the value of data.
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
obstack_alloc (see section Allocation in an Obstack).
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 obstack_object_size,
declared as follows:
obstack_object_size will return zero.
If you have started growing an object and wish to cancel it, you should finish it and then free it, like this:
obstack_free (obstack_ptr, obstack_finish (obstack_ptr));
This has no effect if no object was growing.
You can use 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.
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 obstack_room returns the amount of room available
in the current chunk. It is declared as follows:
While you know there is room, you can use these fast growth functions for adding data to a growing object:
obstack_1grow_fast adds one byte containing the
character c to the growing object in obstack obstack-ptr.
obstack_ptr_grow_fast adds sizeof (void *)
bytes containing the value of data to the growing object in
obstack obstack-ptr.
obstack_int_grow_fast adds sizeof (int) bytes
containing the value of data to the growing object in obstack
obstack-ptr.
obstack_blank_fast adds size bytes to the
growing object in obstack obstack-ptr without initializing them.
When you check for space using 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 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:
void
add_string (struct obstack *obstack, const char *ptr, int len)
{
while (len > 0)
{
int room = obstack_room (obstack);
if (room == 0)
{
/* Not enough room. Add one character slowly,
which may copy to a new chunk and make room. */
obstack_1grow (obstack, *ptr++);
len--;
}
else
{
if (room > len)
room = len;
/* Add fast as much as we have room for. */
len -= room;
while (room-- > 0)
obstack_1grow_fast (obstack, *ptr++);
}
}
}
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.
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).
obstack_next_free
returns the same value as obstack_base.
obstack_next_free (obstack-ptr) - obstack_base (obstack-ptr)
Each obstack has an 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
obstack_alignment_mask, whose function prototype looks like
this:
The expansion of the macro obstack_alignment_mask is an lvalue,
so you can alter the mask by assignment. For example, this statement:
obstack_alignment_mask (obstack_ptr) = 0;
has the effect of turning off alignment processing in the specified obstack.
Note that a change in alignment mask does not take effect until
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 obstack_finish.
This will finish a zero-length object and then do proper alignment for
the next object.
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
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 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 obstack_init (see section Creating Obstacks).
Most often they are defined as macros like this:
#define obstack_chunk_alloc xmalloc #define obstack_chunk_free free
Note that these are simple macros (no arguments). Macro definitions with
arguments will not work! It is necessary that obstack_chunk_alloc
or obstack_chunk_free, alone, expand into a function name if it is
not itself a function name.
If you allocate chunks with 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.
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:
if (obstack_chunk_size (obstack_ptr) < new-chunk-size) obstack_chunk_size (obstack_ptr) = new-chunk-size;
Here is a summary of all the functions associated with obstacks. Each
takes the address of an obstack (struct obstack *) as its first
argument.
void obstack_init (struct obstack *obstack-ptr)
void *obstack_alloc (struct obstack *obstack-ptr, int size)
void *obstack_copy (struct obstack *obstack-ptr, void *address, int size)
void *obstack_copy0 (struct obstack *obstack-ptr, void *address, int size)
void obstack_free (struct obstack *obstack-ptr, void *object)
void obstack_blank (struct obstack *obstack-ptr, int size)
void obstack_grow (struct obstack *obstack-ptr, void *address, int size)
void obstack_grow0 (struct obstack *obstack-ptr, void *address, int size)
void obstack_1grow (struct obstack *obstack-ptr, char data-char)
void *obstack_finish (struct obstack *obstack-ptr)
int obstack_object_size (struct obstack *obstack-ptr)
void obstack_blank_fast (struct obstack *obstack-ptr, int size)
void obstack_1grow_fast (struct obstack *obstack-ptr, char data-char)
int obstack_room (struct obstack *obstack-ptr)
int obstack_alignment_mask (struct obstack *obstack-ptr)
int obstack_chunk_size (struct obstack *obstack-ptr)
void *obstack_base (struct obstack *obstack-ptr)
void *obstack_next_free (struct obstack *obstack-ptr)
The function alloca supports a kind of half-dynamic allocation in
which blocks are allocated dynamically but freed automatically.
Allocating a block with 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 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 alloca is in `stdlib.h'. This function is
a BSD extension.
alloca is the address of a block of size
bytes of storage, allocated in the stack frame of the calling function.
Do not use alloca inside the arguments of a function call--you
will get unpredictable results, because the stack space for the
alloca would appear on the stack in the middle of the space for
the function arguments. An example of what to avoid is foo (x,
alloca (4), y).
alloca Example
As an example of use of 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:
int
open2 (char *str1, char *str2, int flags, int mode)
{
char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1);
stpcpy (stpcpy (name, str1), str2);
return open (name, flags, mode);
}
Here is how you would get the same results with malloc and
free:
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");
stpcpy (stpcpy (name, str1), str2);
desc = open (name, flags, mode);
free (name);
return desc;
}
As you can see, it is simpler with alloca. But alloca has
other, more important advantages, and some disadvantages.
alloca
Here are the reasons why alloca may be preferable to malloc:
alloca wastes very little space and is very fast. (It is
open-coded by the GNU C compiler.)
alloca does not have separate pools for different sizes of
block, space used for any size block can be reused for any other size.
alloca does not cause storage fragmentation.
longjmp (see section Non-Local Exits)
automatically free the space allocated with alloca when they exit
through the function that called alloca. This is the most
important reason to use alloca.
To illustrate this, suppose you have a function
open_or_report_error which returns a descriptor, like
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 longjmp.
Let's change open2 (see section alloca Example) to use this
subroutine:
int
open2 (char *str1, char *str2, int flags, int mode)
{
char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1);
stpcpy (stpcpy (name, str1), str2);
return open_or_report_error (name, flags, mode);
}
Because of the way alloca works, the storage it allocates is
freed even when an error occurs, with no special effort required.
By contrast, the previous definition of open2 (which uses
malloc and 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.
alloca
These are the disadvantages of alloca in comparison with
malloc:
alloca, so it is less
portable. However, a slower emulation of alloca written in C
is available for use on systems with this deficiency.
In GNU C, you can replace most uses of alloca with an array of
variable size. Here is how open2 would look then:
int open2 (char *str1, char *str2, int flags, int mode)
{
char name[strlen (str1) + strlen (str2) + 1];
stpcpy (stpcpy (name, str1), str2);
return open (name, flags, mode);
}
But alloca is not always equivalent to a variable-sized array, for
several reasons:
alloca
remains until the end of the function.
alloca within a loop, allocating an
additional block on each iteration. This is impossible with
variable-sized arrays.
Note: If you mix use of alloca and variable-sized arrays
within one function, exiting a scope in which a variable-sized array was
declared frees all blocks allocated with alloca during the
execution of that scope.
Any system of dynamic memory allocation has overhead: the amount of space it uses is more than the amount the program asks for. The relocating memory allocator achieves very low overhead by moving blocks in memory as necessary, on its own initiative.
When you allocate a block with malloc, the address of the block
never changes unless you use 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 malloc
or realloc can move the relocatable blocks.
For each relocatable block, you must make a 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. See section Signal Handling and Nonreentrant Functions.
In the descriptions below, handleptr designates the address of the handle. All the functions are declared in `malloc.h'; all are GNU extensions.
*handleptr and returns
a non-null pointer to indicate success.
If r_alloc can't get the space needed, it stores a null pointer
in *handleptr, and returns a null pointer.
*handleptr points to, and stores a null pointer
in *handleptr to show it doesn't point to an allocated
block any more.
r_re_alloc adjusts the size of the block that
*handleptr points to, making it size bytes long. It
stores the address of the resized block in *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 *handleptr.
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