.
The GNU system provides several methods for allocating memory space
under explicit program control. They vary in generality and in
efficiency.
-
The
malloc facility allows fully general dynamic allocation.
See section Unconstrained Allocation.
-
Obstacks are another facility, less general than
malloc but more
efficient and convenient for stacklike allocation. See section Obstacks.
-
The function
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:
-
Static allocation is what happens when you declare a static or
global variable. Each static or global variable defines one block of
space, of a fixed size. The space is allocated once, when your program
is started, and is never freed.
-
Automatic allocation happens when you declare an automatic
variable, such as a function argument or a local variable. The space
for an automatic variable is allocated when the compound statement
containing the declaration is entered, and is freed when that
compound statement is exited.
In GNU C, the length of the automatic storage can be an expression
that varies. In other C implementations, it must be a constant.
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'.
- Function: void * malloc (size_t size)
-
This function returns a pointer to a newly allocated block size
bytes long, or a null pointer if the block could not be allocated.
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.
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);
value[len] = '\0';
return (char *) memcpy (value, ptr, len);
}
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).
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'.
- Function: void free (void *ptr)
-
The
free function deallocates the block of storage pointed at
by ptr.
- Function: void cfree (void *ptr)
-
This function does the same thing as
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'.
- Function: void * realloc (void *ptr, size_t newsize)
-
The
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
would do this is to avoid tying up a lot of memory space when only a little
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'.
- Function: void * calloc (size_t count, size_t eltsize)
-
This function allocates a block long enough to contain a vector of
count elements, each of size eltsize. Its contents are
cleared to zero before
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.
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.
- Function: void * memalign (size_t boundary, size_t size)
-
The
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.
- Function: void * valloc (size_t size)
-
Using
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'.
- Function: int mallopt (int param, int value)
-
When calling
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
-
This is the minimum size (in bytes) of the top-most, releaseable chunk
that will cause
sbrk to be called with a negative argument in
order to return memory to the system.
M_TOP_PAD
-
This parameter determines the amount of extra memory to obtain from the
system when a call to
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
-
All chunks larger than this value are allocated outside the normal
heap, using the
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
-
The maximum number of chunks to allocate with
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 `mcheck.h'.
- Function: int mcheck (void (*abortfn) (enum mcheck_status status))
-
Calling
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. Alternately you might use
a debugger to insert a call to mcheck whenever the program is
started, for example these gdb commands will automatically call mcheck
whenever the program starts:
(gdb) break main
Breakpoint 1, main (argc=2, argv=0xbffff964) at whatever.c:10
(gdb) command 1
Type commands for when breakpoint 1 is hit, one per line.
End with a line saying just "end".
>call mcheck(0)
>continue
>end
(gdb) ...
This will however only work if no initialization function of any object
involved calls any of the malloc functions since mcheck
must be called before the first such function.
- Function: enum mcheck_status mprobe (void *pointer)
-
The
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.
- Data Type: enum mcheck_status
-
This enumerated type describes what kind of inconsistency was detected
in an allocated block, if any. Here are the possible values:
MCHECK_DISABLED
-
mcheck was not called before the first allocation.
No consistency checking can be done.
MCHECK_OK
-
No inconsistency detected.
MCHECK_HEAD
-
The data immediately before the block was modified.
This commonly happens when an array index or pointer
is decremented too far.
MCHECK_TAIL
-
The data immediately after the block was modified.
This commonly happens when an array index or pointer
is incremented too far.
MCHECK_FREE
-
The block was already freed.
Another possibility to check for and guard against bugs in the use of
malloc, realloc and free is to set the environment
variable MALLOC_CHECK_. When MALLOC_CHECK_ is set, a
special (less efficient) implementation is used which is designed to be
tolerant against simple errors, such as double calls of free with
the same argument, or overruns of a single byte (off-by-one bugs). Not
all such errors can be proteced against, however, and memory leaks can
result. If MALLOC_CHECK_ is set to 0, any detected heap
corruption is silently ignored; if set to 1, a diagnostic is
printed on stderr; if set to 2, abort is called
immediately. This can be useful because otherwise a crash may happen
much later, and the true cause for the problem is then very hard to
track down.
So, what's the difference between using MALLOC_CHECK_ and linking
with `-lmcheck'? MALLOC_CHECK_ is orthognal with respect to
`-lmcheck'. `-lmcheck' has been added for backward
compatibility. Both MALLOC_CHECK_ and `-lmcheck' should
uncover the same bugs - but using MALLOC_CHECK_ you don't need to
recompile your application.
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'.
- Variable: __malloc_hook
-
The value of this variable is a pointer to function that
malloc
uses whenever it is called. You should define this function to look
like malloc; that is, like:
void *function (size_t size, void *caller)
The value of caller is the return address found on the stack when
the malloc function was called. This value allows to trace the
memory consumption of the program.
- Variable: __realloc_hook
-
The value of this variable is a pointer to function that
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, void *caller)
The value of caller is the return address found on the stack when
the realloc function was called. This value allows to trace the
memory consumption of the program.
- Variable: __free_hook
-
The value of this variable is a pointer to function that
free
uses whenever it is called. You should define this function to look
like free; that is, like:
void function (void *ptr, void *caller)
The value of caller is the return address found on the stack when
the free function was called. This value allows to trace the
memory consumption of the program.
- Variable: __memalign_hook
-
The value of this variable is a pointer to function that
memalign
uses whenever it is called. You should define this function to look
like memalign; that is, like:
void *function (size_t size, size_t alignment)
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. Before calling the function recursively, one
should make sure to restore all the hooks to their previous value. When
coming back from the recursive call, all the hooks should be resaved
since a hook might modify itself.
Here is an example showing how to use __malloc_hook and
__free_hook properly. It installs a function that prints out
information every time malloc or free is called. We just
assume here that realloc and memalign are not used in our
program.
/* Global variables used to hold underlaying hook values. */
static void *(*old_malloc_hook) (size_t);
static void (*old_free_hook) (void*);
/* Prototypes for our hooks. */
static void *my_malloc_hook (size_t);
static void my_free_hook(void*);
static void *
my_malloc_hook (size_t size)
{
void *result;
/* Restore all old hooks */
__malloc_hook = old_malloc_hook;
__free_hook = old_free_hook;
/* Call recursively */
result = malloc (size);
/* Save underlaying hooks */
old_malloc_hook = __malloc_hook;
old_free_hook = __free_hook;
/* printf might call malloc, so protect it too. */
printf ("malloc (%u) returns %p\n", (unsigned int) size, result);
/* Restore our own hooks */
__malloc_hook = my_malloc_hook;
__free_hook = my_free_hook;
return result;
}
static void *
my_free_hook (void *ptr)
{
/* Restore all old hooks */
__malloc_hook = old_malloc_hook;
__free_hook = old_free_hook;
/* Call recursively */
free (ptr);
/* Save underlaying hooks */
old_malloc_hook = __malloc_hook;
old_free_hook = __free_hook;
/* printf might call free, so protect it too. */
printf ("freed pointer %p\n", ptr);
/* Restore our own hooks */
__malloc_hook = my_malloc_hook;
__free_hook = my_free_hook;
}
main ()
{
...
old_malloc_hook = __malloc_hook;
old_free_hook = __free_hook;
__malloc_hook = my_malloc_hook;
__free_hook = my_free_hook;
...
}
The mcheck function (see section Heap Consistency Checking) works by
installing such hooks.
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.
- Data Type: struct mallinfo
-
This structure type is used to return information about the dynamic
storage allocator. It contains the following members:
int arena
-
This is the total size of memory allocated with
sbrk by
malloc, in bytes.
int ordblks
-
This is the number of chunks not in use. (The storage allocator
internally gets chunks of memory from the operating system, and then
carves them up to satisfy individual
malloc requests; see
section Efficiency Considerations for malloc.)
int smblks
-
This field is unused.
int hblks
-
This is the total number of chunks allocated with
mmap.
int hblkhd
-
This is the total size of memory allocated with
mmap, in bytes.
int usmblks
-
This field is unused.
int fsmblks
-
This field is unused.
int uordblks
-
This is the total size of memory occupied by chunks handed out by
malloc.
int fordblks
-
This is the total size of memory occupied by free (not in use) chunks.
int keepcost
-
This is the size of the top-most, releaseable chunk that normally
borders the end of the heap (i.e. the "brk" of the process).
- Function: struct mallinfo mallinfo (void)
-
This function returns information about the current dynamic memory usage
in a structure of type
struct mallinfo.
Here is a summary of the functions that work with malloc:
void *malloc (size_t size)
-
Allocate a block of size bytes. See section Basic Storage Allocation.
void free (void *addr)
-
Free a block previously allocated by
malloc. See section Freeing Memory Allocated with malloc.
void *realloc (void *addr, size_t size)
-
Make a block previously allocated by
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)
-
Allocate a block of count * eltsize bytes using
malloc, and set its contents to zero. See section Allocating Cleared Space.
void *valloc (size_t size)
-
Allocate a block of size bytes, starting on a page boundary.
See section Allocating Aligned Memory Blocks.
void *memalign (size_t size, size_t boundary)
-
Allocate a block of size bytes, starting on an address that is a
multiple of boundary. See section Allocating Aligned Memory Blocks.
int mallopt (int param, int value)
-
Adjust a tunable parameter. See section Malloc Tunable Parameters.
int mcheck (void (*abortfn) (void))
-
Tell
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, void *caller)
-
A pointer to a function that
malloc uses whenever it is called.
void *(*__realloc_hook) (void *ptr, size_t size, void *caller)
-
A pointer to a function that
realloc uses whenever it is called.
void (*__free_hook) (void *ptr, void *caller)
-
A pointer to a function that
free uses whenever it is called.
void (*__memalign_hook) (size_t size, size_t alignment)
-
A pointer to a function that
memalign uses whenever it is called.
struct mallinfo mallinfo (void)
-
Return information about the current dynamic memory usage.
See section Statistics for Storage Allocation with
malloc.
An complicated task when programming with languages which do not use
garbage collected dynamic memory allocation is to find memory leaks.
Long running programs must assure that dynamically allocated objects are
freed at the end of their lifetime. If this does not happen the system
runs out of memory, sooner or later.
The malloc implementation in the GNU C library provides some
simple means to detect sich leaks and provide some information to find
the location. To do this the application must be started in a special
mode which is enabled by an environment variable. There are no speed
penalties if the program is compiled in preparation of the debugging if
the debug mode is not enabled.
- Function: void mtrace (void)
-
When the
mtrace function is called it looks for an environment
variable named MALLOC_TRACE. This variable is supposed to
contain a valid file name. The user must have write access. If the
file already exists it is truncated. If the environment variable is not
set or it does not name a valid file which can be opened for writing
nothing is done. The behaviour of malloc etc. is not changed.
For obvious reasons this also happens if the application is install SUID
or SGID.
If the named file is successfully opened mtrace installs special
handlers for the functions malloc, realloc, and
free (see section Storage Allocation Hooks). From now on all uses of these
functions are traced and protocolled into the file. There is now of
course a speed penalty for all calls to the traced functions so that the
tracing should not be enabled during their normal use.
This function is a GNU extension and generally not available on other
systems. The prototype can be found in `mcheck.h'.
- Function: void muntrace (void)
-
The
muntrace function can be called after mtrace was used
to enable tracing the malloc calls. If no (succesful) call of
mtrace was made muntrace does nothing.
Otherwise it deinstalls the handlers for malloc, realloc,
and free and then closes the protocol file. No calls are
protocolled anymore and the programs runs again with the full speed.
This function is a GNU extension and generally not available on other
systems. The prototype can be found in `mcheck.h'.
Even though the tracing functionality does not influence the runtime
behaviour of the program it is no wise idea to call mtrace in all
programs. Just imagine you debug a program using mtrace and all
other programs used in the debug sessions also trace their malloc
calls. The output file would be the same for all programs and so is
unusable. Therefore one should call mtrace only if compiled for
debugging. A program could therefore start like this:
#include <mcheck.h>
int
main (int argc, char *argv[])
{
#ifdef DEBUGGING
mtrace ();
#endif
...
}
This is all what is needed if you want to trace the calls during the
whole runtime of the program. Alternatively you can stop the tracing at
any time with a call to muntrace. It is even possible to restart
the tracing again with a new call to mtrace. But this can course
unreliable results since there are possibly calls of the functions which
are not called. Please note that not only the application uses the
traced functions, also libraries (including the C library itself) use
this function.
This last point is also why it is no good idea to call muntrace
before the program terminated. The libraries are informed about the
termination of the program only after the program returns from
main or calls exit and so cannot free the memory they use
before this time.
So the best thing one can do is to call mtrace as the very first
function in the program and never call muntrace. So the program
traces almost all uses of the malloc functions (except those
calls which are executed by constructors of the program or used
libraries).
You know the situation. The program is prepared for debugging and in
all debugging sessions it runs well. But once it is started without
debugging the error shows up. In our situation here: the memory leaks
becomes visible only when we just turned off the debugging. If you
foresee such situations you can still win. Simply use something
equivalent to the following little program:
#include <mcheck.h>
#include <signal.h>
static void
enable (int sig)
{
mtrace ();
signal (SIGUSR1, enable);
}
static void
disable (int sig)
{
muntrace ();
signal (SIGUSR2, disable);
}
int
main (int argc, char *argv[])
{
...
signal (SIGUSR1, enable);
signal (SIGUSR2, disable);
...
}
I.e., the user can start the memory debugger any time s/he wants if the
program was started with MALLOC_TRACE set in the environment.
The output will of course not show the allocations which happened before
the first signal but if there is a memory leak this will show up
nevertheless.
If you take a look at the output it will look similar to this:
= Start
[0x8048209] - 0x8064cc8
[0x8048209] - 0x8064ce0
[0x8048209] - 0x8064cf8
[0x80481eb] + 0x8064c48 0x14
[0x80481eb] + 0x8064c60 0x14
[0x80481eb] + 0x8064c78 0x14
[0x80481eb] + 0x8064c90 0x14
= End
What this all means is not really important since the trace file is not
meant to be read by a human. Therefore no attention is payed to good
readability. Instead there is a program which comes with the GNU C
library which interprets the traces and outputs a summary in on
user-friendly way. The program is called mtrace (it is in fact a
Perl script) and it takes one or two arguments. In any case the name of
the file with the trace output must be specified. If an optional argument
precedes the name of the trace file this must be the name of the program
which generated the trace.
drepper$ mtrace tst-mtrace log
No memory leaks.
In this case the program tst-mtrace was run and it produced a
trace file `log'. The message printed by mtrace shows there
are no problems with the code, all allocated memory was freed
afterwards.
If we call mtrace on the example trace given above we would get a
different outout:
drepper$ mtrace errlog
- 0x08064cc8 Free 2 was never alloc'd 0x8048209
- 0x08064ce0 Free 3 was never alloc'd 0x8048209
- 0x08064cf8 Free 4 was never alloc'd 0x8048209
Memory not freed:
-----------------
Address Size Caller
0x08064c48 0x14 at 0x80481eb
0x08064c60 0x14 at 0x80481eb
0x08064c78 0x14 at 0x80481eb
0x08064c90 0x14 at 0x80481eb
We have called mtrace with only one argument and so the script
has no chance to find out what is meant with the addresses given in the
trace. We can do better:
drepper$ mtrace tst-mtrace errlog
- 0x08064cc8 Free 2 was never alloc'd /home/drepper/tst-mtrace.c:39
- 0x08064ce0 Free 3 was never alloc'd /home/drepper/tst-mtrace.c:39
- 0x08064cf8 Free 4 was never alloc'd /home/drepper/tst-mtrace.c:39
Memory not freed:
-----------------
Address Size Caller
0x08064c48 0x14 at /home/drepper/tst-mtrace.c:33
0x08064c60 0x14 at /home/drepper/tst-mtrace.c:33
0x08064c78 0x14 at /home/drepper/tst-mtrace.c:33
0x08064c90 0x14 at /home/drepper/tst-mtrace.c:33
Suddenly the output makes much more sense and the user can see
immediately where the function calls causing the trouble can be found.
Interpreting this output is not complicated. There are at most two
different situations being detected. First, free was called for
pointers which were never returned by one of the allocation functions.
This is usually a very bad problem and how this looks like is shown in
the first three lines of the output. Situations like this are quite
rare and if they appear they show up very drastically: the program
normally crashes.
The other situation which is much harder to detect are memory leaks. As
you can see in the output the mtrace function collects all this
information and so can say that the program calls an allocation function
from line 33 in the source file `/home/drepper/tst-mtrace.c' four
times without freeing this memory before the program terminates.
Whether this is a real problem keeps to be investigated.
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'.
- Data Type: struct obstack
-
An obstack is represented by a data structure of type
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.
- Function: int obstack_init (struct obstack *obstack-ptr)
-
Initialize obstack obstack-ptr for allocation of objects. This
function calls the obstack's
obstack_chunk_alloc function. If
allocation of memory fails, the function pointed to by
obstack_alloc_failed_handler is called. The obstack_init
function always returns 1 (Compatibility notice: Former versions of
obstack returned 0 if allocation failed).
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);
- Variable: obstack_alloc_failed_handler
-
The value of this variable is a pointer to a function that
obstack uses when 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 exit
(see section Program Termination) or longjmp (see section Non-Local Exits) and doesn't return.
void my_obstack_alloc_failed (void)
...
obstack_alloc_failed_handler = &my_obstack_alloc_failed;
The most direct way to allocate an object in an obstack is with
obstack_alloc, which is invoked almost like malloc.
- Function: void * obstack_alloc (struct obstack *obstack-ptr, int size)
-
This allocates an uninitialized block of size bytes in an obstack
and returns its address. Here obstack-ptr specifies which obstack
to allocate the block in; it is the address of the
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 calls
obstack_alloc_failed_handler if allocation of memory by
obstack_chunk_alloc failed.
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:
- Function: void * obstack_copy (struct obstack *obstack-ptr, void *address, int size)
-
This allocates a block and initializes it by copying size
bytes of data starting at address. It calls
obstack_alloc_failed_handler if allocation of memory by
obstack_chunk_alloc failed.
- Function: void * obstack_copy0 (struct obstack *obstack-ptr, void *address, int size)
-
Like
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.
- Function: void obstack_free (struct obstack *obstack-ptr, void *object)
-
If object is a null pointer, everything allocated in the obstack
is freed. Otherwise, object must be the address of an object
allocated in the obstack. Then object is freed, along with
everything allocated in obstack since object.
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.
- Function: void obstack_blank (struct obstack *obstack-ptr, int size)
-
The most basic function for adding to a growing object is
obstack_blank, which adds space without initializing it.
- Function: void obstack_grow (struct obstack *obstack-ptr, void *data, int size)
-
To add a block of initialized space, use
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.
- Function: void obstack_grow0 (struct obstack *obstack-ptr, void *data, int size)
-
This is the growing-object analogue of
obstack_copy0. It adds
size bytes copied from data, followed by an additional null
character.
- Function: void obstack_1grow (struct obstack *obstack-ptr, char c)
-
To add one character at a time, use the function
obstack_1grow.
It adds a single byte containing c to the growing object.
- Function: void obstack_ptr_grow (struct obstack *obstack-ptr, void *data)
-
Adding the value of a pointer one can use the function
obstack_ptr_grow. It adds sizeof (void *) bytes
containing the value of data.
- Function: void obstack_int_grow (struct obstack *obstack-ptr, int data)
-
A single value of type
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.
- Function: void * obstack_finish (struct obstack *obstack-ptr)
-
When you are finished growing the object, use the function
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:
- Function: int obstack_object_size (struct obstack *obstack-ptr)
-
This function returns the current size of the growing object, in bytes.
Remember to call this function before finishing the object.
After it is finished,
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:
- Function: int obstack_room (struct obstack *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
obstack using the fast growth functions.
While you know there is room, you can use these fast growth functions
for adding data to a growing object:
- Function: void obstack_1grow_fast (struct obstack *obstack-ptr, char c)
-
The function
obstack_1grow_fast adds one byte containing the
character c to the growing object in obstack obstack-ptr.
- Function: void obstack_ptr_grow_fast (struct obstack *obstack-ptr, void *data)
-
The function
obstack_ptr_grow_fast adds sizeof (void *)
bytes containing the value of data to the growing object in
obstack obstack-ptr.
- Function: void obstack_int_grow_fast (struct obstack *obstack-ptr, int data)
-
The function
obstack_int_grow_fast adds sizeof (int) bytes
containing the value of data to the growing object in obstack
obstack-ptr.
- Function: void obstack_blank_fast (struct obstack *obstack-ptr, int size)
-
The function
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.
- Function: void * obstack_base (struct obstack *obstack-ptr)
-
This function returns the tentative address of the beginning of the
currently growing object in 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).
- Function: void * obstack_next_free (struct obstack *obstack-ptr)
-
This function returns the address of the first free byte in the current
chunk of obstack obstack-ptr. This is the end of the currently
growing object. If no object is growing,
obstack_next_free
returns the same value as obstack_base.
- Function: int obstack_object_size (struct obstack *obstack-ptr)
-
This function returns the size in bytes of the currently growing object.
This is equivalent to
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:
- Macro: int obstack_alignment_mask (struct obstack *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 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 malloc
#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.
- Macro: int obstack_chunk_size (struct obstack *obstack-ptr)
-
This returns the chunk size of the given obstack.
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)
-
Initialize use of an obstack. See section Creating Obstacks.
void *obstack_alloc (struct obstack *obstack-ptr, int size)
-
Allocate an object of size uninitialized bytes.
See section Allocation in an Obstack.
void *obstack_copy (struct obstack *obstack-ptr, void *address, int size)
-
Allocate an object of size bytes, with contents copied from
address. See section Allocation in an Obstack.
void *obstack_copy0 (struct obstack *obstack-ptr, void *address, int size)
-
Allocate an object of size+1 bytes, with size of them copied
from address, followed by a null character at the end.
See section Allocation in an Obstack.
void obstack_free (struct obstack *obstack-ptr, void *object)
-
Free object (and everything allocated in the specified obstack
more recently than object). See section Freeing Objects in an Obstack.
void obstack_blank (struct obstack *obstack-ptr, int size)
-
Add size uninitialized bytes to a growing object.
See section Growing Objects.
void obstack_grow (struct obstack *obstack-ptr, void *address, int size)
-
Add size bytes, copied from address, to a growing object.
See section Growing Objects.
void obstack_grow0 (struct obstack *obstack-ptr, void *address, int size)
-
Add size bytes, copied from address, to a growing object,
and then add another byte containing a null character. See section Growing Objects.
void obstack_1grow (struct obstack *obstack-ptr, char data-char)
-
Add one byte containing data-char to a growing object.
See section Growing Objects.
void *obstack_finish (struct obstack *obstack-ptr)
-
Finalize the object that is growing and return its permanent address.
See section Growing Objects.
int obstack_object_size (struct obstack *obstack-ptr)
-
Get the current size of the currently growing object. See section Growing Objects.
void obstack_blank_fast (struct obstack *obstack-ptr, int size)
-
Add size uninitialized bytes to a growing object without checking
that there is enough room. See section Extra Fast Growing Objects.
void obstack_1grow_fast (struct obstack *obstack-ptr, char data-char)
-
Add one byte containing data-char to a growing object without
checking that there is enough room. See section Extra Fast Growing Objects.
int obstack_room (struct obstack *obstack-ptr)
-
Get the amount of room now available for growing the current object.
See section Extra Fast Growing Objects.
int obstack_alignment_mask (struct obstack *obstack-ptr)
-
The mask used for aligning the beginning of an object. This is an
lvalue. See section Alignment of Data in Obstacks.
int obstack_chunk_size (struct obstack *obstack-ptr)
-
The size for allocating chunks. This is an lvalue. See section Obstack Chunks.
void *obstack_base (struct obstack *obstack-ptr)
-
Tentative starting address of the currently growing object.
See section Status of an Obstack.
void *obstack_next_free (struct obstack *obstack-ptr)
-
Address just after the end of the currently growing object.
See section Status of an Obstack.
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.
- Function: void * alloca (size_t size);
-
The return value of
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).
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.
Here are the reasons why alloca may be preferable to malloc:
-
Using
alloca wastes very little space and is very fast. (It is
open-coded by the GNU C compiler.)
-
Since
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.
-
Nonlocal exits done with
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.
These are the disadvantages of alloca in comparison with
malloc:
-
If you try to allocate more storage than the machine can provide, you
don't get a clean error message. Instead you get a fatal signal like
the one you would get from an infinite recursion; probably a
segmentation violation (see section Program Error Signals).
-
Some non-GNU systems fail to support
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:
-
A variable size array's space is freed at the end of the scope of the
name of the array. The space allocated with
alloca
remains until the end of the function.
-
It is possible to use
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.
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