This is libc.info, produced by makeinfo version 4.0b from libc.texinfo.

INFO-DIR-SECTION GNU libraries
START-INFO-DIR-ENTRY
* Libc: (libc).                 C library.
END-INFO-DIR-ENTRY

   This file documents the GNU C library.

   This is Edition 0.10, last updated 2001-07-06, of `The GNU C Library
Reference Manual', for Version 2.2.x.

   Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001 Free
Software Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "GNU Lesser General Public License", the
Front-Cover texts being (a) (see below), and with the Back-Cover Texts
being (b) (see below).  A copy of the license is included in the section
entitled "GNU Free Documentation License".

   (a) The FSF's Front-Cover Text is:

   A GNU Manual

   (b) The FSF's Back-Cover Text is:

   You have freedom to copy and modify this GNU Manual, like GNU
software.  Copies published by the Free Software Foundation raise
funds for GNU development.


File: libc.info,  Node: Malloc Tunable Parameters,  Next: Heap Consistency Checking,  Prev: Aligned Memory Blocks,  Up: Unconstrained Allocation

Malloc Tunable Parameters
.........................

   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,
          releasable 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'.



File: libc.info,  Node: Heap Consistency Checking,  Next: Hooks for Malloc,  Prev: Malloc Tunable Parameters,  Up: Unconstrained Allocation

Heap Consistency Checking
.........................

   You can ask `malloc' to check the consistency of dynamic memory 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' (*note
     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.  Alternatively 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 protected 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.

   There is one problem with `MALLOC_CHECK_': in SUID or SGID binaries
it could possibly be exploited since diverging from the normal programs
behavior it now writes something to the standard error descriptor.
Therefore the use of `MALLOC_CHECK_' is disabled by default for SUID
and SGID binaries.  It can be enabled again by the system administrator
by adding a file `/etc/suid-debug' (the content is not important it
could be empty).

   So, what's the difference between using `MALLOC_CHECK_' and linking
with `-lmcheck'?  `MALLOC_CHECK_' is orthogonal 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.


File: libc.info,  Node: Hooks for Malloc,  Next: Statistics of Malloc,  Prev: Heap Consistency Checking,  Up: Unconstrained Allocation

Memory Allocation Hooks
.......................

   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 memory
allocation, for example.

   The hook variables are declared in `malloc.h'.

 - Variable: __malloc_hook
     The value of this variable is a pointer to the 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, const void *CALLER)

     The value of CALLER is the return address found on the stack when
     the `malloc' function was called.  This value allows you 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, const void *CALLER)

     The value of CALLER is the return address found on the stack when
     the `realloc' function was called.  This value allows you 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, const void *CALLER)

     The value of CALLER is the return address found on the stack when
     the `free' function was called.  This value allows you 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, const void *CALLER)

     The value of CALLER is the return address found on the stack when
     the `memalign' function was called.  This value allows you to
     trace the memory consumption of the program.

   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.

 - Variable: __malloc_initialize_hook
     The value of this variable is a pointer to a function that is
     called once when the malloc implementation is initialized.  This
     is a weak variable, so it can be overridden in the application
     with a definition like the following:

          void (*__MALLOC_INITIALIZE_HOOK) (void) = my_init_hook;

   An issue to look out for is the time at which the malloc hook
functions can be safely installed.  If the hook functions call the
malloc-related functions recursively, it is necessary that malloc has
already properly initialized itself at the time when `__malloc_hook'
etc. is assigned to.  On the other hand, if the hook functions provide a
complete malloc implementation of their own, it is vital that the hooks
are assigned to _before_ the very first `malloc' call has completed,
because otherwise a chunk obtained from the ordinary, un-hooked malloc
may later be handed to `__free_hook', for example.

   In both cases, the problem can be solved by setting up the hooks from
within a user-defined function pointed to by
`__malloc_initialize_hook'--then the hooks will be set up safely at the
right time.

   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.

     /* Prototypes for __malloc_hook, __free_hook */
     #include <malloc.h>
     
     /* Prototypes for our hooks.  */
     static void *my_init_hook (void);
     static void *my_malloc_hook (size_t, const void *);
     static void my_free_hook (void*, const void *);
     
     /* Override initializing hook from the C library. */
     void (*__malloc_initialize_hook) (void) = my_init_hook;
     
     static void
     my_init_hook (void)
     {
       old_malloc_hook = __malloc_hook;
       old_free_hook = __free_hook;
       __malloc_hook = my_malloc_hook;
       __free_hook = my_free_hook;
     }
     
     static void *
     my_malloc_hook (size_t size, const void *caller)
     {
       void *result;
       /* Restore all old hooks */
       __malloc_hook = old_malloc_hook;
       __free_hook = old_free_hook;
       /* Call recursively */
       result = malloc (size);
       /* Save underlying 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, const void *caller)
     {
       /* Restore all old hooks */
       __malloc_hook = old_malloc_hook;
       __free_hook = old_free_hook;
       /* Call recursively */
       free (ptr);
       /* Save underlying 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 ()
     {
       ...
     }

   The `mcheck' function (*note Heap Consistency Checking::) works by
installing such hooks.


File: libc.info,  Node: Statistics of Malloc,  Next: Summary of Malloc,  Prev: Hooks for Malloc,  Up: Unconstrained Allocation

Statistics for Memory Allocation with `malloc'
..............................................

   You can get information about dynamic memory 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
     memory 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 memory
          allocator internally gets chunks of memory from the operating
          system, and then carves them up to satisfy individual
          `malloc' requests; see *Note Efficiency and 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 releasable chunk that
          normally borders the end of the heap (i.e. the high end of
          the virtual address space's data segment).


 - Function: struct mallinfo mallinfo (void)
     This function returns information about the current dynamic memory
     usage in a structure of type `struct mallinfo'.


File: libc.info,  Node: Summary of Malloc,  Prev: Statistics of Malloc,  Up: Unconstrained Allocation

Summary of `malloc'-Related Functions
.....................................

   Here is a summary of the functions that work with `malloc':

`void *malloc (size_t SIZE)'
     Allocate a block of SIZE bytes.  *Note Basic Allocation::.

`void free (void *ADDR)'
     Free a block previously allocated by `malloc'.  *Note Freeing
     after 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.  *Note Changing Block
     Size::.

`void *calloc (size_t COUNT, size_t ELTSIZE)'
     Allocate a block of COUNT * ELTSIZE bytes using `malloc', and set
     its contents to zero.  *Note Allocating Cleared Space::.

`void *valloc (size_t SIZE)'
     Allocate a block of SIZE bytes, starting on a page boundary.
     *Note 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.  *Note Aligned Memory Blocks::.

`int mallopt (int PARAM, int VALUE)'
     Adjust a tunable parameter.  *Note 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.  *Note Heap Consistency Checking::.

`void *(*__malloc_hook) (size_t SIZE, const void *CALLER)'
     A pointer to a function that `malloc' uses whenever it is called.

`void *(*__realloc_hook) (void *PTR, size_t SIZE, const void *CALLER)'
     A pointer to a function that `realloc' uses whenever it is called.

`void (*__free_hook) (void *PTR, const void *CALLER)'
     A pointer to a function that `free' uses whenever it is called.

`void (*__memalign_hook) (size_t SIZE, size_t ALIGNMENT, const void *CALLER)'
     A pointer to a function that `memalign' uses whenever it is called.

`struct mallinfo mallinfo (void)'
     Return information about the current dynamic memory usage.  *Note
     Statistics of Malloc::.


File: libc.info,  Node: Allocation Debugging,  Next: Obstacks,  Prev: Unconstrained Allocation,  Up: Memory Allocation

Allocation Debugging
--------------------

   A 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 such leaks and obtain 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 for the program if the debugging mode is not enabled.

* Menu:

* Tracing malloc::               How to install the tracing functionality.
* Using the Memory Debugger::    Example programs excerpts.
* Tips for the Memory Debugger:: Some more or less clever ideas.
* Interpreting the traces::      What do all these lines mean?


File: libc.info,  Node: Tracing malloc,  Next: Using the Memory Debugger,  Up: Allocation Debugging

How to install the tracing functionality
........................................

 - 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 behavior of `malloc' etc.
     is not changed.  For obvious reasons this also happens if the
     application is installed with the SUID or SGID bit set.

     If the named file is successfully opened, `mtrace' installs special
     handlers for the functions `malloc', `realloc', and `free' (*note
     Hooks for Malloc::).  From then 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 tracing
     should not be enabled during 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 (successful) 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 program runs again at full speed.

     This function is a GNU extension and generally not available on
     other systems.  The prototype can be found in `mcheck.h'.


File: libc.info,  Node: Using the Memory Debugger,  Next: Tips for the Memory Debugger,  Prev: Tracing malloc,  Up: Allocation Debugging

Example program excerpts
........................

   Even though the tracing functionality does not influence the runtime
behavior of the program it is not a good idea to call `mtrace' in all
programs.  Just imagine that you debug a program using `mtrace' and all
other programs used in the debugging session also trace their `malloc'
calls.  The output file would be the same for all programs and thus 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 cause
unreliable results since there may be 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 these
functions.

   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).


File: libc.info,  Node: Tips for the Memory Debugger,  Next: Interpreting the traces,  Prev: Using the Memory Debugger,  Up: Allocation Debugging

Some more or less clever ideas
..............................

   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.  A typical example is a memory leak that
becomes visible only when we turn 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.


File: libc.info,  Node: Interpreting the traces,  Prev: Tips for the Memory Debugger,  Up: Allocation Debugging

Interpreting the traces
.......................

   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 given to
readability.  Instead there is a program which comes with the GNU C
library which interprets the traces and outputs a summary in an
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 errlog
     - 0x08064cc8 Free 2 was never alloc'd /home/drepper/tst.c:39
     - 0x08064ce0 Free 3 was never alloc'd /home/drepper/tst.c:39
     - 0x08064cf8 Free 4 was never alloc'd /home/drepper/tst.c:39
     
     Memory not freed:
     -----------------
        Address     Size     Caller
     0x08064c48     0x14  at /home/drepper/tst.c:33
     0x08064c60     0x14  at /home/drepper/tst.c:33
     0x08064c78     0x14  at /home/drepper/tst.c:33
     0x08064c90     0x14  at /home/drepper/tst.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 what 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 remains to be investigated.


File: libc.info,  Node: Obstacks,  Next: Variable Size Automatic,  Prev: Allocation Debugging,  Up: Memory Allocation

Obstacks
--------

   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.

* Menu:

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


File: libc.info,  Node: Creating Obstacks,  Next: Preparing for Obstacks,  Up: Obstacks

Creating Obstacks
.................

   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.


File: libc.info,  Node: Preparing for Obstacks,  Next: Allocation in an Obstack,  Prev: Creating Obstacks,  Up: Obstacks

Preparing for Using Obstacks
............................

   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' (*note Unconstrained Allocation::).  This is done with the
following pair of macro definitions:

     #define obstack_chunk_alloc xmalloc
     #define obstack_chunk_free free

Though the memory you get using obstacks really comes from `malloc',
using obstacks is faster because `malloc' is called less often, for
larger blocks of memory.  *Note 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' (*note Program
     Termination::) or `longjmp' (*note Non-Local Exits::) and doesn't
     return.

          void my_obstack_alloc_failed (void)
          ...
          obstack_alloc_failed_handler = &my_obstack_alloc_failed;



File: libc.info,  Node: Allocation in an Obstack,  Next: Freeing Obstack Objects,  Prev: Preparing for Obstacks,  Up: Obstacks

Allocation in an Obstack
........................

   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'
(*note Basic Allocation::).


File: libc.info,  Node: Freeing Obstack Objects,  Next: Obstack Functions,  Prev: Allocation in an Obstack,  Up: Obstacks

Freeing Objects in an Obstack
.............................

   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 memory 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 (*note Preparing for Obstacks::).  Then
other obstacks, or non-obstack allocation, can reuse the space of the
chunk.


File: libc.info,  Node: Obstack Functions,  Next: Growing Objects,  Prev: Freeing Obstack Objects,  Up: Obstacks

Obstack Functions and Macros
............................

   The interfaces for using obstacks may be defined either as functions
or as macros, depending on the compiler.  The obstack facility works
with all C compilers, including both 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.  *Note Macro Definitions::.

   *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.


File: libc.info,  Node: Growing Objects,  Next: Extra Fast Growing,  Prev: Obstack Functions,  Up: Obstacks

Growing Objects
...............

   Because memory in obstack chunks is used sequentially, it is
possible to build up an object step by step, adding one or more bytes
at a time to the end of the object.  With this technique, you do not
need to know how much data you will put in the object until you come to
the end of it.  We call this the technique of "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' (*note 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.


File: libc.info,  Node: Extra Fast Growing,  Next: Status of an Obstack,  Prev: Growing Objects,  Up: Obstacks

Extra Fast Growing Objects
..........................

   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++);
             }
         }
     }


File: libc.info,  Node: Status of an Obstack,  Next: Obstacks Data Alignment,  Prev: Extra Fast Growing,  Up: Obstacks

Status of an Obstack
....................

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

 - 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)


File: libc.info,  Node: Obstacks Data Alignment,  Next: Obstack Chunks,  Prev: Status of an Obstack,  Up: Obstacks

Alignment of Data in Obstacks
.............................

   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.

