This is libc.info, produced by makeinfo version 4.3 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, 2002
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 "Free Software Needs Free Documentation" and
"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: 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.


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

Obstack Chunks
..............

   Obstacks work by allocating space for themselves in large chunks, and
then parceling out space in the chunks to satisfy your requests.  Chunks
are normally 4096 bytes long unless you specify a different chunk size.
The chunk size includes 8 bytes of overhead that are not actually used
for storing objects.  Regardless of the specified size, longer chunks
will be allocated when necessary for long objects.

   The obstack library allocates chunks by calling the function
`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' (*note 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;


File: libc.info,  Node: Summary of Obstacks,  Prev: Obstack Chunks,  Up: Obstacks

Summary of Obstack Functions
............................

   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.  *Note Creating Obstacks::.

`void *obstack_alloc (struct obstack *OBSTACK-PTR, int SIZE)'
     Allocate an object of SIZE uninitialized bytes.  *Note 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.  *Note 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.  *Note
     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).  *Note Freeing Obstack Objects::.

`void obstack_blank (struct obstack *OBSTACK-PTR, int SIZE)'
     Add SIZE uninitialized bytes to a growing object.  *Note Growing
     Objects::.

`void obstack_grow (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)'
     Add SIZE bytes, copied from ADDRESS, to a growing object.  *Note
     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.  *Note Growing
     Objects::.

`void obstack_1grow (struct obstack *OBSTACK-PTR, char DATA-CHAR)'
     Add one byte containing DATA-CHAR to a growing object.  *Note
     Growing Objects::.

`void *obstack_finish (struct obstack *OBSTACK-PTR)'
     Finalize the object that is growing and return its permanent
     address.  *Note Growing Objects::.

`int obstack_object_size (struct obstack *OBSTACK-PTR)'
     Get the current size of the currently growing object.  *Note
     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.  *Note Extra Fast Growing::.

`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.  *Note Extra Fast Growing::.

`int obstack_room (struct obstack *OBSTACK-PTR)'
     Get the amount of room now available for growing the current
     object.  *Note Extra Fast Growing::.

`int obstack_alignment_mask (struct obstack *OBSTACK-PTR)'
     The mask used for aligning the beginning of an object.  This is an
     lvalue.  *Note Obstacks Data Alignment::.

`int obstack_chunk_size (struct obstack *OBSTACK-PTR)'
     The size for allocating chunks.  This is an lvalue.  *Note Obstack
     Chunks::.

`void *obstack_base (struct obstack *OBSTACK-PTR)'
     Tentative starting address of the currently growing object.  *Note
     Status of an Obstack::.

`void *obstack_next_free (struct obstack *OBSTACK-PTR)'
     Address just after the end of the currently growing object.  *Note
     Status of an Obstack::.


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

Automatic Storage with Variable Size
------------------------------------

   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 memory, 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)'.

* Menu:

* Alloca Example::              Example of using `alloca'.
* Advantages of Alloca::        Reasons to use `alloca'.
* Disadvantages of Alloca::     Reasons to avoid `alloca'.
* GNU C Variable-Size Arrays::  Only in GNU C, here is an alternative
				 method of allocating dynamically and
				 freeing automatically.


File: libc.info,  Node: Alloca Example,  Next: Advantages of Alloca,  Up: Variable Size Automatic

`alloca' Example
................

   As an example of the 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.


File: libc.info,  Node: Advantages of Alloca,  Next: Disadvantages of Alloca,  Prev: Alloca Example,  Up: Variable Size Automatic

Advantages of `alloca'
......................

   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 memory fragmentation.

   * Nonlocal exits done with `longjmp' (*note 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' (*note 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 memory 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 memory 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.


File: libc.info,  Node: Disadvantages of Alloca,  Next: GNU C Variable-Size Arrays,  Prev: Advantages of Alloca,  Up: Variable Size Automatic

Disadvantages of `alloca'
.........................

   These are the disadvantages of `alloca' in comparison with `malloc':

   * If you try to allocate more memory 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 (*note 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.


File: libc.info,  Node: GNU C Variable-Size Arrays,  Prev: Disadvantages of Alloca,  Up: Variable Size Automatic

GNU C Variable-Size Arrays
..........................

   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.


File: libc.info,  Node: Resizing the Data Segment,  Prev: Locking Pages,  Up: Memory

Resizing the Data Segment
=========================

   The symbols in this section are declared in `unistd.h'.

   You will not normally use the functions in this section, because the
functions described in *Note Memory Allocation:: are easier to use.
Those are interfaces to a GNU C Library memory allocator that uses the
functions below itself.  The functions below are simple interfaces to
system calls.

 - Function: int brk (void *ADDR)
     `brk' sets the high end of the calling process' data segment to
     ADDR.

     The address of the end of a segment is defined to be the address
     of the last byte in the segment plus 1.

     The function has no effect if ADDR is lower than the low end of
     the data segment.  (This is considered success, by the way).

     The function fails if it would cause the data segment to overlap
     another segment or exceed the process' data storage limit (*note
     Limits on Resources::).

     The function is named for a common historical case where data
     storage and the stack are in the same segment.  Data storage
     allocation grows upward from the bottom of the segment while the
     stack grows downward toward it from the top of the segment and the
     curtain between them is called the "break".

     The return value is zero on success.  On failure, the return value
     is `-1' and `errno' is set accordingly.  The following `errno'
     values are specific to this function:

    `ENOMEM'
          The request would cause the data segment to overlap another
          segment or exceed the process' data storage limit.


 - Function: int sbrk (ptrdiff_t DELTA)
     This function is the same as `brk' except that you specify the new
     end of the data segment as an offset DELTA from the current end
     and on success the return value is the address of the resulting
     end of the data segment instead of zero.

     This means you can use `sbrk(0)' to find out what the current end
     of the data segment is.



File: libc.info,  Node: Locking Pages,  Next: Resizing the Data Segment,  Prev: Memory Allocation,  Up: Memory

Locking Pages
=============

   You can tell the system to associate a particular virtual memory page
with a real page frame and keep it that way -- i.e. cause the page to
be paged in if it isn't already and mark it so it will never be paged
out and consequently will never cause a page fault.  This is called
"locking" a page.

   The functions in this chapter lock and unlock the calling process'
pages.

* Menu:

* Why Lock Pages::                Reasons to read this section.
* Locked Memory Details::         Everything you need to know locked
                                    memory
* Page Lock Functions::           Here's how to do it.


File: libc.info,  Node: Why Lock Pages,  Next: Locked Memory Details,  Up: Locking Pages

Why Lock Pages
--------------

   Because page faults cause paged out pages to be paged in
transparently, a process rarely needs to be concerned about locking
pages.  However, there are two reasons people sometimes are:

   * Speed.  A page fault is transparent only insofar as the process is
     not sensitive to how long it takes to do a simple memory access.
     Time-critical processes, especially realtime processes, may not be
     able to wait or may not be able to tolerate variance in execution
     speed.

     A process that needs to lock pages for this reason probably also
     needs priority among other processes for use of the CPU.  *Note
     Priority::.

     In some cases, the programmer knows better than the system's demand
     paging allocator which pages should remain in real memory to
     optimize system performance.  In this case, locking pages can help.

   * Privacy.  If you keep secrets in virtual memory and that virtual
     memory gets paged out, that increases the chance that the secrets
     will get out.  If a password gets written out to disk swap space,
     for example, it might still be there long after virtual and real
     memory have been wiped clean.


   Be aware that when you lock a page, that's one fewer page frame that
can be used to back other virtual memory (by the same or other
processes), which can mean more page faults, which means the system
runs more slowly.  In fact, if you lock enough memory, some programs
may not be able to run at all for lack of real memory.


File: libc.info,  Node: Locked Memory Details,  Next: Page Lock Functions,  Prev: Why Lock Pages,  Up: Locking Pages

Locked Memory Details
---------------------

   A memory lock is associated with a virtual page, not a real frame.
The paging rule is: If a frame backs at least one locked page, don't
page it out.

   Memory locks do not stack.  I.e. you can't lock a particular page
twice so that it has to be unlocked twice before it is truly unlocked.
It is either locked or it isn't.

   A memory lock persists until the process that owns the memory
explicitly unlocks it.  (But process termination and exec cause the
virtual memory to cease to exist, which you might say means it isn't
locked any more).

   Memory locks are not inherited by child processes.  (But note that
on a modern Unix system, immediately after a fork, the parent's and the
child's virtual address space are backed by the same real page frames,
so the child enjoys the parent's locks).  *Note Creating a Process::.

   Because of its ability to impact other processes, only the superuser
can lock a page.  Any process can unlock its own page.

   The system sets limits on the amount of memory a process can have
locked and the amount of real memory it can have dedicated to it.
*Note Limits on Resources::.

   In Linux, locked pages aren't as locked as you might think.  Two
virtual pages that are not shared memory can nonetheless be backed by
the same real frame.  The kernel does this in the name of efficiency
when it knows both virtual pages contain identical data, and does it
even if one or both of the virtual pages are locked.

   But when a process modifies one of those pages, the kernel must get
it a separate frame and fill it with the page's data.  This is known as
a "copy-on-write page fault".  It takes a small amount of time and in a
pathological case, getting that frame may require I/O.

   To make sure this doesn't happen to your program, don't just lock the
pages.  Write to them as well, unless you know you won't write to them
ever.  And to make sure you have pre-allocated frames for your stack,
enter a scope that declares a C automatic variable larger than the
maximum stack size you will need, set it to something, then return from
its scope.


File: libc.info,  Node: Page Lock Functions,  Prev: Locked Memory Details,  Up: Locking Pages

Functions To Lock And Unlock Pages
----------------------------------

   The symbols in this section are declared in `sys/mman.h'.  These
functions are defined by POSIX.1b, but their availability depends on
your kernel.  If your kernel doesn't allow these functions, they exist
but always fail.  They _are_ available with a Linux kernel.

   *Portability Note:* POSIX.1b requires that when the `mlock' and
`munlock' functions are available, the file `unistd.h' define the macro
`_POSIX_MEMLOCK_RANGE' and the file `limits.h' define the macro
`PAGESIZE' to be the size of a memory page in bytes.  It requires that
when the `mlockall' and `munlockall' functions are available, the
`unistd.h' file define the macro `_POSIX_MEMLOCK'.  The GNU C library
conforms to this requirement.

 - Function: int mlock (const void *ADDR, size_t LEN)
     `mlock' locks a range of the calling process' virtual pages.

     The range of memory starts at address ADDR and is LEN bytes long.
     Actually, since you must lock whole pages, it is the range of
     pages that include any part of the specified range.

     When the function returns successfully, each of those pages is
     backed by (connected to) a real frame (is resident) and is marked
     to stay that way.  This means the function may cause page-ins and
     have to wait for them.

     When the function fails, it does not affect the lock status of any
     pages.

     The return value is zero if the function succeeds.  Otherwise, it
     is `-1' and `errno' is set accordingly.  `errno' values specific
     to this function are:

    `ENOMEM'
             * At least some of the specified address range does not
               exist in the calling process' virtual address space.

             * The locking would cause the process to exceed its locked
               page limit.

    `EPERM'
          The calling process is not superuser.

    `EINVAL'
          LEN is not positive.

    `ENOSYS'
          The kernel does not provide `mlock' capability.

     You can lock _all_ a process' memory with `mlockall'.  You unlock
     memory with `munlock' or `munlockall'.

     To avoid all page faults in a C program, you have to use
     `mlockall', because some of the memory a program uses is hidden
     from the C code, e.g. the stack and automatic variables, and you
     wouldn't know what address to tell `mlock'.


 - Function: int munlock (const void *ADDR, size_t LEN)
     `mlock' unlocks a range of the calling process' virtual pages.

     `munlock' is the inverse of `mlock' and functions completely
     analogously to `mlock', except that there is no `EPERM' failure.


 - Function: int mlockall (int FLAGS)
     `mlockall' locks all the pages in a process' virtual memory address
     space, and/or any that are added to it in the future.  This
     includes the pages of the code, data and stack segment, as well as
     shared libraries, user space kernel data, shared memory, and
     memory mapped files.

     FLAGS is a string of single bit flags represented by the following
     macros.  They tell `mlockall' which of its functions you want.  All
     other bits must be zero.

    `MCL_CURRENT'
          Lock all pages which currently exist in the calling process'
          virtual address space.

    `MCL_FUTURE'
          Set a mode such that any pages added to the process' virtual
          address space in the future will be locked from birth.  This
          mode does not affect future address spaces owned by the same
          process so exec, which replaces a process' address space,
          wipes out `MCL_FUTURE'.  *Note Executing a File::.

     When the function returns successfully, and you specified
     `MCL_CURRENT', all of the process' pages are backed by (connected
     to) real frames (they are resident) and are marked to stay that
     way.  This means the function may cause page-ins and have to wait
     for them.

     When the process is in `MCL_FUTURE' mode because it successfully
     executed this function and specified `MCL_CURRENT', any system call
     by the process that requires space be added to its virtual address
     space fails with `errno' = `ENOMEM' if locking the additional space
     would cause the process to exceed its locked page limit.  In the
     case that the address space addition that can't be accommodated is
     stack expansion, the stack expansion fails and the kernel sends a
     `SIGSEGV' signal to the process.

     When the function fails, it does not affect the lock status of any
     pages or the future locking mode.

     The return value is zero if the function succeeds.  Otherwise, it
     is `-1' and `errno' is set accordingly.  `errno' values specific
     to this function are:

    `ENOMEM'
             * At least some of the specified address range does not
               exist in the calling process' virtual address space.

             * The locking would cause the process to exceed its locked
               page limit.

    `EPERM'
          The calling process is not superuser.

    `EINVAL'
          Undefined bits in FLAGS are not zero.

    `ENOSYS'
          The kernel does not provide `mlockall' capability.

     You can lock just specific pages with `mlock'.  You unlock pages
     with `munlockall' and `munlock'.


 - Function: int munlockall (void)
     `munlockall' unlocks every page in the calling process' virtual
     address space and turn off `MCL_FUTURE' future locking mode.

     The return value is zero if the function succeeds.  Otherwise, it
     is `-1' and `errno' is set accordingly.  The only way this
     function can fail is for generic reasons that all functions and
     system calls can fail, so there are no specific `errno' values.



File: libc.info,  Node: Character Handling,  Next: String and Array Utilities,  Prev: Memory,  Up: Top

Character Handling
******************

   Programs that work with characters and strings often need to
classify a character--is it alphabetic, is it a digit, is it
whitespace, and so on--and perform case conversion operations on
characters.  The functions in the header file `ctype.h' are provided
for this purpose.

   Since the choice of locale and character set can alter the
classifications of particular character codes, all of these functions
are affected by the current locale.  (More precisely, they are affected
by the locale currently selected for character classification--the
`LC_CTYPE' category; see *Note Locale Categories::.)

   The ISO C standard specifies two different sets of functions.  The
one set works on `char' type characters, the other one on `wchar_t'
wide characters (*note Extended Char Intro::).

* Menu:

* Classification of Characters::       Testing whether characters are
			                letters, digits, punctuation, etc.

* Case Conversion::                    Case mapping, and the like.
* Classification of Wide Characters::  Character class determination for
                                        wide characters.
* Using Wide Char Classes::            Notes on using the wide character
                                        classes.
* Wide Character Case Conversion::     Mapping of wide characters.


File: libc.info,  Node: Classification of Characters,  Next: Case Conversion,  Up: Character Handling

Classification of Characters
============================

   This section explains the library functions for classifying
characters.  For example, `isalpha' is the function to test for an
alphabetic character.  It takes one argument, the character to test,
and returns a nonzero integer if the character is alphabetic, and zero
otherwise.  You would use it like this:

     if (isalpha (c))
       printf ("The character `%c' is alphabetic.\n", c);

   Each of the functions in this section tests for membership in a
particular class of characters; each has a name starting with `is'.
Each of them takes one argument, which is a character to test, and
returns an `int' which is treated as a boolean value.  The character
argument is passed as an `int', and it may be the constant value `EOF'
instead of a real character.

   The attributes of any given character can vary between locales.
*Note Locales::, for more information on locales.

   These functions are declared in the header file `ctype.h'.

 - Function: int islower (int C)
     Returns true if C is a lower-case letter.  The letter need not be
     from the Latin alphabet, any alphabet representable is valid.

 - Function: int isupper (int C)
     Returns true if C is an upper-case letter.  The letter need not be
     from the Latin alphabet, any alphabet representable is valid.

 - Function: int isalpha (int C)
     Returns true if C is an alphabetic character (a letter).  If
     `islower' or `isupper' is true of a character, then `isalpha' is
     also true.

     In some locales, there may be additional characters for which
     `isalpha' is true--letters which are neither upper case nor lower
     case.  But in the standard `"C"' locale, there are no such
     additional characters.

 - Function: int isdigit (int C)
     Returns true if C is a decimal digit (`0' through `9').

 - Function: int isalnum (int C)
     Returns true if C is an alphanumeric character (a letter or
     number); in other words, if either `isalpha' or `isdigit' is true
     of a character, then `isalnum' is also true.

 - Function: int isxdigit (int C)
     Returns true if C is a hexadecimal digit.  Hexadecimal digits
     include the normal decimal digits `0' through `9' and the letters
     `A' through `F' and `a' through `f'.

 - Function: int ispunct (int C)
     Returns true if C is a punctuation character.  This means any
     printing character that is not alphanumeric or a space character.

 - Function: int isspace (int C)
     Returns true if C is a "whitespace" character.  In the standard
     `"C"' locale, `isspace' returns true for only the standard
     whitespace characters:

    `' ''
          space

    `'\f''
          formfeed

    `'\n''
          newline

    `'\r''
          carriage return

    `'\t''
          horizontal tab

    `'\v''
          vertical tab

 - Function: int isblank (int C)
     Returns true if C is a blank character; that is, a space or a tab.
     This function is a GNU extension.

 - Function: int isgraph (int C)
     Returns true if C is a graphic character; that is, a character
     that has a glyph associated with it.  The whitespace characters
     are not considered graphic.

 - Function: int isprint (int C)
     Returns true if C is a printing character.  Printing characters
     include all the graphic characters, plus the space (` ') character.

 - Function: int iscntrl (int C)
     Returns true if C is a control character (that is, a character that
     is not a printing character).

 - Function: int isascii (int C)
     Returns true if C is a 7-bit `unsigned char' value that fits into
     the US/UK ASCII character set.  This function is a BSD extension
     and is also an SVID extension.


File: libc.info,  Node: Case Conversion,  Next: Classification of Wide Characters,  Prev: Classification of Characters,  Up: Character Handling

Case Conversion
===============

   This section explains the library functions for performing
conversions such as case mappings on characters.  For example, `toupper'
converts any character to upper case if possible.  If the character
can't be converted, `toupper' returns it unchanged.

   These functions take one argument of type `int', which is the
character to convert, and return the converted character as an `int'.
If the conversion is not applicable to the argument given, the argument
is returned unchanged.

   *Compatibility Note:* In pre-ISO C dialects, instead of returning
the argument unchanged, these functions may fail when the argument is
not suitable for the conversion.  Thus for portability, you may need to
write `islower(c) ? toupper(c) : c' rather than just `toupper(c)'.

   These functions are declared in the header file `ctype.h'.

 - Function: int tolower (int C)
     If C is an upper-case letter, `tolower' returns the corresponding
     lower-case letter.  If C is not an upper-case letter, C is
     returned unchanged.

 - Function: int toupper (int C)
     If C is a lower-case letter, `toupper' returns the corresponding
     upper-case letter.  Otherwise C is returned unchanged.

 - Function: int toascii (int C)
     This function converts C to a 7-bit `unsigned char' value that
     fits into the US/UK ASCII character set, by clearing the high-order
     bits.  This function is a BSD extension and is also an SVID
     extension.

 - Function: int _tolower (int C)
     This is identical to `tolower', and is provided for compatibility
     with the SVID.  *Note SVID::.

 - Function: int _toupper (int C)
     This is identical to `toupper', and is provided for compatibility
     with the SVID.

