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(libc.info)Changing Block Size

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Changing the Size of a Block
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   Often you do not know for certain how big a block you will
ultimately need at the time you must begin to use the block.  For
example, the block might be a buffer that you use to hold a line being
read from a file; no matter how long you make the buffer initially, you
may encounter a line that is longer.

   You can make the block longer by calling `realloc'.  This function
is declared in `stdlib.h'.

 - Function: void * realloc (void *PTR, size_t NEWSIZE)
     The `realloc' function changes the size of the block whose address
     is PTR to be NEWSIZE.

     Since the space after the end of the block may be in use, `realloc'
     may find it necessary to copy the block to a new address where
     more free space is available.  The value of `realloc' is the new
     address of the block.  If the block needs to be moved, `realloc'
     copies the old contents.

     If you pass a null pointer for PTR, `realloc' behaves just like
     `malloc (NEWSIZE)'.  This can be convenient, but beware that older
     implementations (before ISO C) may not support this behavior, and
     will probably crash when `realloc' is passed a null pointer.

   Like `malloc', `realloc' may return a null pointer if no memory
space is available to make the block bigger.  When this happens, the
original block is untouched; it has not been modified or relocated.

   In most cases it makes no difference what happens to the original
block when `realloc' fails, because the application program cannot
continue when it is out of memory, and the only thing to do is to give
a fatal error message.  Often it is convenient to write and use a
subroutine, conventionally called `xrealloc', that takes care of the
error message as `xmalloc' does for `malloc':

     void *
     xrealloc (void *ptr, size_t size)
     {
       register void *value = realloc (ptr, size);
       if (value == 0)
         fatal ("Virtual memory exhausted");
       return value;
     }

   You can also use `realloc' to make a block smaller.  The reason you
would do this is to avoid tying up a lot of memory space when only a
little is needed.  In several allocation implementations, making a
block smaller sometimes necessitates copying it, so it can fail if no
other space is available.

   If the new size you specify is the same as the old size, `realloc'
is guaranteed to change nothing and return the same address that you
gave.