Often you do not know for certain how big a block you will ultimately need at the time you must begin to use the block. For example, the block might be a buffer that you use to hold a line being read from a file; no matter how long you make the buffer initially, you may encounter a line that is longer.
You can make the block longer by calling realloc
. This function
is declared in `stdlib.h'.
realloc
function changes the size of the block whose address is
ptr to be newsize.
Since the space after the end of the block may be in use, realloc
may find it necessary to copy the block to a new address where more free
space is available. The value of realloc
is the new address of the
block. If the block needs to be moved, realloc
copies the old
contents.
If you pass a null pointer for ptr, realloc
behaves just
like `malloc (newsize)'. This can be convenient, but beware
that older implementations (before ISO C) may not support this
behavior, and will probably crash when realloc
is passed a null
pointer.
Like malloc
, realloc
may return a null pointer if no
memory space is available to make the block bigger. When this happens,
the original block is untouched; it has not been modified or relocated.
In most cases it makes no difference what happens to the original block
when realloc
fails, because the application program cannot continue
when it is out of memory, and the only thing to do is to give a fatal error
message. Often it is convenient to write and use a subroutine,
conventionally called xrealloc
, that takes care of the error message
as xmalloc
does for malloc
:
void * xrealloc (void *ptr, size_t size) { register void *value = realloc (ptr, size); if (value == 0) fatal ("Virtual memory exhausted"); return value; }
You can also use realloc
to make a block smaller. The reason you
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.
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