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| <h5 class="subsubsection">3.2.2.4 Changing the Size of a Block</h5> |
| |
| <p><a name="index-changing-the-size-of-a-block-_0028_0040code_007bmalloc_007d_0029-265"></a> |
| 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. |
| |
| <p>You can make the block longer by calling <code>realloc</code>. This function |
| is declared in <samp><span class="file">stdlib.h</span></samp>. |
| <a name="index-stdlib_002eh-266"></a> |
| <!-- malloc.h stdlib.h --> |
| <!-- ISO --> |
| |
| <div class="defun"> |
| — Function: void * <b>realloc</b> (<var>void *ptr, size_t newsize</var>)<var><a name="index-realloc-267"></a></var><br> |
| <blockquote><p>The <code>realloc</code> function changes the size of the block whose address is |
| <var>ptr</var> to be <var>newsize</var>. |
| |
| <p>Since the space after the end of the block may be in use, <code>realloc</code> |
| may find it necessary to copy the block to a new address where more free |
| space is available. The value of <code>realloc</code> is the new address of the |
| block. If the block needs to be moved, <code>realloc</code> copies the old |
| contents. |
| |
| <p>If you pass a null pointer for <var>ptr</var>, <code>realloc</code> behaves just |
| like ‘<samp><span class="samp">malloc (</span><var>newsize</var><span class="samp">)</span></samp>’. This can be convenient, but beware |
| that older implementations (before ISO C<!-- /@w -->) may not support this |
| behavior, and will probably crash when <code>realloc</code> is passed a null |
| pointer. |
| </p></blockquote></div> |
| |
| <p>Like <code>malloc</code>, <code>realloc</code> 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. |
| |
| <p>In most cases it makes no difference what happens to the original block |
| when <code>realloc</code> 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 <code>xrealloc</code>, that takes care of the error message |
| as <code>xmalloc</code> does for <code>malloc</code>: |
| |
| <pre class="smallexample"> void * |
| xrealloc (void *ptr, size_t size) |
| { |
| register void *value = realloc (ptr, size); |
| if (value == 0) |
| fatal ("Virtual memory exhausted"); |
| return value; |
| } |
| </pre> |
| <p>You can also use <code>realloc</code> 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. |
| <!-- The following is no longer true with the new malloc. --> |
| <!-- But it seems wise to keep the warning for other implementations. --> |
| In several allocation implementations, making a block smaller sometimes |
| necessitates copying it, so it can fail if no other space is available. |
| |
| <p>If the new size you specify is the same as the old size, <code>realloc</code> |
| is guaranteed to change nothing and return the same address that you gave. |
| |
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