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<h5 class="subsubsection">3.2.2.2 Examples of <code>malloc</code></h5>

<p>If no more space is available, <code>malloc</code> returns a null pointer. 
You should check the value of <em>every</em> call to <code>malloc</code>.  It is
useful to write a subroutine that calls <code>malloc</code> and reports an
error if the value is a null pointer, returning only if the value is
nonzero.  This function is conventionally called <code>xmalloc</code>.  Here
it is:

<pre class="smallexample">     void *
     xmalloc (size_t size)
     {
       register void *value = malloc (size);
       if (value == 0)
         fatal ("virtual memory exhausted");
       return value;
     }
</pre>
   <p>Here is a real example of using <code>malloc</code> (by way of <code>xmalloc</code>). 
The function <code>savestring</code> will copy a sequence of characters into
a newly allocated null-terminated string:

<pre class="smallexample">     char *
     savestring (const char *ptr, size_t len)
     {
       register char *value = (char *) xmalloc (len + 1);
       value[len] = '\0';
       return (char *) memcpy (value, ptr, len);
     }
</pre>
   <p>The block that <code>malloc</code> gives you is guaranteed to be aligned so
that it can hold any type of data.  In the GNU system, the address is
always a multiple of eight on most systems, and a multiple of 16 on
64-bit systems.  Only rarely is any higher boundary (such as a page
boundary) necessary; for those cases, use <code>memalign</code>,
<code>posix_memalign</code> or <code>valloc</code> (see <a href="Aligned-Memory-Blocks.html#Aligned-Memory-Blocks">Aligned Memory Blocks</a>).

   <p>Note that the memory located after the end of the block is likely to be
in use for something else; perhaps a block already allocated by another
call to <code>malloc</code>.  If you attempt to treat the block as longer than
you asked for it to be, you are liable to destroy the data that
<code>malloc</code> uses to keep track of its blocks, or you may destroy the
contents of another block.  If you have already allocated a block and
discover you want it to be bigger, use <code>realloc</code> (see <a href="Changing-Block-Size.html#Changing-Block-Size">Changing Block Size</a>).

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