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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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