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| <h5 class="subsubsection">3.2.5.2 Advantages of <code>alloca</code></h5> |
| |
| <p>Here are the reasons why <code>alloca</code> may be preferable to <code>malloc</code>: |
| |
| <ul> |
| <li>Using <code>alloca</code> wastes very little space and is very fast. (It is |
| open-coded by the GNU C compiler.) |
| |
| <li>Since <code>alloca</code> does not have separate pools for different sizes of |
| block, space used for any size block can be reused for any other size. |
| <code>alloca</code> does not cause memory fragmentation. |
| |
| <li><a name="index-longjmp-346"></a>Nonlocal exits done with <code>longjmp</code> (see <a href="Non_002dLocal-Exits.html#Non_002dLocal-Exits">Non-Local Exits</a>) |
| automatically free the space allocated with <code>alloca</code> when they exit |
| through the function that called <code>alloca</code>. This is the most |
| important reason to use <code>alloca</code>. |
| |
| <p>To illustrate this, suppose you have a function |
| <code>open_or_report_error</code> which returns a descriptor, like |
| <code>open</code>, if it succeeds, but does not return to its caller if it |
| fails. If the file cannot be opened, it prints an error message and |
| jumps out to the command level of your program using <code>longjmp</code>. |
| Let's change <code>open2</code> (see <a href="Alloca-Example.html#Alloca-Example">Alloca Example</a>) to use this |
| subroutine: |
| |
| <pre class="smallexample"> int |
| open2 (char *str1, char *str2, int flags, int mode) |
| { |
| char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1); |
| stpcpy (stpcpy (name, str1), str2); |
| return open_or_report_error (name, flags, mode); |
| } |
| </pre> |
| <p class="noindent">Because of the way <code>alloca</code> works, the memory it allocates is |
| freed even when an error occurs, with no special effort required. |
| |
| <p>By contrast, the previous definition of <code>open2</code> (which uses |
| <code>malloc</code> and <code>free</code>) would develop a memory leak if it were |
| changed in this way. Even if you are willing to make more changes to |
| fix it, there is no easy way to do so. |
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