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(libc.info)GNU C Variable-Size Arrays

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GNU C Variable-Size Arrays
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   In GNU C, you can replace most uses of `alloca' with an array of
variable size.  Here is how `open2' would look then:

     int open2 (char *str1, char *str2, int flags, int mode)
     {
       char name[strlen (str1) + strlen (str2) + 1];
       stpcpy (stpcpy (name, str1), str2);
       return open (name, flags, mode);
     }

   But `alloca' is not always equivalent to a variable-sized array, for
several reasons:

   * A variable size array's space is freed at the end of the scope of
     the name of the array.  The space allocated with `alloca' remains
     until the end of the function.

   * It is possible to use `alloca' within a loop, allocating an
     additional block on each iteration.  This is impossible with
     variable-sized arrays.

   *Note:* If you mix use of `alloca' and variable-sized arrays within
one function, exiting a scope in which a variable-sized array was
declared frees all blocks allocated with `alloca' during the execution
of that scope.