3.2.5.4 GNU C Variable-Size Arrays

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.

NB: 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.