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3.2.5.1 alloca Example

As an example of the use of alloca, here is a function that opens a file name made from concatenating two argument strings, and returns a file descriptor or minus one signifying failure: 

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 (name, flags, mode);
}

Here is how you would get the same results with malloc and free: 

int
open2 (char *str1, char *str2, int flags, int mode)
{
  char *name = (char *) malloc (strlen (str1) + strlen (str2) + 1);
  int desc;
  if (name == 0)
    fatal ("virtual memory exceeded");
  stpcpy (stpcpy (name, str1), str2);
  desc = open (name, flags, mode);
  free (name);
  return desc;
}

As you can see, it is simpler with alloca. But alloca has other, more important advantages, and some disadvantages.