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.