GNU C Library (libc) Programming Guide - Alloca Example

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.