Eina tmpstr is intended for being able to conveniently pass strings back to a calling parent without having to use single static buffers (which don't work with multiple threads or when returning multiple times as parameters to a single function.

The traditional way to "return" a string in C is either to provide a buffer as a parameter to return it in, return a pointer to a single static buffer, which has issues, or return a duplicated string. All cases are inconvenient and return special handling. This is intended to make this easier. Now you can do something like this:

 Eina_Tmpstr *my_homedir(void) {
   return eina_tmpstr_add(getenv("HOME"));
 }

 Eina_Tmpstr *my_tmpdir(void) {
   return eina_tmpstr_add(getenv("TMP"));
 }

 void my_movefile(Eina_Tmpstr *src, Eina_Tmpstr *dst) {
   rename(src, dst);
   eina_tmpstr_del(src);
   eina_tmpstr_del(dst);
 }

 char buf[500];
 my_movefile(my_homedir(), my_tmpdir());
 my_movefile("/tmp/file", "/tmp/newname");
 my_movefile(my_homedir(), "/var/tmp");
 snprintf(buf, sizeof(buf), "/tmp/%i.file", rand());
 my_movefile("/tmp.file", buf);

Notice that you can interchange standard C strings (static ones or even generated buffers) with tmpstrings. The Eina_Tmpstr type is merely a type marker letting you know that the function cleans up those strings after use, and it is totally interchangeable with const char.