malloc’ing strcat

["Alexandre François Garreau" <galex-713@galex-713.eu>]

Hi,

strcat needs a buffer already wide enough to contain concatenation of both 
strings, hence I deduce idiomatic use is to first malloc(strlen(str1) + 
strlen(str2) + 1)…

But then the usage must always be something like:
#define strcat(a, b) strcat (strcpy (malloc (strlen(a) + strlen (b) + 1) 
a), b)
(which, of course, requires further free())…

So is there already a function or macro which does that? if not so, why? 
asprintf&cie already exist, after all…