itoa is not ansi C standard and you should probably avoid it. Here are some roll-your-own implementations if you really want to use it anyway:
http://www.strudel.org.uk/itoa/
If you need in memory string formatting, a better option is to use snprintf. Working from your example:
#include <stdio.h> #include <stdlib.h> int main () { int i; char buffer [33]; printf ("Enter a number: "); scanf ("%d",&i); snprintf(buffer, sizeof(buffer), "%d", i); printf ("decimal: %s\n",buffer); return 0; }