Replace

[jim mcnamara]

Somebody somewhere sometime somehow wants this algorithm [translation: I'm too tired to remember]

(please note: algorithm - no letter y; non-rhythmic in all ways)

// returns number of replacements, -1 for error
int replacech(char *t, unsigned char old, unsigned char new){
             char *buf;
             int i=0;
             if(t==NULL || old < 1 || new < 1) return (-1);
             while (strchr(t,old) != NULL){
                      buf = strchr(t,old);
                      *buf = new;
                      i++;
             }
             return i;
} 

[CornedBee]

Is this the solution or do yoú want help?
There is a failure in it:
You may not call a variable new (unless it's C, but then you can't use it in a C++ program.

Also, to make it faster, use this:

while ((buf =strchr(t,old)) != NULL){

// instead of

while (strchr(t,old) != NULL){
     buf = strchr...

because there is only one call to strchr.