warning: incompatible implicit declaration of built-in function 'malloc'

[Robert Dell <dellr@xxxxxxx>]

can somebody please explain why I'm getting this warning?
warning: incompatible implicit declaration of built-in function 'malloc'

char *returnval = 0;
long outputBytes = 0;
... outputBytes gets changed and tested to ensure it's valid (non zero) ...

the offending line is here.
returnval = (char *) malloc(outputBytes + 2);