what is this statement //while (s && *s)send_char (*s++);

it would be easy to understand and explain it to if you can send the whole code with declaration of 's'

"while (s && *s)send_char (*s++);"

This statement simply says that send characters until you find identifier to terminate, to form a string.  here the identifier is null pointer, it means when all characters are displayed then it stops execution until it gets next character. it is the another example of character declaration similar to

char p[4] = "four"; 

or char *s = "four";

above both declarations will work same except that s++ is valid but p++ is not valid.

also in,

char p[4] = "four"; 

The "four" is stored in stack section of memory.

where as in,

char *s = "four"; 

The 's'  is stored in stack but "four" is stored in code section of memory.

Also,

"while (s && *s)send_char (*s++);"

and 

"while (s && *s){

send_char (*s++);"

} 

both statement are valid in C.

If there is one statement after condition then you can write a function just ahead of condition. 

e.g:

if(a=b) a++;

or if (a=b){

a++;

}

to tell you more clearly, just post your code.

If '&&' is logical-AND and '*s' is "not-s" then the logic 
is "never" and nothing happens.