C - While and IF statements - Trying to timeout after X time

I am having difficulties with the below code;

int i = 0;
int x = 0;
int ch ;
int n;

    while((i < sizeof(buffer) - 1) && (x < (TIMER_FREQ*30)))
    {
        //getkey_serial0 returns either a (int)character or 0 if nothing on                                            
        //UART0
        if((ch = getkey_serial0) == 0)
        {
             x++;    //Increment X as there is nothing being received.
        }
        else
        {
            if(ch == '\n')
            {
                 n++;
            }

            if(n < 8){    //Yes I can simplify this but for some reason
            }             //I only just noticed this :/ Anyway, it is
            else{         //just here to avoid saving info I don't need
                 buffer[i] = ch ;
                 i++;
            }
        }
    }

As the input it is reading in is the results of a wireless scan the number of entries scanned can vary greatly, and so I need to be able to avoid infinitely looping. Originally I just read up to 11 \n's but this was rubbish as I kept missing SSID's which I needed, so I decided I needed some sort of timer or method to help me break after X amount of time.

TIMER_FREQ is defined as 10.

Clearly I am doing something stupid so any suggestions or tips would be greatly appreciated. I generally prefer suggestions to help me try and think out the problem as opposed to fixed code posts :) I always seem to miss some开发者_开发知识库thing simple despite my best efforts!

Thanks

EDIT: I should mention, this is on an embedded system (ARM7)

You should have access to a general purpose timer interrupt -- commonly called sys_tick().

The general practice in such "bare metal" applications is to configure the interrupt to fire every n milliseconds (10 ms is frequently used on my Cortex M3). Then, have the ISR update a counter. You'll want to ensure the counter update is atomic, so use a 32-bit, properly-aligned variable. (I'm assuming your processor is 32-bit, I can't recall for certain). Then your "application" code can poll the elapsed time as needed.

BUT - this timer discussion might be moot. In my ARM9 applications, we tie an interrupt to the UART's receive buffer. The associated ISR captures the keystroke and then performs any buffer management. Is that an option for you?

Do you really mean:

if((ch = getkey_serial0) == 0) { ...

Or do you actually mean:

if((ch = getkey_serial0()) == 0) { ...

If the latter, this is why your program never returns zero as you are giving it a function pointer. Does your program have many warnings at build?

If you want to time things, look into time(). It will let you see the system's wall clock, so you can determine if too many seconds have elapsed.