AVR Freaks

I've checked and double checked.  All the pointers are volatile whenever they're in the list.  When I pull the objects out of the list, I cast away their volatile.  They include a volatile status flag to indicate free/inuse.

THe purpose of the ISR is to pass messages back to main to be processed in the main loop.  The serial data is stuffed into a string.  The string gets shoved into a list and then a callback is called.  Main can then parse that string at leisure, and return the string/list item setup to the pool with a simple function call.  I trimmed out some unnecesarry code to keep a "Free list", since the status flag is enough to ensure I can always find a free list node if one exists.

Static allocation is because the application is very memory constrained.  9x% RAM usage.  I need to start counting bits.  Most of this is output buffers :-S

Overall the project has sufficient RAM.  communication with the outside world being the largest offender.  The end of build report is:

Program Memory Usage : 17392 bytes   53.1 % Full
Data Memory Usage : 1783 bytes   87.1 % Full

With the data memory including: 

#define AERIAL_RECV_LINE_LEN 80
#define AERIAL_RECV_NUM_LINES 2
#define AERIAL_SEND_LINE_LEN 240
#define AERIAL_SEND_NUM_LINES 2

These are defined like so:

typedef struct ae_recv_line ae_recv_line;
struct ae_recv_line
{
    volatile char status;
    size_t len;
    char data[AERIAL_RECV_LINE_LEN];
    volatile ae_recv_line* next;
};

typedef struct ae_send_line ae_send_line;
struct ae_send_line
{
    volatile char status;
    size_t len;
    size_t idx;
    char data[AERIAL_SEND_LINE_LEN];
    volatile ae_send_line* next;
};

and these use up a significant portion of available RAM.  As for other functions throughout the project, they mainly pass pointers and indices around to globally defined state variables and structures.  memory copies are generally kept to a minimum, there is no heap usage.  No malloc or friends.  No recursion.  Maximum call depth hasn't been rigorously analyzed, but should run approximately 6-ish functions deep, the bottom levels of which are all defined static inline since I've plenty of program space to play with.  The big problem is the passing back of long status reports and data dumping of complex control structures.

That's why I'm dynamically selecting an available object into which to stuff a string for send or receive.  Anyway, last night I pretty much sorted everything out.  It's all working perfectly, and if my employer will let me, I'll release the full code of the serial library I had to write since nobody else has a good one out right now.