From 608d693c89f76e242e5491bfa6317d9774637e9f Mon Sep 17 00:00:00 2001
From: Jonathan McCrohan <jmccrohan@gmail.com>
Date: Thu, 5 Jan 2012 02:11:13 +0000
Subject: Add comments

---
 src/modbuslog.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

(limited to 'src')

diff --git a/src/modbuslog.c b/src/modbuslog.c
index be7a4d1..9424936 100644
--- a/src/modbuslog.c
+++ b/src/modbuslog.c
@@ -42,10 +42,13 @@
 #define START_ADDRESS 0
 #define NUMBER_REGISTERS 14
 
+
+// handle SIGALRM by resetting it
 void minute_check(int signum) {
 	alarm(60);
 }
 
+// get mac address of primary interface eth0
 char *mac_address() {
 	int s;
 	struct ifreq ifr;
@@ -74,6 +77,8 @@ int main(int argc, char *argv[]) {
 
 	int DEBUG = 0;
 	int k;
+
+	// check the argv array for strings matching -d
 	for (k = 1; k < argc; k++) {
 		if (strcmp(argv[k], "-d") == 0) {
 			DEBUG = 1;
@@ -92,6 +97,8 @@ int main(int argc, char *argv[]) {
 
 	config_init(&cfg);
 
+	// attempt to read config
+	// loads entire file to memory and destroys file descriptor
 	if (!config_read_file(&cfg, configfile)) {
 		fprintf(stderr, "%s:%d - %s\n", config_error_file(&cfg),
 				config_error_line(&cfg), config_error_text(&cfg));
@@ -100,6 +107,7 @@ int main(int argc, char *argv[]) {
 		return -1;
 	}
 
+	// die if core config file options aren't there
 	if (!(config_lookup_string(&cfg, "modbus.device", &modbus_device_address)
 			&& config_lookup_int(&cfg, "modbus.baud", &modbus_baud_rate)
 			&& config_lookup_int(&cfg, "modbus.data_bits", &modbus_data_bits)
@@ -123,10 +131,14 @@ int main(int argc, char *argv[]) {
 	int rc;
 	int i;
 
+	// SIGALRM used to wake for loop up every 60 secs
+	// sleep puts whole thread to sleep which isn't what we want
+	// other methods involve CPU spinlocks which are inefficient
 	signal(SIGALRM, minute_check);
 	alarm(60);
 
 	for (;;) {
+		// block until SIGARLM
 		select(0, NULL, NULL, NULL, NULL);
 
 		time_t t = time(NULL);
@@ -138,9 +150,12 @@ int main(int argc, char *argv[]) {
 
 		readings = config_lookup(&cfg, "reading");
 
+		// find number of required readings
 		unsigned int num_readings = config_setting_length(readings);
 
 		int i;
+
+		// cycle through each reading and pull info from config file
 		for (i = 0; i < num_readings; ++i) {
 			config_setting_t *register_element = config_setting_get_elem(
 					readings, i);
@@ -176,10 +191,12 @@ int main(int argc, char *argv[]) {
 			else
 				intervalduration = intervalvalue * 60;
 
+			// if we match the required time for the reading
 			if (unixtime_min % intervalduration == 0) {
 				//printf("specified minute.  %d %d\n", unixtime_min,
 				//		intervalduration);
 
+				// attempt to create new modbus connection
 				ctx = modbus_new_rtu(modbus_device_address, modbus_baud_rate,
 						modbus_parity[0], modbus_data_bits, modbus_stop_bits);
 
@@ -199,6 +216,8 @@ int main(int argc, char *argv[]) {
 				}
 
 				int retry = 0;
+
+				// handle timeouts and retries
 				do {
 					switch (registertype) {
 
@@ -227,13 +246,14 @@ int main(int argc, char *argv[]) {
 				//MODBUS_GET_LOW_BYTE(data);
 				//MODBUS_SET_INT16_TO_INT8(tab_int8, index, value);
 
-				//<mac address>_YYYY_MM_DD_HH_MM_SS.log
+				// round forward to next midnight
 				time_t unixtime_day = ((((int) t) / 86400 * 86400) + 86400);
 
 				struct tm midnight = *localtime(&unixtime_day);
 
 				char filename[50];
 
+				//<mac address>_YYYY_MM_DD_HH_MM_SS.log
 				sprintf(filename,
 						"/var/modbuslog/%s_%04i_%02i_%02i_%02i_%02i_%02i.log",
 						mac_address(), midnight.tm_year + 1900,
@@ -259,11 +279,14 @@ int main(int argc, char *argv[]) {
 				int16_t registervalue = 0;
 
 				int p;
+
+				// data arrives in words, split into bytes
 				for (p = 0; p < numregisters; p++) {
 					MODBUS_SET_INT16_TO_INT8(byte, p * 2, tab_reg[p]);
 					//registervalue += tab_reg[i] + tab_reg[i + 1];
 				}
 
+				// bitshifting magic
 				switch (numregisters) {
 				case 1:
 					registervalue = byte[0];
-- 
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