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  1. So, this is partly for me, and partly for others who need a refresher, or just do not know how. But I will be making several post here over time on how to write very simply code, to do one thing, or another. These, used in conjunction with a shell script could be very useful / flexible. After several long talks with many people, including some here on these very forums. I've decided that using C, to communicate with hardware, or hardware interfaces is best as can be for many situations. However, when you need to run several tools all at once, and have output formatted in some fashion, or easily modified. Shell scripts are very good at that sort of thing. Read from a real-time clock This post I will make about reading from a real-time clock. I spent hours messing around code related to I2C communications, and could never get exactly what I wanted. Plus, I wanted something that output date / time that looked very similar to the date Linux command. This could definitely been done using a shell script, but code size would probably be a lot larger. Additionally, a shell script would very likely be a lot slower, as with a script, one would have to be calling external cmdline tools to perform various operations. This example code is very fast, and prints to screen immediately after issuing the command. Since this command is very simple, and only prints the formatted date / time to screen. This could very easily be called from a shell script, and formatted further if need be. The real-time clock I'm using for this demonstration is a Maxim DS3232 real-time clock which is very accurate, and also very expensive compared to other real-time clocks. At $7 + US each, it's not cheap. I also had to write my own device tree overlay for this RTC, which strictly speaking is not necessary. One can set the device up from the command line manually as demonstrated for many different RTC's on the web. In fact, all the device tree overlay that I wrote does, is set all this automatically up at boot. As far as teh actual overlay it's self. All I did was modify an existing overlay from the "official" bb-overlays repo on github. To look something like this: /* * Copyright (C) 2015 Robert Nelson <> * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License version 2 as * published by the Free Software Foundation. */ /dts-v1/; /plugin/; #include <dt-bindings/board/am335x-bbw-bbb-base.h> #include <dt-bindings/gpio/gpio.h> #include <dt-bindings/pinctrl/am33xx.h> / { compatible = "ti,beaglebone", "ti,beaglebone-black", "ti,beaglebone-green"; /* identification */ part-number = "BB-RTC-01"; version = "00A0"; fragment@2 { target = <&i2c2>; __overlay__ { status = "okay"; /* shut up DTC warnings */ #address-cells = <1>; #size-cells = <0>; /* MCP79410 RTC module */ rtc@68 { compatible = "maxim,ds3232"; reg = <0x68>; }; }; }; }; On our cape, the RTC is on bus I2C-2, which is already enabled by default for capemgr. The rest of the above just means that status is okay(load the device ), the kernel module to load is called "ds3232", and the device address on the bus is 0x68. Now on to the actual C code for reading from /dev/rtc1: #include <stdio.h> #include <stdlib.h> #include <linux/rtc.h> #include <sys/ioctl.h> #include <sys/time.h> #include <sys/types.h> #include <fcntl.h> #include <unistd.h> #include <errno.h> void display_date_time(void) { struct rtc_time rtc_tm; int fd = open("/dev/rtc1", O_RDONLY); if(fd == -1){ perror("/dev/rtc"); exit(errno); } /* Read the RTC time/date */ int retval = ioctl(fd, RTC_RD_TIME, &rtc_tm); if (retval == -1) { perror("ioctl"); exit(errno); } int d = rtc_tm.tm_mday; int m = rtc_tm.tm_mon + 1; int y = rtc_tm.tm_year + 1900; const char *wdays[] = {"Sun","Mon","Tues","Wed","Thur","Fri","Sat"}; const char *mnths[] = {"Jan","Feb","Mar","Apr","May","June","July","Aug","Sept","Oct","Nov","Dec"}; int wday = (d += m < 3 ? y-- : y - 2, 23*m/9 + d + 4 + y/4- y/100 + y/400)%7; fprintf(stdout, "%s %s %02d %02d:%02d:%02d %d UTC\n", wdays[wday], mnths[rtc_tm.tm_mon], rtc_tm.tm_mday, rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec, y); } int main( int argc, char **argv ) { display_date_time(); return 0; } As one can see, most of this code is for formatting the output in a specific way. In this case, the output will look exactly like the output one might expect to see after issuing the command "date". However, this output is fixed to output the date / time in the UTC time zone. As for one of the projects I'm using this in is for devices spread out all over the US, in 3 different time zones, and we do not care so much what the local time zone of that system so much, as much as knowing a given time "standard". e.g. if something fails, and we need to tell a customer what failed, and what time it failed, we can, Then if we need to convert that time to their time zone, easy. Notice that the read() is handled by ioctl(). . . Output: root@wgd:~/# gcc -Wall -o read_rtc read_rtc.c root@wgd:~/# ./read_rtc Tues May 09 23:08:49 2017 UTC