read_wattage_linux.c

/**
	Copyright (c) 2020 Nikola Pavlica
	
	Permission is hereby granted, free of charge, to any person obtaining a copy
	of this software and associated documentation files (the "Software"), to deal
	in the Software without restriction, including without limitation the rights
	to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
	copies of the Software, and to permit persons to whom the Software is
	furnished to do so, subject to the following conditions:
	
	The above copyright notice and this permission notice shall be included in all
	copies or substantial portions of the Software.
	
	THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
	IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
	FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
	AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
	LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
	OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
	SOFTWARE.
**/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#if !defined(__linux__)
	#error "Commit suicide pls!"
#endif

#define false 0
#define true 1

#define POWER_DIR "/sys/class/power_supply/"
#define CURR_STAT "/current_now"
#define VOLT_STAT "/voltage_now"

int main(int argc, char *argv[]) {
	// battery device
	char *battery = "BAT0";
	_Bool showWatts = false;

	unsigned int voltage, current;

	// process options
	for(int i = 1; i<argc; i++) {
		if(!strcmp("--battery", argv[i])) {
			battery = argv[i+1];
			i++;
		} else if(!strcmp("--help", argv[i])) {
			printf("Usage:\n"
				"\t--help - shows this\n"
				"\t--watts - shows the 'W' behind the value\n"
				"\t--battery [BAT0] - specify battery to measure from\n");
			return 0;
		} else if(!strcmp("--watts", argv[i])) {
			showWatts = true;
		} else {
			fprintf(stderr, "Invalid parameter %s\n"
				"For usage information try --help\n",
				argv[i]);
			return -1;
		}
	}

	// obligatory C memory managment
	const size_t strLen = sizeof(POWER_DIR) + sizeof(CURR_STAT) + sizeof(battery);

	// read voltage
	char *vS = (char*)malloc(strLen);	
	strcpy(vS, POWER_DIR);
	strcat(vS, battery);
	strcat(vS, VOLT_STAT);
	FILE *vF = fopen(vS, "r");
	if(vF == NULL) {
		fprintf(stderr, "Unable to read battery voltage!\n");
		return -1;
	}
	fscanf(vF, "%u", &voltage);

	// read current
	char *cS = (char*)malloc(strLen);	
	strcpy(cS, POWER_DIR);
	strcat(cS, battery);
	strcat(cS, CURR_STAT);
	FILE *cF = fopen(cS, "r");
	if(cF == NULL) {
		fprintf(stderr, "Unable to read battery current!\n");
		return -1;
	}
	fscanf(cF, "%u", &current);

	float power = current/1000000.0 * voltage/1000000.0;

	// show value
	printf("%.2f", power);
	if(showWatts) printf(" W");
	putchar('\n');

	// safely close everything
	fclose(vF);
	fclose(cF);
	free(vS);
	free(cS);
}