Helpful code for using Microchip's TC74 I2C temperature sensors on the Raspberry Pi

TC74.c

/*!
 * @file
 * 
 * @brief Library for the TC47A* I2C temperature sensors
 * 
 * @author Blake Bourque
 * 
 * @date Mar 5, 2014
 * 
 * Provides readTempInC and readTempInF 
 * 
 */
 
#include <stdio.h> 	//*printf, pclose/popen
#include <string.h> //strcpy
#include <stdlib.h> //strtol

#include "TC74.h"

// Private functions
int exec(char *command, char *out, int out_len);
float c2f(float f);

// Public function documented in header
int readTempInC(int bus, TC74_t addy)
{
	#define C_LEN	20	//length of command
	#define B_LEN	3	//length of bus
	#define A_LEN	10	//length of addy
	#define PADD	6	//leave some extra space

	#define T_LEN	6	//Max length of temperature string

	char i2c_command[A_LEN+B_LEN+C_LEN+PADD];
	strncpy(i2c_command, "/usr/sbin/i2cget -y", 20);
	
	if (bus == 1) {
		strncat(i2c_command, " 1", B_LEN);
	}
	else if (bus == 0) {
		strncat(i2c_command, " 0", B_LEN);
	}
	else {
		fprintf(stderr, "Valid values for bus are [0,1] %d is invalid.\n", bus);
		return -1;
	}

	char addystr[A_LEN];
	int ret = snprintf(addystr, A_LEN, " 0x%x", addy);
	if (ret < 0) {
		fprintf(stderr, "Unable to generate command string.\n");
		return -2;
	}
	else if (ret > A_LEN) {
		fprintf(stderr, "%d characters were truncated.\n", ret-A_LEN);
	}
	strncat(i2c_command, addystr, A_LEN);
	//printf("%s\n", i2c_command);

	char temp[T_LEN];
	ret = exec(i2c_command, temp, T_LEN);
	if (ret < 0)
	{
		return -3;
	}
	//parse the temperature

	return strtol(temp, NULL, 16); //@todo This might not handle negative tempertatures well
}
// Public function documented in header
float readTempInF(int bus, TC74_t addy)
{
	int ret = readTempInC(bus, addy); //@todo what happens to negative return values
	return c2f(ret);
}

/**
 * Execute the command on the shell and give the first out_len characters from the
 * first line of output to the caller by placing them in the caller allocated buffer.
 * @param  command command to run on the shell
 * @param  out     a caller allocated buffer of out_len length
 * @param  out_len the max number of characters to return
 * @return         0 on success, < 0 on error
 */
int exec(char *command, char *out, int out_len)
{
	FILE *fp;

	//Open the command for reading.
	fp = popen(command, "r");
	if (fp == NULL) {
		fprintf(stderr, "Failed to run command %s\n", command);
		return -1;
	}

	// Read the output a line at a time - output it.
	// while (fgets(path, sizeof(path)-1, fp) != NULL) {
	// 	printf("%s", path);
	// }

	//read the first line and send it back to the caller
	char *ret = fgets(out, out_len-1, fp);
	if (ret == NULL)
	{
		fprintf(stderr, "Unable to read output from command %s\n", command);
		return -2;
	}

	//close
	pclose(fp);

	return 0;
}

/**
 * Convert Celsius to Fahrenheit
 * @param  c Celsius temperature
 * @return   Fahrenheit temperature
 */
float c2f(float c)
{
	float _temp = c*9.0;
	_temp /= 5;
	_temp += 32;
	return _temp;
}

TC74.h

/*!
 * @file
 * 
 * @brief Library for the TC47A* I2C temperature sensors
 * 
 * @author Blake Bourque
 * 
 * @date Mar 5, 2014
 * 
 * Provides readTempInC and readTempInF (header)
 * 
 */

#ifndef __TC74_h__
#define __TC74_h__

typedef enum {
	TC74A0 = 0x48,
	TC74A1 = 0x49,
	TC74A2 = 0x4a,
	TC74A3 = 0x4b,
	TC74A4 = 0x4c,
	TC74A5 = 0x4d,
	TC74A6 = 0x4e,
	TC74A7 = 0x4f
} TC74_t;

/**
 * Reads the temperature from the sensor at address on the given bus.
 * Accurate to plus or minus 2 deg C.
 * @param  bus  0 or 1: RPIv2 bus 1 is broken out on P1 header
 * @param  addy address of a TC74 sensor
 * @return      temperature in Celsius
 */
int readTempInC(int bus, TC74_t addy);

/**
 * Reads the temperature from the sensor at address on the given bus and converts
 * the temp to Fahrenheit.
 * @param  bus  0 or 1: RPIv2 bus 1 is broken out on P1 header
 * @param  addy address of a TC74 sensor
 * @return      temperature in Fahrenheit
 */
float readTempInF(int bus, TC74_t addy);


#endif //__TC74_h__