Make function to read Temperature from DS18B20 and show on LCD - c

Question

I want to create a new function that will return me a actual temperature from DS18B20 as a float variable. I need that kind of variable to place a string on LCD.
Already I'm using function to read temperature like that:
int8_t readTemp(struct ds18b20 *d) {
struct ds18b20 *newTemp;
char tempAct[5];
while (d->next != NULL)
{
d = d->next;
int fd = open(d->devPath, O_RDONLY);
if (fd == -1)
{
perror("Couldn't open the w1 device.");
return 1;
}
char buf[256];
ssize_t numRead;
while ((numRead = read(fd, buf, 256)) > 0)
{
newTemp = malloc(sizeof(struct ds18b20));
strncpy(newTemp->tempData, strstr(buf, "t=") + 2, 5);
//float tempC = strtof(d->tempData, NULL);
sprintf(tempAct, "%s C", newTemp->tempData);
//printf("Device: %s - ", d->devID);
//printf("Temp: %.3f C ", tempC / 1000);
//printf("%.3f F\n\n", (tempC / 1000) * 9 / 5 + 32);
}
close(fd);
}
return 1;}
I have problem with sprintf from this line:
sprintf(tempAct, "%s C", newTemp->tempData);
In the main code:
int main(void) {
struct ds18b20 *rootNode;
struct ds18b20 *devNode;
struct ds18b20 *getTemp;
// Load pin configuration. Ignore error if already loaded
system("echo w1 > /sys/devices/bone_capemgr.9/slots>/dev/null");
while (1) {
rootNode = malloc(sizeof(struct ds18b20));
devNode = rootNode;
getTemp = rootNode;
int8_t devCnt = findDevices(devNode);
printf("\n Found %d devices\n\n", devCnt);
int8_t tempAct = readTemp(getTemp);
printf("\n Temp Act: %d \n\n", tempAct);
//readTemp(rootNode);
// Free linked list memory
while (rootNode) {
// Start with current value of root node
devNode = rootNode;
// Save address of next devNode to rootNode before deleting current devNode
rootNode = devNode->next;
// Free current devNode.
free(devNode);
}
free(rootNode);
}
return 0;}
I'm trying to recreate the functions from finddevices:
int8_t devCnt = findDevices(devNode);
printf("\n Found %d devices\n\n", devCnt);
int8_t tempAct = readTemp(getTemp);
printf("\n Temp Act: %d \n\n", tempAct);
But the tempData* isn't imported from readTemp fuction to main code.

Answer 1

The code is littered with issues that are not explicitly part of your question. Just some:
readTemp() does not return temperature. In fact it returns 1 if it fails and 1 if it succeeds. That is probably an error.
readTemp() appears to read all available temperature sensors, passed to it as a linked list of struct ds18b20 not just one. But actually skips the root node and accesses the second and subsequent without checking it is valid. If the root node contains no device (just a pointer to the first device), then you need only pass it pass rootNode->next instead.
It is not clear why it dynamically allocates a new struct ds18b20 or why it fails to later de-allocate. That is a serious memory leak.
At a guess the following is more likely to be close to what you need (without knowing anything about struct ds18b20 or what the expected output of the sensor is - just going by the evidence in the code (and commented out code) you have posted - so some bold assumptions have been made.
int readTemp( struct ds18b20 *d, // List of sensors
float* temperatures, // Pointer to array to receive temperatures
int8_t max_temperatures ) // Max number of temperatures to receive
{
struct ds18b20* sensor = d ;
int count = 0 ;
while( sensor != NULL && count < max_temperatures )
{
int fd = open( sensor->devPath, O_RDONLY ) ;
if( fd >= 0 )
{
if( read( fd, buf, sizeof(buf) ) > 0 )
{
char buf[256];
char* temp_str = strstr(buf, "t=") + 2 ;
sscanf( temp_str, "%f", &temperatures[count] ) ;
temperatures[count] /= 1000 ;
count++ ;
}
close( fd ) ;
sensor = sensor->next ;
}
else
{
perror("Couldn't open the w1 device.");
}
}
return count ;
}
Then in you can call it thus:
int8_t devCount = findDevices( rootNode ) ;
float* temperatures = malloc( devCnt * sizeof(float) ) ;
int8_t tempCount = readTemp( rootNode->next, temperatures, devCount ) ;
for( int i = 0; i < tempCount; i++ )
{
printf( "Temp Act: %f\n", temperatures[i] ) ;
}
free( temperatures ) ;
If you know there is only one device or you only need print the first, then this can be simplified:
int8_t devCount = findDevices( rootNode ) ;
if( devCount > 0 )
{
float temperature = 0f ;
int8_t tempCount = readTemp( rootNode->next, temperature, 1 ) ;
if( tempCount > 0 )
{
printf( "Temp Act: %f\n", temperature ) ;
}
}
Issues in your main() are also varied, including:
The root node is needlessly dynamically allocated
several aliases to the root node are needlessly created
The device list is repeatedly re-enumerated and the device list allocated and free'd on every iteration of a non-terminating loop. Assuming the number of sensors does not change while the code is running, this is unnecessary.
The loop never ends - even in the event of an error.
A better implementation might look something like:
int main(void)
{
// Load pin configuration. Ignore error if already loaded
system("echo w1 > /sys/devices/bone_capemgr.9/slots>/dev/null");
struct ds18b20 rootNode = { 0 } ;
int8_t devCount = findDevices( rootNode ) ;
if( devCount > 0)
{
struct ds18b20* devlist = rootNode->next ;
float* temperatures = malloc( devCount * sizeof(float) ) ;
int8_t tempCount = 0 ;
do
{
tempCount = readTemp( devList, temperatures, devCount ) ;
for( int i = 0; i < tempCount; i++ )
{
printf( "Temp Act: %f\n", temperatures[i] ) ;
}
} while (tempCount > 0 ) ;
// Free resources after a temperature read fails
free( temperatures ) ;
// Free linked list memory
while( devlist != NULL )
{
// Save address of next before deleting current
struct ds18b20* next = devlist->next ;
// Free current devNode.
free( devlist ) ;
// get next
devlist = next ;
}
}
return 0 ;
}