how can I read input from terminal in a C program

Question

I have a circuit connected to a thermistor and using system(command) I can get it to read the value into the terminal but I can't get it to send the value properly to my C program.

I'm sending code to the terminal to run, and this code is returning a value in the terminal. I want to take this value into my C program

Here is my code

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>       //for Log

float Vo;                     //voltage out
float Ro = 5000;        //value of resistance at 25C in the thermistor
float R ;               //value of fixed resistor
float B = 3977;         //Beta constant
float T0 = 298.15;      //25 degrees C in Kelvin
//float logR2,  R2,T;
//float A = 1.281426510e-03, B = 2.368116050e-04, C = 0.9002008458e-07;  // Ste$


    int main(){
    
    char command[50];
    
    //get the Analog value
    sprintf(command, "cd /sys/bus/iio/devices/iio\\:device0 && cat in_voltage0_raw"$
    Vo = system(command);
    printf("Analog Reading: %d\n", Vo);
return 0;
}

An example output would be

3973

Analog Reading: 4319452

where is the 4319452 coming from? I want it to read 3973

After reading the comments I tried to use fopen in the following way

//#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>       //for Log

float Vo;                       //voltage out
float Ro = 5000;        //value of resistance at 25C in the thermistor
float R ;               //value of fixed resistor
float B = 3977;         //Beta constant
float T0 = 298.15;      //25 degrees C in Kelvin
//float logR2,  R2,T;
//float A = 1.281426510e-03, B = 2.368116050e-04, C = 0.9002008458e-07;  // Steinhart-Hart and Hart Coefficients



int main(){

FILE *fp;
char command[50];
int c;
//get the Analog value
//sprintf(command, "cd /sys/bus/iio/devices/iio\\:device0 && cat in_voltage0_raw" );
fp = fopen("/sys/bus/iio/devices/iio\\:device0/in_voltage0_raw","r");
while(1) {
      c = fgetc(fp);
      if( feof(fp) ) {
         break ;
      }
      printf("%c", c);
   }
   fclose(fp);
//Vo[i] = system(command);

printf("Analog Reading: %d\n", c);

return 0;
}

I get

Segmentation fault

after running the code

I edited the code in the following way

sprintf(command, "cd /sys/bus/iio/devices/iio:device0 && cat in_voltage0_raw"$

After I run it I get the following output

3 9 6 8

Analog Reading: ▒

It reads the output correctly once but in the second

printf("Analog Reading: %d\n", c);

it gives me

Analog Reading: ▒

Full code up to this point

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>       //for Log

int Vo;                 //voltage out
float Ro = 5000;        //value of resistance at 25C in the thermistor
float R ;               //value of fixed resistor
float B = 3977;         //Beta constant
float T0 = 298.15;      //25 degrees C in Kelvin
//float logR2,  R2,T;
//float A = 1.281426510e-03, B = 2.368116050e-04, C = 0.9002008458e-07;  // Steinhart-Hart and Hart Coefficients



int main(){

FILE *fp;
char command[50];
int c;
//get the Analog value
//sprintf(command, "cd /sys/bus/iio/devices/iio\\:device0 && cat in_voltage0_raw" );
fp = fopen("/sys/bus/iio/devices/iio:device0/in_voltage0_raw","r");
if ( fp == NULL ){
perror("Error: ");
return(-1);
}
while(1) {
      c = fgetc(fp);
      if( feof(fp) ) {
         break ;
      }
      printf("%c ", c);
//      Vo = c;
   }

   fclose(fp);
//Vo[i] = system(command);

printf("Analog Reading: %c\n", c);

return 0;

}

I solved the problem by taking every character read and putting it in an array

Here the working code

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>       //for Log

char Vo[10];                    //voltage out
float Ro = 5000;        //value of resistance at 25C in the thermistor
float R ;               //value of fixed resistor
float B = 3977;         //Beta constant
float T0 = 298.15;      //25 degrees C in Kelvin
//float logR2,  R2,T;
//float A = 1.281426510e-03, B = 2.368116050e-04, C = 0.9002008458e-07;  // Steinhart-Hart and Hart Coefficients



int main(){

FILE *fp;
char command[50];
int c;
//get the Analog value
//sprintf(command, "cd /sys/bus/iio/devices/iio\\:device0 && cat in_voltage0_raw" );
fp = fopen("/sys/bus/iio/devices/iio:device0/in_voltage0_raw","r");
if ( fp == NULL ){
perror("Error: ");
return(-1);
}
int i=0;
while(1) {
      c = fgetc(fp);
      if( feof(fp) ) {
         break ;
      }
      printf("%c ", c);
if (c != '\0'){
  Vo[i] = c;
++i;
}
//      Vo = c;
   }

   fclose(fp);
//Vo[i] = system(command);

printf("Analog Reading: %s\n", Vo);

return 0;
}

Output:

3 9 7 0

Analog Reading: 3970

Answer 1

It looks like you're assuming the output from the system function is your value; it is not. The return value from system() is the exit status of the shell and it is an int and not a float.

I'm not sure what that return code is but you are not casting correctly and I'm surprised you're not getting a compile Warning like:

t.c:22:36: warning: format specifies type 'int' but the argument has type 'float' [-Wformat]
    printf("Analog Reading: %d\n", Vo);
                            ~~     ^~
                            %f
1 warning generated.

Basically this means that you are taking an int and putting it not a float without a proper cast or conversion. Its not actually the data you want. You want the content of the file.

What you might want to do is to read /sys/bus/iio/devices/iio\\:device0/in_voltage0_raw directly as a string and then convert it to the appropriate numeric value.

Technically what you're trying to do is read the stdout from the shell through a return value which is not possible with the function you are using.