sprintf returns random values upon unsuccessful conversion

After running the following code 6 times and for different values of number, I get :

number = 908
str = 908.000000
check = 10

number = 98.7
str = 98.699997
check = 9

number = 09870.123
str = 9870.123047
check = 11

number = how
str = 0.000000
check = 8

number = hi
str = 0.000000
check = 8

number = 0
str = 0.000000
check = 8

Why do I get random values for invalid floats instead of a negative number as specified on the man page? And how can I overcome this and properly check error?

Code:

---


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

int main()
{
    do
    {
        char str[20];
        float number;

              printf("Enter a number: ");
              scanf("%f",&number);

              int check = sprintf(str,"%f",number);

        //Check for error
        if (check<0)
        {
            printf("Error occured\n");
            exit(EXIT_FAILURE);
        }

        printf("You entered: %s\n",str);
     
        //Reveal check
        printf("sprintf returned %d\n",check);
     
        getch(); //halt

      }while (1);

      return 0;

}

---

Your quesiton is senseless.
The values you display are not "random".
And the code you display is not what generated the output.
In particular, you have an error that your compiler should've warned you about on line 24.

And sprintf doesn't return -1 for the kind of "error" you are talking about. For example, if you enter "hello" it will return 0, not -1. It returns the length of the string that it creates, which is 0 in that case. It returns -1 if some kind of "encoding error" occurs, which is not likely to happen in your case.

Sorry, I forgot that.

So can you answer me now?

Check the return value of scanf(). It gives you the number of successful conversions.

scanf? How is it related to sprintf?

It's not. You want to check for an input error, correct? Checking the return value of sprintf won't do that, but checking the return value of scanf will.

Quote:

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Originally Posted by SergeP

scanf? How is it related to sprintf?

---

If you read carefully the definition of printf() functions. All returns the number of printer characters. A negative value is returned only in case of "output error" (like using fprintf() and trying to write to an invalid stream). The only exception is snprintf() functions. They will return the number of characters that would be printed regardless of the size of the bufffer.

scanf() will return the number of successful convertions.

I see! It makes sense now.
Gracias!