Block I/O problems with reading float - C

Question

I have recently been trying to write a file writing program that saves inventory statistics of part number, the quantity and a price for the part. While writing to my binary file my scanf saves my prices, but when I read them in my next program, it comes out with a slew of meaningless numbers, that are not what I input. Compiler with write program:(* * is user input)

This program stores a business inventory.
Please enter item data (part number, quantity, price): *2, 3, 1.6*
Please enter item data (part number, quantity, price): *3, 1, 5.3*
Please enter item data (part number, quantity, price): *0*
Thank you. Inventory stored in file inventory.txt

Write Program Code

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

int main(int argc, int argv[])
{
int pnum=1, quantity;
float price;
FILE *fp1;

fp1 = fopen("inventory.txt", "wb+");
if(fp1 == NULL)
{
    printf("Can't open!\n");
    exit(EXIT_FAILURE);
}

printf("This program stores a business inventory.\n");
while(pnum != 0)
{
printf("Please enter item data (part number, quantity, price): ");
scanf("%d, %d, %f", &pnum, &quantity, &price);
printf("%d, %d, %f", pnum, quantity, price);
fwrite(&pnum, sizeof(int), 1, fp1);// Is there a way to combine these 3 fwrites into 1?
fwrite(&quantity, sizeof(int), 1, fp1);
fwrite(&price, sizeof(float), 1, fp1);
}
printf("Thank you. Inventory stored in file inventory.txt");
fclose(fp1);
return 0;


}

Compiler with read program (* * is user input)

Below are the items in your inventory.
Part#    Quantity        Item Price
2         3                1070386381?
3         1                1084856730?
0?        1                1084856730?

Read Program Code

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

int main()
{
int pnum, quantity;
float price;
FILE *fp1 = fopen("inventory.txt", "rb");
if(fp1 == NULL)
{
    printf("Can't open!");
    exit(EXIT_FAILURE);
}
printf("Below are the items in your inventory.\n");

printf("Part#\tQuantity\t Item Price\n");
while (fread(&pnum, sizeof(int), 1, fp1) == 1)//Is there a way to combine these 3 freads into 1 line of code?
{
    printf("%5d\t", pnum);
}
while (fread(&quantity, sizeof(int), 1, fp1) == 1)
{
    printf("%8d\t", quantity);
}
while (fread(&price, sizeof(float), 1, fp1) == 1)
{
    printf("$");
    printf("%9.2f\n", price);

}
fclose(fp1);
return 0;

}

As you may of seen, scanf is being scanf and must have to do with my float, but I haven't been able to figure out how to fix it, because without scanf nothing gets saved to my inventory.txt file (I didn't include the .txt file because it's binary), and for some reason when I type in 0 to break the loop, it saves the 0 in the file. If any other info is needed I can supply it, but I think I've supplied everything. Thank you for any help, and happy coding :)

Answer 1

• The zero is inserted into the array because your while loop only breaks after the fwrite calls. You can use break I believe to exit right after the scan.
• To combine the writes and reads you can use a struct, but notice that you should probably serialize and deserialize from/into exactly the same struct because of padding.
• I suspect that your freads into float yield strange results because of wrong write/read order or padding. Try reading and writing into a struct and see if that solves your issue. If that doesn't work try printing the hexadcimal value of your float with %x. Then compare to the expected result.

Consider the following code:

#include <stdio.h>
#include <fcntl.h> // for open
#include <unistd.h> // for close


typedef struct
{
    float price;
    int pnum;
    int quantity;
} shoppingItem;


void writeToFile(FILE *fp) {    
    shoppingItem input1 = {1.1,2,3};
    shoppingItem input2 = {3.1412,42,666};
    fwrite(&input1, sizeof(shoppingItem), 1, fp);
    fwrite(&input2, sizeof(shoppingItem), 1, fp);
    printf("%f %d %d\n", input1.price, input1.pnum, input1.quantity);
    printf("%f %d %d\n", input2.price, input2.pnum, input2.quantity);
}

void readFromFile(FILE *fp) {
    shoppingItem output1 = {0.0,0,0};
    shoppingItem output2 = {0.0,0,0};
    fread(&output1, sizeof(shoppingItem), 1, fp);
    fread(&output2, sizeof(shoppingItem), 1, fp);
    printf("%f %d %d\n", output1.price, output1.pnum, output1.quantity);
    printf("%f %d %d\n", output2.price, output2.pnum, output2.quantity);
}

int main()
{
    FILE *fp = fopen("inventory.txt", "wb+");
    if(fp == NULL)
    {
        printf("Can't open!\n");
        return 1;
    }

    writeToFile(fp);
    rewind(fp);
    readFromFile(fp);

    if (fclose(fp) == 0) {
        printf("Done\n");
        return 0;
    } else {
        printf("Error on closing file\n");
        return 1;
    }
}

Answer 2

The strange values you get in you "reading program" are completely valid but... They are encoded as IEEE-754 floating point value. If you encode 5.3 as IEEE-754, you'll get 0x40a9999a which is equal to 1084856730, so your readings are good and something's bad with outputting it.

Change your printf format from %9.2f to just %f. If that doesn't help, try to force casting of price: fread((void*)(&price), sizeof(float), 1, fp1).

For the future - avoid storing data in a format like that. If you would try to read that file on a machine with a different endianess, the result will be completely different.