Why does fseek work when you are writing (fwrite) to a binary file, but when reading (fread) it doesn't? - C

Question

basically my problem is that when I use the following program:

#include <stdlib.h>
#include <stdio.h>
#define SIZE 1000


int main() {
FILE *fp;
int r, i;

char fp_string[600] = "/Users/mac/Library/Mobile Documents/com~apple~CloudDocs/College/Program With Persistent Data/Lab 3/num1000.bin";

fp = fopen(fp_string, "rb+");

r = 11;


fseek(fp, 3 * sizeof(int), SEEK_SET);
fwrite(&r, sizeof(int), 1, fp);
fseek(fp, 10 * sizeof(int), SEEK_SET);
fwrite(&r, sizeof(int), 1, fp);


fclose(fp);
return 0;
}

It updates the binary file (which is 1000 integers) with the 3rd and 10th digits to be 11.

But when I do the following:

#include <stdlib.h>
#include <stdio.h>
#define SIZE 1000


int main() {
FILE *fp;
int r, i;

char fp_string[600] = "/Users/mac/Library/Mobile Documents/com~apple~CloudDocs/College/Program With Persistent Data/Lab 3/num1000.bin";

fp = fopen(fp_string, "rb+");

r = 11;

printf("\n\n Before making any changes:\n");

for (i = 0; i < SIZE; i++) {

    fseek(fp, i * sizeof(int), SEEK_SET);
    fread(&r, sizeof(int), 1, fp);
    printf("%d ", r);


}

fseek(fp, 3 * sizeof(int), SEEK_SET);
fwrite(&r, sizeof(int), 1, fp);
fseek(fp, 10 * sizeof(int), SEEK_SET);
fwrite(&r, sizeof(int), 1, fp);


printf("\n\n After making changes:\n");

fseek(fp, 0, SEEK_SET);
for (i = 0; i < SIZE; i++) {

    fread(&r, sizeof(int), 1, fp);
    printf("%d ", r);


}

fclose(fp);
return 0;
}

It doesn't change anything at all. Just in case you where wondering, to check if the first program worked what I did is:

1. I would run the program you have underneath this text to check the integers stored in the binary file.
2. I would run the program you have on top of this text (the second one I posted) to change the 3rd and 10th integer to 11.
3. I would run the program you have underneath to check that those integers were changed to 11.

That way it worked, but the first program doesn't seem to change anything, it shows the exact same numbers again.

#include <stdlib.h>
#include <stdio.h>
#define SIZE 1000

int main() {
FILE *fp;
int r, i;

char fp_string[600] = "/Users/mac/Library/Mobile Documents/com~apple~CloudDocs/College/Program With Persistent Data/Lab 3/num1000.bin";

fp = fopen(fp_string, "rb");

for (i=0;i<SIZE;i++) {

    fread(&r, sizeof(int), 1, fp);
    printf("%d ", r);

}

fclose(fp);
return 0;
}

Answer 1

The first for loop is reading every number in the file into r before printing them. At the end of the loop, r contains the last number in the file, and that gets written into the places where you seek.

Either use a different variable in the loop, or put the r = 11; assignment after the loop.