[Solved] C binary read and write file

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Okay, I don’t really understand what’s going on, but it seems that you cannot trust fseek with SEEK_CUR when using with read/write files in that case (I’m running Windows, and the standard functions are notoriously different from Linux that may be the issue)

EDIT: Andrew’s answer confirms my suspicions. My solution complies to what the standards recommend.

What I have done to workaround the problem is to manage file position myself and seek to that position instead of implicitly relying on current file position when calling fseek.

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

int main(void)
{
 FILE *f;

f = fopen("inputData.txt", "w+b");
if (!f) { perror("cannot create input"); exit(1); }

int n = 5;
int i;
for (i = 1; i <= n; ++i) {
    fwrite(&i, sizeof(int), 1, f);
}


int x;
int pos=0;
fseek(f, 0, SEEK_SET);
while (fread(&x, sizeof(int), 1, f) == 1) {
    if (fseek(f, pos, SEEK_SET)) {perror("cannot seek");exit(1);}
    pos += sizeof(int);
    printf("%d %lu\n", x, ftell(f));
    if (x % 2 == 0) {
        x = x * 2;
    } else {
        x = 3 * x;
    }
    if (fwrite(&x, sizeof(int), 1, f) != 1) {perror("cannot write");exit(1);}
    if (fseek(f, pos, SEEK_SET)) {perror("cannot seek");exit(1);}
}

fclose(f);
}

now the output of the program is (with current offset)

1 0
2 4
3 8
4 12
5 16

contents of the binary file is now (as expected on a little endian architecture):

03 00 00 00 04 00 00 00 09 00 00 00 08 00 00 00 0F 00 00 00

So this is a workaround but at least it works properly.

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solved C binary read and write file