#include <stdio.h>

int main(int argc, char *argv[])
{
    FILE *file=fopen("test.dat","r+");
        int dat;

    if (!file)
    {
        printf("Failed to open file for RW\n");
        return 1;
    }

    if (fread(&dat,4,1,file)<1)
        {
                printf("Failed to read 4 bytes from file\n");
                fclose(file);
                return 2;
        }

        if (fwrite(&dat,4,1,file)<1)
        {
                printf("Failed to write 4 bytes to file\n");
                fclose(file);
                return 3;
        }

        fclose(file);
        return 0;
}

quote:

Reads and writes cannot be arbitrarily intermixed on read/write streams.
ANSI C requires that a file positioning function intervene between output
and input, unless an input operation encounters end-of-file.

quote:

Reads and writes may be intermixed on read/write streams in any order, and
do not require an intermediate seek as in previous versions of stdio. This
is not portable to other systems, however; ANSI C requires that a file
positioning function intervene between output and input, unless an input
operation encounters end-of-file.

quote:

ORIGINAL: Rodney

I see this behavior with Interix 3.5. But at the same time I see this behavior on an older version of OpenBSD.