interflow of two sorted files (binary)

interflow.cpp

FILE* interflow(FILE* f1, FILE* f2)
{
    if (!f1 || !f2) return NULL;

    FILE *f3 = fopen("3","wb");

    fseek(f2,0,2);
    fseek(f1,0,2);
    fpos_t pos1 = 0, pos2;
    pos2 = ftell(f2) - sizeof(int);
    int i1, i2, exit1 = 0;
    fsetpos(f1,&pos1);
    fread(&i1, sizeof(int), 1, f1);
    fsetpos(f2, &pos2);
    fread(&i2, sizeof(int), 1, f2);
    while ( 1 )
    {
        while ( i1 < i2 )
        {
            fwrite(&i1, sizeof(int), 1, f3);
            if ( feof(f1)) {
                exit1 = 1;
                break;
            }
            fread(&i1, sizeof(int), 1, f1);
        }
        if ( exit1 ) break;
        while ( i2 < i1 )
        {
            fwrite(&i2, sizeof(int), 1, f3);
            if ( ftell(f2) == sizeof(int)) break;
            fseek(f2, -2*sizeof(int), 1);
            fread(&i2, sizeof(int), 1, f2);
        }
        if ( ftell(f2) == sizeof(int)) break;
    }

    if ( ftell(f2) == sizeof(int))
        while (1)
        {
            fwrite(&i1, sizeof(int), 1, f3);
            if (feof(f1)) return f3;
            fread(&i1, sizeof(int), 1, f1);
        }

    if (exit1)
        while (1) {
            fwrite(&i2, sizeof(int), 1, f3);
            if ( ftell(f2) == sizeof(int)) return f3;
            fseek(f2, -2*sizeof(int), 1);
            fread(&i2, sizeof(int), 1, f2);
        }
}