ofstream writing extra zero bytes to file on Unix server

Question

Why does the following code write four bytes when run on my local Windows machine using Visual C++, yet writes 4 extra 0 bytes when I upload to my school's Unix server?

unsigned long temp = 1025;
ofstream file("test", ofstream::binary);
file.write((char*)&temp, sizeof(temp));

Here is the result using xxd locally: 0104 0000

Here is the result on the Unix server: 0104 0000 0000 0000

Is this a problem with what mode I am opening the file in?

Answer 1

long has implementation-defined size. Your Windows system uses a 32 bit (4 byte) long (even 64 bit Windows uses 32 bit long; otherwise it's rare to see on 64 bit OSes), while most if not all 64 bit UNIX-like systems I'm aware of have 64 bit (8 byte) longs. Since you're writing out sizeof(temp) bytes, and temp is unsigned long, you'll get different output sizes on different systems (you may also encounter byte order issues if you move from little endian to big endian machines).

If you need a consistent size, use the fixed width types from <cstdint>, e.g. uint32_t or uint64_t.