It is currently a flaw in many library implementations (and, IMHO, the specification itself).
There are supposed to be two positions associated with a file:
• the “get” position (for reading)
• the “put” position (for writing)
They are supposed to be independent and separately modifiable. They might not be.
Also, they are useless if you did not open the file in “binary” mode, which is another dumb flaw in the standard library.
Use seekp() to position the file position before writing.
Use seekg() to position the file position before reading.
If you use one, don’t for get to use the other.
Make sure to open in binary mode.