euid , _open (C under Linux) for a newbie

hi

I have several questions . I have just started with C programming
Can someone expain what means euid(effective uid)
and I didn't find a place where i get information about the parameters of the _open() functions.
And my last problem is how to save structures in a file
with the open() function or with _open("file" , _O_BINARY)

thanks in advance

i can answer one of those for you.
euid: effective user id, how is this different from your user id? your euid can change, while your uid stays the same. for example, if you execute a program that is suid root, then you effectively assume the root id during the execution of that program. So, before you execute the program your uid is 34 lets say, and your euid is also 34; but once you run /bin/ping your euid will become root, while your uid stays at 34. once /bin/ping is done, your uid and euid will both be 34 again.