This is because of /proc
is not real file system so pid_max
writes are handled in a way you don't need any seek
. I even don't know if seeks are supported here.
Just to give you feeling of how different /proc
files are here is reference for pretty old but illustrative kernel bug specially related to pid_max
: https://bugzilla.kernel.org/show_bug.cgi?id=13090
This link should explain you even more details: T H E /proc F I L E S Y S T E M
And finally developerWorks article "Access the Linux kernel using the /proc filesystem" with step-by-step illustration of kernel module code which have /proc FS API. This looks like 100% what you need.