Writing and reading to linux /proc/... filesystem without lseek()

Question

In this source code http://man7.org/tlpi/code/online/dist/sysinfo/procfs_pidmax.c.html the file /proc/sys/kernel/pid_max is first simply read (using the read syscall) and then simply written (using the write syscall).

Why is it no necessary to lseek to the beginning before writing? I thought the file-offset pointer is the same for read's and write's (that's what the author of the associated books says).

Answer 1

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.

Answer 2

I've looked at kernel source, files under /proc/sys/ is under sysctl table control, read/write callbacks for each entry support file offset. "pid_max entry" has one int value to operate and, hence, offset in those callbacks actually is not using.