HI,
I am trying to add a new sysctl variable used for tcp.
So I defined the variable in sysctl.h, and in sysctl_net_ipv4.c i defined the variable in this way:

code:
{
.ctl_name = NET_IPV4_TCP_VAR,
.procname = "tcp_var",
.data = &sysctl_tcp_var,
.maxlen = sizeof(int),
.mode = 0644,
.proc_handler = &proc_dointvec
},

So when I am compiling my Kernel, i got the following error:

net/ipv4/sysctl_net_ipv4.c:193: error: 'NET_IPV4_TCP_VAR' undeclared here (not in a function)

what is problem here? Any idea?

You should check the #includes for both the header and the source. It's entirely possible that sysctl.h is just a wrapper for another header and that the source includes that other header directly. Also check the preprocessor conditionals in the header to make sure you didn't inadvertently put the #define within a block that doesn't get compiled.
Kevin Barry

edit: Did you modify sys/sysctl.h or linux/sysctl.h? Was it in the kernel source tree?

U r right, i ahould declare my variable in linux/sysctl.h
So kernel recompilation was successfull after i corrected my mistake.

But my new kernel variable is not working (error in my variable)
All what i did is declaring my vaiable at the end of the enum containing net_ipv4 variables such as sysctl_tcp_timestamps
Then i defined my var in sysctl_net_ipv4.c (code in first msg) and i used my var in other codes
I didnt find my varible in /etc/sysctl.conf and add to this other sysctl variables e.g. Tcp_timestamps are not working after recompilation ( they were working before)

Am i missing anything in defining my variable? What r the steps to declare sysctl varable?

Can the problem be that in sysctl.h we must give a number to the variable (in the enum) and sysctl_net_ipv4.c I must take in consideration the order to declare my var??

Thanks in advance, i hope i were enough clear in my explanation

I have to admit I've never added a sysctl; however, it might help if you post a userspace program that highlights the problem you're having.
Kevin Barry