How to set ulimit -n from a golang program? - network-programming

Question

My purspose was to set ulimit -n from within a golang program so that I do not have to set it globally but restrict it within the program.
Found systemcalls setrlimit and get rlimit for the same. (http://linux.die.net/man/2/setrlimit)
But when I tried a sample program for the same I was getting an error saying invalid argument while setting the value.
package main
import (
"fmt"
"syscall"
)
func main() {
var rLimit syscall.Rlimit
err := syscall.Getrlimit(syscall.RLIMIT_NOFILE, &rLimit)
if err != nil {
fmt.Println("Error Getting Rlimit ", err)
}
fmt.Println(rLimit)
rLimit.Max = 999999
rLimit.Cur = 999999
err = syscall.Setrlimit(syscall.RLIMIT_NOFILE, &rLimit)
if err != nil {
fmt.Println("Error Setting Rlimit ", err)
}
err = syscall.Getrlimit(syscall.RLIMIT_NOFILE, &rLimit)
if err != nil {
fmt.Println("Error Getting Rlimit ", err)
}
fmt.Println("Rlimit Final", rLimit)
}
The output obtained was:
george#george-Not-Specified ~/work/odesk/progium/trial $ ./getRlimit
{4294963002032703 0}
Error Setting Rlimit invalid argument
Rlimit Final {4294963002032703 999999}
george#george-Not-Specified ~/work/odesk/progium/trial $ sudo ./getRlimit
[sudo] password for george:
{4294963002032703 0}
Error Setting Rlimit invalid argument
Rlimit Final {4294963002032703 999999}
george#george-Not-Specified ~/work/odesk/progium/trial $ uname -a
Linux george-Not-Specified 3.5.0-17-generic #28-Ubuntu SMP Tue Oct 9 19:32:08 UTC 2012 i686 i686 i686 GNU/Linux
george#george-Not-Specified ~/work/odesk/progium/trial $
So I was able to get the rlimit
Setting the limit failed and returned an error.
Even though it failed the MAX value got changed when I took the value again but CUR value remains the same.
Can this error be due to some issue with my kernel or is it a bad program? Where can I find more information and how to deal with an issue like this?
Update:
Works after the fix has been made.
george#george-Not-Specified ~/work/odesk/progium/trial $ go build getRlimit.go
george#george-Not-Specified ~/work/odesk/progium/trial $ ./getRlimit
{1024 4096}
Error Setting Rlimit operation not permitted
Rlimit Final {1024 4096}
george#george-Not-Specified ~/work/odesk/progium/trial $ sudo ./getRlimit
[sudo] password for george:
{1024 4096}
Rlimit Final {999999 999999}
george#george-Not-Specified ~/work/odesk/progium/trial $ uname -a
Linux george-Not-Specified 3.5.0-17-generic #28-Ubuntu SMP Tue Oct 9 19:32:08 UTC 2012 i686 i686 i686 GNU/Linux
george#george-Not-Specified ~/work/odesk/progium/trial $ go version
go version devel +7c42cfa28e24 Tue Jul 30 14:22:14 2013 +1000 linux/386

Answer 1

It works as expected.
setrlimit(2).
The soft limit is the value that the kernel enforces for the
corresponding resource. The hard limit acts as a ceiling for the soft
limit: an unprivileged process may only set its soft limit to a value
in the range from 0 up to the hard limit, and (irreversibly) lower its
hard limit. A privileged process (under Linux: one with the
CAP_SYS_RESOURCE capability) may make arbitrary changes to either
limit value.
rlimit.go:
package main
import (
"fmt"
"syscall"
)
func main() {
var rLimit syscall.Rlimit
err := syscall.Getrlimit(syscall.RLIMIT_NOFILE, &rLimit)
if err != nil {
fmt.Println("Error Getting Rlimit ", err)
}
fmt.Println(rLimit)
rLimit.Max = 999999
rLimit.Cur = 999999
err = syscall.Setrlimit(syscall.RLIMIT_NOFILE, &rLimit)
if err != nil {
fmt.Println("Error Setting Rlimit ", err)
}
err = syscall.Getrlimit(syscall.RLIMIT_NOFILE, &rLimit)
if err != nil {
fmt.Println("Error Getting Rlimit ", err)
}
fmt.Println("Rlimit Final", rLimit)
}
Output:
$ uname -a
Linux peterSO 3.8.0-26-generic #38-Ubuntu SMP Mon Jun 17 21:43:33 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux
$ go build rlimit.go
$ ./rlimit
{1024 4096}
Error Setting Rlimit operation not permitted
Rlimit Final {1024 4096}
$ sudo ./rlimit
[sudo] password for peterSO:
{1024 4096}
Rlimit Final {999999 999999}
UPDATE:
I successfully ran rlimit.go for linux/amd64, you failed for linux/386. There were a Go bugs in Getrlimit and Setrlimit for Linux 32-bit distributions. These bugs have been fixed.
Using the Go default branch tip (to include the bug fixes), run the following, and update your question with the results.
$ uname -a
Linux peterSO 3.8.0-26-generic #38-Ubuntu SMP Mon Jun 17 21:46:08 UTC 2013 i686 i686 i686 GNU/Linux
$ go version
go version devel +ba52f6399462 Thu Jul 25 09:56:06 2013 -0400 linux/386
$ ulimit -Sn
1024
$ ulimit -Hn
4096
$ go build rlimit.go
$ ./rlimit
{1024 4096}
Error Setting Rlimit operation not permitted
Rlimit Final {1024 4096}
$ sudo ./rlimit
[sudo] password for peterSO:
{1024 4096}
Rlimit Final {999999 999999}
$