Linux / Unix Command: sysctl

Command Library

NAME

sysctl - read/write system parameters  

SYNOPSIS

#include <unistd.h>

#include <linux/unistd.h>

#include <linux/sysctl.h>

_syscall1(int, _sysctl, struct __sysctl_args *, args);

int _sysctl(struct __sysctl_args *args);  

DESCRIPTION

The _sysctl call reads and/or writes kernel parameters. For example, the hostname, or the maximum number of open files. The argument has the form

struct __sysctl_args {
        int *name;        /* integer vector describing variable */
        int nlen;         /* length of this vector */
        void *oldval;     /* 0 or address where to store old value */
        size_t *oldlenp;  /* available room for old value,
                             overwritten by actual size of old value */
        void *newval;     /* 0 or address of new value */
        size_t newlen;    /* size of new value */
};

This call does a search in a tree structure, possibly resembling a directory tree under /proc/sys, and if the requested item is found calls some appropriate routine to read or modify the value.

 

EXAMPLE

#include <linux/unistd.h>
#include <linux/types.h>
#include <linux/sysctl.h>

_syscall1(int, _sysctl, struct __sysctl_args *, args);
int sysctl(int *name, int nlen, void *oldval, size_t *oldlenp,
           void *newval, size_t newlen)
{
        struct __sysctl_args args={name,nlen,oldval,oldlenp,newval,newlen};
        return _sysctl(&args);
}

#define SIZE(x) sizeof(x)/sizeof(x[0])
#define OSNAMESZ 100

char osname[OSNAMESZ];
int osnamelth;
int name[] = { CTL_KERN, KERN_OSTYPE };

main(){
        osnamelth = SIZE(osname);
        if (sysctl(name, SIZE(name), osname, &osnamelth, 0, 0))
                perror("sysctl");
        else
                printf("This machine is running %*s\n", osnamelth, osname);
        return 0;
}

 

RETURN VALUE

Upon successful completion, _sysctl returns 0. Otherwise, a value of -1 is returned and errno is set to indicate the error.  

ERRORS

ENOTDIR

name was not found.

EPERM

No search permission for one of the encountered `directories', or no read permission where oldval was nonzero, or no write permission where newval was nonzero.

EFAULT

The invocation asked for the previous value by setting oldval non-NULL, but allowed zero room in oldlenp.

 

CONFORMING TO

This call is Linux-specific, and should not be used in programs intended to be portable. A sysctl call has been present in Linux since version 1.3.57. It originated in 4.4BSD. Only Linux has the /proc/sys mirror, and the object naming schemes differ between Linux and BSD 4.4, but the declaration of the sysctl(2) function is the same in both.  

SEE ALSO

proc(5)

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Important: Use the man command (% man) to see how a command is used on your particular computer.

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