sysctl

Sommaire

NAME

sysctl - read/write system parameters

SYNOPSIS

 #include <unistd.h>
 

 #include <linux/sysctl.h>
 
 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.

RETURN VALUE

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

ERRORS

EFAULT
The invocation asked for the previous value by setting oldval non-NULL, but allowed zero room in oldlenp.
ENOTDIR
name was not found.
EPERM
No search permission for one of the encountered `directories', or no read permission where oldval was non-zero, or no write permission where newval was non-zero.

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 4.4BSD, but the declaration of the sysctl(2) function is the same in both.

NOTES

Glibc does not provide a wrapper for this system call; call it using syscall(2).

BUGS

The object names vary between kernel versions. This makes this system call worthless for applications. Use the /proc/sys interface instead.

Not all available objects are properly documented.

It is not yet possible to change operating system by writing to /proc/sys/kernel/ostype.

EXAMPLE

 #define _GNU_SOURCE
 #include <unistd.h>
 #include <sys/syscall.h>
 #include <string.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <linux/sysctl.h>
 
 int _sysctl(struct __sysctl_args *args );
 
 #define OSNAMESZ 100
 
 int
 main(void)
 {
     struct __sysctl_args args;
     char osname[OSNAMESZ];
     size_t osnamelth;
     int name[] = { CTL_KERN, KERN_OSTYPE };
 
     memset(&args, 0, sizeof(struct __sysctl_args));
     args.name = name;
     args.nlen = sizeof(name)/sizeof(name[0]);
     args.oldval = osname;
     args.oldlenp = &osnamelth;
 
     osnamelth = sizeof(osname);
 
     if (syscall(SYS__sysctl, &args) == -1) {
         perror("_sysctl");
         exit(EXIT_FAILURE);
     }
     printf("This machine is running %*s\n", osnamelth, osname);
     exit(EXIT_SUCCESS);
 }

SEE ALSO

proc(5)