# sysctl

## NAME



           sysctl - read/write system parameters
    


## SYNOPSIS



           **#include <unistd.h>**
           **#include <linux/sysctl.h>**
    
           **[[deprecated]] int _sysctl(struct __sysctl_args ***_args_**);**
    


## DESCRIPTION



           **This system call no longer exists on current kernels!** See NOTES.
    
           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;    /* number of elements 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



           **EACCES**
           **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_.
    
           **ENOTDIR**
                  _name_ was not found.
    


## STANDARDS



           Linux.
    


## NOTES



           Use of this system call was long discouraged: since Linux 2.6.24,
           uses of this system call result in warnings in the kernel log, and
           in Linux 5.5, the system call was finally removed.  Use the
           _/proc/sys_ interface instead.
    
           Note that on older kernels where this system call still exists, it
           is available only if the kernel was configured with the
           **CONFIG_SYSCTL_SYSCALL** option.  Furthermore, glibc does not provide
           a wrapper for this system call, necessitating the use of
           syscall(2).
    


## BUGS



           The object names vary between kernel versions, making this system
           call worthless for applications.
    
           Not all available objects are properly documented.
    
           It is not yet possible to change operating system by writing to
           _/proc/sys/kernel/ostype_.
    


## EXAMPLES



           #define _GNU_SOURCE
           #include <stdio.h>
           #include <stdlib.h>
           #include <string.h>
           #include <sys/syscall.h>
           #include <unistd.h>
    
           #include <linux/sysctl.h>
    
           #define ARRAY_SIZE(arr)  (sizeof(arr) / sizeof((arr)[0]))
    
           int _sysctl(struct __sysctl_args *args);
    
           #define OSNAMESZ 100
    
           int
           main(void)
           {
               int                   name[] = { CTL_KERN, KERN_OSTYPE };
               char                  osname[OSNAMESZ];
               size_t                osnamelth;
               struct __sysctl_args  args;
    
               memset(&args, 0, sizeof(args));
               args.name = name;
               args.nlen = ARRAY_SIZE(name);
               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", (int) osnamelth, osname);
               exit(EXIT_SUCCESS);
           }
    


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