SYSCTL(2)                                                               Linux Programmer's Manual                                                              SYSCTL(2)

NAME
       sysctl - read/write system parameters

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

       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;    /* 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 rou‐
       tine 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.

VERSIONS
       This system call first appeared in Linux 1.3.57.  It was removed in Linux 5.5; glibc support was removed in version 2.32.

CONFORMING TO
       This  call is Linux-specific, and should not be used in programs intended to be portable.  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() function is the same in both.

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.  Fur‐
       thermore, 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 <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(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)

Linux                                                                          2021-03-22                                                                      SYSCTL(2)