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 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.

VERSIONS

       This system call first appeared in Linux 1.3.57.  It was removed in Linux 5.5.

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.  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.

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)

COLOPHON

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       https://www.kernel.org/doc/man-pages/.