sysctl(2) — Linux manual page

sysctl(2)                  System Calls Manual                 sysctl(2)

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;    /* 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.

STANDARDS

       Linux.

HISTORY

       Linux 1.3.57.  Removed in Linux 5.5, glibc 2.32.

       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.

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);
       }

SEE ALSO

       proc(5)

COLOPHON

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Linux man-pages 6.9.1          2024-06-15                      sysctl(2)

Pages that refer to this page: syscalls(2), proc_sys(5), sctp(7)

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