NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON

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

NAME         top

       sysctl - read/write system parameters

SYNOPSIS         top

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

       int _sysctl(struct __sysctl_args *args);

       Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION         top

       Do not use this system call!  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         top

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

ERRORS         top

       EFAULT The invocation asked for the previous value by setting oldval
              non-NULL, but allowed zero room in oldlenp.

       ENOTDIR
              name was not found.

       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.

CONFORMING TO         top

       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() function is the same
       in both.

NOTES         top

       Glibc does not provide a wrapper for this system call; call it using
       syscall(2).  Or rather...  don't call it: use of this system call has
       long been discouraged, and it is so unloved that it is likely to
       disappear in a future kernel version.  Since Linux 2.6.24, uses of
       this system call result in warnings in the kernel log.  Remove it
       from your programs now; use the /proc/sys interface instead.

       This system call is available only if the kernel was configured with
       the CONFIG_SYSCTL_SYSCALL option.

BUGS         top

       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         top

       #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         top

       proc(5)

COLOPHON         top

       This page is part of release 3.73 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       http://www.kernel.org/doc/man-pages/.

Linux                            2012-12-22                        SYSCTL(2)