• NAME
  
  sysctl - read/write system parameters
• SYNOPSIS
  

  
   #include <unistd.h>
  
   #include <linux/sysctl.h>
  
   int _sysctl(struct __sysctl_args * args );
  

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

  EFAULT

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

  ENOTDIR

  name was not found.

  EPERM

  No search permission for one of the encountered "directories", or no read permission where oldval was non-zero, or no write permission where newval was non-zero.

• CONFORMING TO
  
  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
  
  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. Remove it from your programs now; use the /proc/sys interface instead.
• 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
  
  This page is part of release 3.19 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

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