bionic (2) sysctl.2.gz

NAME

       sysctl - read/write system parameters

SYNOPSIS

       #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

       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

       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.

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.  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

       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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