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


     sysctl - read/write system parameters


     #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.


     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.


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


            No search permission for one of the  encountered  "directories",
            or no read permission where oldval was nonzero, or no write per-
            mission where newval was nonzero.
     EFAULT The invocation asked for the previous value  by  setting  oldval
            non-NULL, but allowed zero room in oldlenp.
            name was not found.


     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


     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 disap-
     pear in a future kernel version.  Since Linux 2.6.24, uses of this sys-
     tem 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.


     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


     #define  _GNU_SOURCE  #include  <unistd.h>   #include   <sys/syscall.h>
     #include  <string.h>  #include  <stdio.h>  #include <stdlib.h> #include
     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)); = name;
         args.nlen = sizeof(name)/sizeof(name[0]);
         args.oldval = osname;
         args.oldlenp = &osnamelth;
         osnamelth = sizeof(osname);
         if (syscall(SYS__sysctl, &args) == -1) {
         printf("This machine is running %*s\n", osnamelth, osname);
         exit(EXIT_SUCCESS); }




     This page is part of release 4.16 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

Linux 2017-09-15 SYSCTL(2)

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