_sysctl

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.

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

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.

Linux.

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.

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).

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.

#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);
}