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31.4 System Parameters

This section describes the sysctl function, which gets and sets a variety of system parameters.

The symbols used in this section are declared in the file sys/sysctl.h.

Function: int sysctl (int *names, int nlen, void *oldval, size_t *oldlenp, void *newval, size_t newlen)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

sysctl gets or sets a specified system parameter. There are so many of these parameters that it is not practical to list them all here, but here are some examples:

  • network domain name
  • paging parameters
  • network Address Resolution Protocol timeout time
  • maximum number of files that may be open
  • root filesystem device
  • when kernel was built

The set of available parameters depends on the kernel configuration and can change while the system is running, particularly when you load and unload loadable kernel modules.

The system parameters with which sysctl is concerned are arranged in a hierarchical structure like a hierarchical filesystem. To identify a particular parameter, you specify a path through the structure in a way analogous to specifying the pathname of a file. Each component of the path is specified by an integer and each of these integers has a macro defined for it by sys/sysctl.h. names is the path, in the form of an array of integers. Each component of the path is one element of the array, in order. nlen is the number of components in the path.

For example, the first component of the path for all the paging parameters is the value CTL_VM. For the free page thresholds, the second component of the path is VM_FREEPG. So to get the free page threshold values, make names an array containing the two elements CTL_VM and VM_FREEPG and make nlen = 2.

The format of the value of a parameter depends on the parameter. Sometimes it is an integer; sometimes it is an ASCII string; sometimes it is an elaborate structure. In the case of the free page thresholds used in the example above, the parameter value is a structure containing several integers.

In any case, you identify a place to return the parameter’s value with oldval and specify the amount of storage available at that location as *oldlenp. *oldlenp does double duty because it is also the output location that contains the actual length of the returned value.

If you don’t want the parameter value returned, specify a null pointer for oldval.

To set the parameter, specify the address and length of the new value as newval and newlen. If you don’t want to set the parameter, specify a null pointer as newval.

If you get and set a parameter in the same sysctl call, the value returned is the value of the parameter before it was set.

Each system parameter has a set of permissions similar to the permissions for a file (including the permissions on directories in its path) that determine whether you may get or set it. For the purposes of these permissions, every parameter is considered to be owned by the superuser and Group 0 so processes with that effective uid or gid may have more access to system parameters. Unlike with files, the superuser does not invariably have full permission to all system parameters, because some of them are designed not to be changed ever.

sysctl returns a zero return value if it succeeds. Otherwise, it returns -1 and sets errno appropriately. Besides the failures that apply to all system calls, the following are the errno codes for all possible failures:

EPERM

The process is not permitted to access one of the components of the path of the system parameter or is not permitted to access the system parameter itself in the way (read or write) that it requested.

ENOTDIR

There is no system parameter corresponding to name.

EFAULT

oldval is not null, which means the process wanted to read the parameter, but *oldlenp is zero, so there is no place to return it.

EINVAL
  • The process attempted to set a system parameter to a value that is not valid for that parameter.
  • The space provided for the return of the system parameter is not the right size for that parameter.
ENOMEM

This value may be returned instead of the more correct EINVAL in some cases where the space provided for the return of the system parameter is too small.

If you have a Linux kernel with the proc filesystem, you can get and set most of the same parameters by reading and writing to files in the sys directory of the proc filesystem. In the sys directory, the directory structure represents the hierarchical structure of the parameters. E.g. you can display the free page thresholds with 

cat /proc/sys/vm/freepages

Some more traditional and more widely available, though less general, GNU C Library functions for getting and setting some of the same system parameters are:

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