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.
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:
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
syslog 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
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
-1 and sets
errno appropriately. Besides the
failures that apply to all system calls, the following are the
errno codes for all possible failures:
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.
There is no system parameter corresponding to name.
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.
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
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
sys directory of the
proc filesystem. In the
directory, the directory structure represents the hierarchical structure
of the parameters. E.g. you can display the free page thresholds with
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:
sethostname(See Host Identification.)
uname(See Platform Type.)