/etc/system File (Writing Device Drivers)

Writing Device Drivers

`/etc/system` File

The /etc/system file serves several purposes, but for driver development, the most important is that it allows you to set the value of kernel variables at boot time. This can be used to toggle different behaviors in a driver, or to enable certain debugging features made available by the kernel.

/etc/system is read only once, while the kernel is booting. After this file is modified, the system must be rebooted for the changes to take effect. If a change in the file causes the system not to work, boot with the ask (-a) option and specify /dev/null as the system file.

The set command is used to change the value of module or kernel variables:

To set module variables, specify the module name and the variable:
```
        set module_name:variable=value
```
For example, to set the variable test_debug in the driver test, use the following set command:
```
        set test:test_debug=1
```
To set a variable exported by the kernel itself, omit the module name. Other assignments are also supported, such as bitwise OR'ing a value into an existing value:
```
        set moddebug | 0x80000000
```

See the system(4) man page for more information.

Note –

Most kernel variables are not guaranteed to be present in subsequent releases.

Controlling Module Loading with `moddebug`

moddebug is a kernel variable that controls the module loading process. The possible values are:

`0x80000000`	Prints messages to the console when loading or unloading modules.
`0x40000000`	Gives more detailed error messages.
`0x20000000`	Prints more detail when loading or unloading (such as including the address and size).
`0x00001000`	No auto-unloading drivers: the system will not attempt to unload the device driver when the system resources become low.
`0x00000080`	No auto-unloading streams: the system will not attempt to unload the streams module when the system resources become low.
`0x00000010`	No auto-unloading of kernel modules of any type.
`0x00000001`	If running with `kadb`, `moddebug` causes a breakpoint to be executed and a return to `kadb` immediately before each module's _`init`(9E) routine is called. Also generates additional debug messages when the module's `_info` and `_fini` routines are executed.

`kmem_flags`

kmem_flags is a kernel variable used to enable debugging features in the kernel's memory allocator. Setting kmem_flags to 0xf enables the allocator's debugging features. These include runtime checks to find:

Code that writes to a buffer after it is freed
Code using memory before it is initialized
Code that writes past the end of a buffer

The “Debugging With the Kernel Memory Allocator” section of the Solaris Modular Debugger Guide describes how the kernel memory allocator can be used to determine the root cause of these problems.

Note –

Testing and developing with kmem_flags set to 0xf is extremely valuable because it can detect latent memory corruption bugs. Because setting kmem_flags to 0xf changes the internal behavior of the kernel memory allocator, you should thoroughly test without kmem_flags as well.

/etc/system File

Controlling Module Loading with moddebug

kmem_flags

`/etc/system` File

Controlling Module Loading with `moddebug`

`kmem_flags`