The lock can either be designed to always give priority to readers read-preferringto always give priority to writers write-preferring or be unspecified with regards to priority. These policies lead to different tradeoffs with regards to concurrency and starvation. Read-preferring RW locks allow for maximum concurrency, but can lead to write-starvation if contention is high.
This document is Copyright Riku Saikkonen. If you have corrections or something to add, feel free to e-mail me Riku. For debugging, you can use gcc -g -O at least with modern versions of gccthough optimisation can sometimes make the debugger behave a bit strangely.
Permissions Before you access any ports, you must give your program permission to do so. This is done by calling the ioperm function declared in unistd.
For example, ioperm 0x, 5, 1 would give access to ports 0x through 0x a total of 5 ports. The last argument is a Boolean value specifying whether to give access to the program to the ports true 1 or to remove access false 0. You can call ioperm multiple times to enable multiple non-consecutive ports.
See the ioperm 2 manual page for details on the syntax. The ioperm call requires your program to have root privileges; thus you need to either run it as the root user, or make it setuid root.
You can drop the root privileges after you have called ioperm to enable the ports you want to use. You are not required to explicitly drop your port access privileges with ioperm A setuid to a non-root user does not disable the port access granted by iopermbut a fork does the child process does not get access, but the parent retains it.
Use the level argument 3 i. Again, you need root privileges to call iopl. See the iopl 2 manual page for details. Accessing the ports To input a byte 8 bits from a port, call inb portit returns the byte it got. To output a byte, call outb value, port please note the order of the parameters. To output a word to the two ports, use outw value, x.
Note that all port access instructions take at least about a microsecond to execute.
For more versatile methods of delaying, read on. There are manual pages for ioperm 2iopl 2and the above macros in reasonably recent releases of the Linux manual page collection.
This method is probably slower than the normal method above, but does not need compiler optimisation nor ioperm. You can disable interrupts from within a user-mode program, though it can be dangerous even kernel drivers do it for as short a time as possible.
After calling iopl 3you can disable interrupts simply by calling asm "cli" ;, and re-enable them with asm "sti". Your process might be scheduled out at any time for anything from about 10 milliseconds to a few seconds on a system with very high load.Write-back support.
UBIFS supports write-back, which means that file changes do not go to the flash media straight away, but they are cached and go to the flash later, when it is absolutely necessary.
chmod is a command in Linux and other Unix-like not a directory, and you grant write permission (just so as to give an example of granting every permission).
Text method shortcuts and user to have, and then total their values up. For example, if you want to grant the owner of a directory read write and execution permissions, and you.
An easy to follow tutorial with screenshots to teach you how to set write permission on ext4 partition In Ubuntu Linux. Linux Installation, Step by Step¶.
If you have come directly to this page hoping to install Linux without doing any more reading, I suggest that you reconsider. You seem to live in a world of hacking together toy software. One day when you become a software ENGINEER who has to build complex, long-lived software as part of a team of people, following modern engineering practices (model-driven development, design-by-contract), upon which people's well-being depends, you will change your tune, I suspect.
If the OS or the filesystem does not support ACLs, another way is to use groups.. Create a group. Some Linux distributions create a separate group for each user: tom would automatically be in a group also named tom.
If not, create a group.