C / C ++ fundamental types like int, double, etc. atomic, for example, is thread-safe?

whether they are free from data races; that is, if one thread writes to an object of this type and another thread reads from it, is the behavior well defined?

If not, is it compiler dependent or something else?

4 57

Peter mortensen

4 answers:

you can use std :: atomic instead or std :: atomic .

Note: std :: atomic was introduced with C ++ 11, and I understand that prior to C ++ 11, the C ++ standard did not recognize the existence of multithreading at all.

program execution 5.1.2.3

when the processing of an abstract machine is interrupted by the arrival of a signal, object values ​​that are not atomic non-locking objects, nor volatile sig_atomic_t type, are specified, as is the floating point environment state. This value of any object is changed by a handler that is neither a lock-free atomic object nor a volatile sig_atomic_t type becomes undefined when the handler exits, as does the floating point environment state if so changed by the handler and not restored to its original state.

5.1.2.4 multithreading and data races

two expression evaluations conflict if one of them modifies a region of memory and the other reads or modifies the same region of memory.

[several pages of standards - some paragraphs explicitly related to atomic types]

program execution contains data race if it contains two conflicting activities in different threads, at least one of which is not atomic and neither happens before the other. what are the results of a data race in undefined behavior.

a processor can also reorder memory accesses according to that processor's memory ordering constraints. The memory ordering limits for x86 architecture can be found in the Intel 64 and ia32 Architectures software developer guide section 8.2, starting on page 2212.

primitive types (int, char etc) are not atomic

i hope this explains why primitive types are not atomic.

Additional Information which I haven't seen is mentioned in other answers so far:

if you are using std :: atomic for example bool is actually atomic in the target architecture, then the compiler will not generate redundant fences or locks. The same code will be generated as for a simple bool.

in other words, using std :: atomic only makes the code less efficient if it really is required for platform correctness. So there is no reason to avoid it.

- in the process of creating a website, we have to work a lot with text blocks, or text phrases. Once, I needed to add a text phrase, so that before this text phrase there was some indent text from html edge the block to which the given text was added.

So, the first method is the most natural and simple. To indent before the text, add before the text html code spaces - You can add html space code in any html editor.

Here is an example of a working code for setting the indentation for text, using the space code:

Html indent text to the left, use the space code

In this example, before the text we have selected, the space code is -& nbsp; added four times, as a result, we get the indent we need.

We know that whitespace code is handled by any browser. Therefore, by adding the required number of spaces before the text, you can get the required indentation for the text.

The good thing about this method is that it is guaranteed to work in any browser.

But have this method there is a significant drawback. In order to make the indentation for the text large enough in length, in the html code, before the text, you will have to add a large number of space characters, which may not look pretty, cumbersome and not professional.

One of the options for solving this problem is next way set indentation.

HTML text indentation, method two - this method is based on the properties of the blockquote tag. This tag sets an indent of approximately 40 pixels to the left and right for the text placed in it. In addition, the padding is set at the top and bottom. Html example the code for using this method is given below:

on the left, use the tag blockquote

As you can see, this method of setting text indentation is very easy to use, but this method also has a significant drawback. The indentation that the tag sets blockquote, is fixed and is always the same value - 40px.

To remedy the situation, and when adding html indent and for text be able to set any value, we will use the third method.

HTML text indentation, method three.

Here we will use the CSS Cascading Style Sheets' text-indent property.

If we open the CSS reference, on the page with the description of the properties of text-indent, we will see that with the help of text-indent, we can set the amount of indentation of the first line or the first paragraph of any block of text.

We are considering an example with a short text block, so the text-indent properties are fine for our case.

Below is the html code that indents text using text-indent. From the example you can see that by setting different argument values ​​for text-indent, we can change the amount of text indentation:

Htmlindenttext, working CSS style - text-indent

In my opinion, this way of setting the indentation for the text is the most optimal, but, nevertheless, consider another way of setting the indentation, using images.

HTML text indentation, the fourth method - here we will use an image to set the indentation.

As a working example, let's take a look at the html code that demonstrates how this method works:

on the left, use the image

It is clearly seen from the given example that by changing the width of the image, we can change the position of the text, that is, set the required indentation for the text.

As an image, you can use an image whose width and height are equal to one pixel. So that the image we are using is not visible on the page, set the color for it exactly the same as the background of the site page.

In my opinion, the above methods of setting text indents are quite enough to organize the indentation you need in any situation when adding content to the page of your sites.

Category:. No comments yet. Published: 17.05.2012.

Hello, today we will consider a very necessary topic such as structuring text in HTML and after reading this article we will be able to correctly and beautifully structure the text that is on the page of our site. In the article I will tell you and of course I will show you with examples how to create paragraphs for pages, create headings different levels and also add several kinds of lists. Now let's get down to the very process of structuring the text.

Without paragraph tags, our text on the page will look like one large piece of text that is not convenient for the visitor to read, so a tag was created that creates paragraph in html and thanks to this, large text can be broken down into logical parts. This tag looks like this: text should be placed between these two tags, which will then be separated from the main text.

Let's take a look at the paragraph tag with an example. I have already prepared a large piece of text for this, which we will structure. Open the document in a program for editing, you can use notepad, notepad ++. We open the document and see that we have just a large piece of text, without any separation.

Now let's put our tag in the document every four to five lines of text.

And let's now save the file and open it in any browser.

Now we see that our text is divided into many logical parts and now the text has become much more convenient to read. I hope that you understand how you can break the text into logical parts (paragraphs) and it will be much more convenient for your visitors to read the articles.

Each text cannot do without headings and subheadings, which differ in meaning and importance. The h1-h6 tag is responsible for the headers in html, and depending on the number, the size of the header is determined, all headers are highlighted in bold text. Let's take an example using headers.

All of these headings on the site will look like this:

In order to optimize pages on your site, you need to use headers very carefully, especially the h1 and h2 tags. There should be only one h1 header on the page, this should be the main site header and all the rest are subheadings. Use h2 once or twice, not more often, and you can use all other headings on average five to six times and this can have a positive effect on optimization.

Lists in HTML.

In HTML, we can also create lists on the page, lists are very helpful for providing information on the site. Lists are also very useful when creating templates for your website, which we'll cover a little later. Well, now we will move on to considering all the list options such as a numbered list, a bulleted list.

Numbered list in HTML.

Let's start our discussion of the topic of lists in HTML with a numbered list in HTML, and now we can safely start exploring. The very first thing we'll do is prepare a list to demonstrate the capabilities of a numbered list in HTML. Let's take a real life example and write a shopping list.

Ketchup
Mayonnaise
Salt
Pepper
Dill
Eggplant
Tomatoes

Here is the list and we are starting to format it. Numbered list in HTML given by paired tag and now, with an example, we will see the process of creating a numbered list.

First, we need to wrap all the list items in a tag.

Task

Indent the first line of each paragraph.

Solution

When you need to set a paragraph indentation, which is also called a red line, it is best to use the text-indent style property. Its value determines how much to shift the text of the first line to the right of starting position... In this case, the width of the text block does not change and remains set initially. The amount of indentation can be specified in pixels, percentage or other available units (example 1).

Example 1. Indenting the first line

HTML5 CSS 2.1 IE Cr Op Sa Fx

The Babylonian pandemonium, according to biblical tradition, is the failed attempt of King Nimrod to build a pillar in Babylon up to the sky. God, angry with people for their reckless desire, decided to punish the builders: he mixed their language so that they ceased to understand each other, were forced to stop construction and little by little scattered around the world. Hence, as the ancients explained, the difference in languages ​​originated.

In the usual sense, Babylonian pandemonium means disorder, confusion with a large crowd of people.

The result of this example is shown in Fig. one.