Linux get milliseconds c

Как получить текущее время в миллисекундах с C в Linux?

Как узнать текущее время в Linux в миллисекундах?

Этого можно добиться с помощью функции POSIX clock_gettime .

В текущей версии POSIX, gettimeofday будет отмечен устаревшими . Это означает, что он может быть удален из будущей версии спецификации. Разработчикам приложений рекомендуется использовать эту clock_gettime функцию вместо gettimeofday .

Вот пример того, как использовать clock_gettime :

Если ваша цель — измерить прошедшее время, и ваша система поддерживает параметр «монотонные часы», вам следует рассмотреть возможность использования CLOCK_MONOTONIC вместо CLOCK_REALTIME .

Вам нужно сделать что-то вроде этого:

Ниже приведена функция util для получения текущей отметки времени в миллисекундах:

О часовом поясе :

gettimeofday () поддерживает указание часового пояса, я использую NULL , что игнорирует часовой пояс, но вы можете указать часовой пояс, если необходимо.

@Update — часовой пояс

Поскольку long представление времени не имеет отношения к самому tz часовому поясу и не зависит от него , установка param для gettimeofday () не требуется, поскольку это не будет иметь никакого значения.

И, по словам мужчины странице gettimeofday() , использование timezone структуры является устаревшим, поэтому tz аргумент должен нормально быть указан как NULL, для информации , пожалуйста , смотрите страницы.

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C++ — How can we get a millisecond timestamp in linux?

How to convert std::chrono::monotonic_clock::now() to milliseconds and cast it to long?

using steady_clock or high_resolution_clock from chrono is also same. I have seen into std::chrono::duration_cast but I only want the current timestamp and not any duration gaps.

2 Answers 2

The current timestamp is defined with respect to some point in time (hence it is a duration). For instance, it is «typical» to get a timestamp with respect to the beginning of the Epoch (January 1st 1970, in Unix). You can do that by using time_since_epoch() :

To get the value in milliseconds you would need to cast it to std::chrono::milliseconds , instead.

All the built-in clocks have an associated «epoch» which is their base time. The actual date/time of the epoch is not specified, and may vary from clock to clock.

If you just want a number for comparisons then some-clock ::now().time_since_epoch() will give you a duration for the time since the epoch for that clock, which you can convert to an integer with the count() member of the duration type. The units of this will depend on the period of the clock. If you want specific units then use duration_cast first:

As I said, this is only good for comparisons, not as an absolute time stamp, since the epoch is unspecified.

If you need a UNIX timestamp then you need to use std::chrono::system_clock , which has a to_time_t() function for converting a time_point to a time_t .

Alternatively, you can take a baseline count at a particular point in your program, along with the corresponding time from gettimeofday or something, and then use that to convert relative counts to absolute times.

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Time in milliseconds in C

Using the following code:

I get the following output:

Start and stop times are the same. Does it mean that the program hardly takes time to complete execution?

If 1. is false, then atleast the no.of digits beyond the (.) should differ, which does not happen here. Is my logic correct?

Note: I need to calculate the time taken for execution, and hence the above code.

8 Answers 8

Yes, this program has likely used less than a millsecond. Try using microsecond resolution with timeval .

You can then query the difference (in microseconds) between stop.tv_usec — start.tv_usec . Note that this will only work for subsecond times (as tv_usec will loop). For the general case use a combination of tv_sec and tv_usec .

Edit 2016-08-19

A more appropriate approach on system with clock_gettime support would be:

This code snippet can be used for displaying time in seconds,milliseconds and microseconds:

A couple of things might affect the results you’re seeing:

1. You’re treating clock_t as a floating-point type, I don’t think it is.
2. You might be expecting ( 1^4 ) to do something else than compute the bitwise XOR of 1 and 4., i.e. it’s 5.
3. Since the XOR is of constants, it’s probably folded by the compiler, meaning it doesn’t add a lot of work at runtime.
4. Since the output is buffered (it’s just formatting the string and writing it to memory), it completes very quickly indeed.

You’re not specifying how fast your machine is, but it’s not unreasonable for this to run very quickly on modern hardware, no.

If you have it, try adding a call to sleep() between the start/stop snapshots. Note that sleep() is POSIX though, not standard C.

You can use gettimeofday() together with the timedifference_msec() function below to calculate the number of milliseconds elapsed between two samples:

Note that, when using gettimeofday() , you need to take seconds into account even if you only care about microsecond differences because tv_usec will wrap back to zero every second and you have no way of knowing beforehand at which point within a second each sample is obtained.

Here is what I write to get the timestamp in millionseconds.

The clock() function returns an approximation of processor time used by the program.

So there is no indication you should treat it as milliseconds. Some standards require precise value of CLOCKS_PER_SEC , so you could rely on it, but I don’t think it is advisable. Second thing is that, as @unwind stated, it is not float/double. Man times suggests that will be an int. Also note that:

this function will return the same value approximately every 72 minutes

And if you are unlucky you might hit the moment it is just about to start counting from zero, thus getting negative or huge value (depending on whether you store the result as signed or unsigned value).

Will most probably print garbage as treating any int as float is really not defined behaviour (and I think this is where most of your problem comes). If you want to make sure you can always do:

Though I would rather go for printing it as long long int at first:

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How to get the current time in milliseconds from C in Linux?

How do I get the current time on Linux in milliseconds?

7 Answers 7

This can be achieved using the POSIX clock_gettime function.

In the current version of POSIX, gettimeofday is marked obsolete. This means it may be removed from a future version of the specification. Application writers are encouraged to use the clock_gettime function instead of gettimeofday .

Here is an example of how to use clock_gettime :

If your goal is to measure elapsed time, and your system supports the «monotonic clock» option, then you should consider using CLOCK_MONOTONIC instead of CLOCK_REALTIME .

You have to do something like this:

Following is the util function to get current timestamp in milliseconds:

About timezone:

gettimeofday() support to specify timezone, I use NULL, which ignore the timezone, but you can specify a timezone, if need.

@Update — timezone

Since the long representation of time is not relevant to or effected by timezone itself, so setting tz param of gettimeofday() is not necessary, since it won’t make any difference.

And, according to man page of gettimeofday() , the use of the timezone structure is obsolete, thus the tz argument should normally be specified as NULL, for details please check the man page.

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How to get current time in milliseconds?

I am new to C++ and I don’t know much about its library. I need to do time analysis of different sorting algorithms, for which I need to get the current time in milliseconds. Is there any way to do that?

2 Answers 2

Simply use std::chrono. The general example below times the task «of printing 1000 stars»:

Instead of printing the stars, you will place your sorting algorithm there and time measure it.

Do not forget to enable the optimization flags for your compiler, if you intend to do some benchmarking, e.g. for g++, you need -O3 . This is serious, check what happened to me when I didn’t do so: Why emplace_back is faster than push_back?

Ps: If your compiler doesn’t support c++11, then you could look into other methods in my Time Measurements (C++).

A specific (toy) example, by using my Quicksort (C++), would be:

and the output now is:

It’s as simple as that. Happy benchmarking! =)

high_resolution_clock::now() function returns time relative to which time?

A reference to a specific point in time, like one’s birthday, today’s dawn, or when the next train passes. In this library, objects of the time_point class template express this by using a duration relative to an epoch (which is a fixed point in time common to all time_point objects using the same clock).

where one could check this epoch and time_point example, which outputs:

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