{"id":5862,"date":"2023-05-06T00:01:09","date_gmt":"2023-05-06T07:01:09","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=5862"},"modified":"2023-04-29T10:14:22","modified_gmt":"2023-04-29T17:14:22","slug":"checking-the-time","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=5862","title":{"rendered":"Checking the Time"},"content":{"rendered":"

I tend to keep the lessons on this blog specific to the standard C library, C99 release. Yet every so often a newer function draws my attention, which may not be standard but can still be useful. Case in point is the clock_gettime()<\/em> function, which subscriber M.Stumpfl mentioned in a previous post<\/a>.
\n
\nThe clock_gettime()<\/em> function is part of a family of time.h<\/code> functions, including clock_getres()<\/em> and clock_settime()<\/em>. As these are newer functions, they may require linking in the rt<\/code> library for older C compilers.<\/p>\n

Like the time()<\/em> function, clock_gettime()<\/em> returns the number of seconds in the Unix Epoch, the time elapsed since midnight January 1, 1970. It also includes the number of nanoseconds elapsed since the last second. This makes the function far more precise than time()<\/em>, which tracks only seconds (a time_t<\/em> value).<\/p>\n

Here is the function’s man<\/em> page format:<\/p>\n

int clock_gettime(clockid_t clockid, struct timespec *tp);<\/code><\/p>\n

The clockid<\/em> value is a constant representing a clock to examine. The constant CLOCK_REALTIME<\/code> serves as the system clock. In my earlier post, M.Stumpfl used the CLOCK_MONOTONIC<\/code> constant, which isn’t affected by jumps that occur in the system clock. But CLOCK_MONOTONIC<\/code>‘s values represent the number of seconds elapsed since the system was last booted, not the Epoch value.<\/p>\n

The second argument is the address of a timespec<\/em> structure. This structure has two members:<\/p>\n

struct timespec {
\n    time_t   tv_sec;        \/* seconds *\/
\n    long     tv_nsec;       \/* nanoseconds *\/
\n};<\/code><\/p>\n

Member tv_sec<\/em> is the time_t<\/em> value, the number of seconds elapsed since the Epoch. Member tv_nsec<\/em> is the number of nanoseconds (billionths of a second) that have passed since the last clock tick. It’s this second value that adds such precision to the clock_gettime()<\/em> function.<\/p>\n

The following code shows the clock_gettime()<\/em> function in action, outputting the current values retrieved from the system clock:<\/p>\n

2023_05_06-Lesson-a.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <time.h>\r\n\r\nint main()\r\n{\r\n    struct timespec res;\r\n\r\n    clock_gettime(CLOCK_REALTIME,&res);\r\n    printf(\"Seconds: %lu\\nNanoseconds: %lu\\n\",\r\n            res.tv_sec,\r\n            res.tv_nsec\r\n          );\r\n\r\n    return(0);\r\n}<\/pre>\n
The function is called with the CLOCK_REALTIME<\/code> argument, using the system clock. The timespec<\/em> structure res<\/code> holds the values returned, which are output in the printf()<\/em> statement. The %lu<\/code> (long unsigned<\/em>) placeholder is used for both values. Here is the output:<\/p>\n
Seconds: 1682175617
\nNanoseconds: 242989728<\/code><\/p>\n
The Seconds value is the Unix Epoch, the same value that would be returned from the time()<\/em> function. The value I’m more interested in is Nanoseconds. Obviously, billionths of seconds are going to fly by more quickly than your reservation time at the Apple Store Genius Bar.<\/p>\n
To monitor the nanosecond progress, I updated the code as shown below.<\/p>\n
2023_05_06-Lesson-b.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <time.h>\r\n\r\nint main()\r\n{\r\n    struct timespec res;\r\n    int count;\r\n\r\n    \/* loop to read nanosecond values *\/<\/span>\r\n    count = 20;\r\n    while( count-- )\r\n    {\r\n        clock_gettime(CLOCK_REALTIME,&res);\r\n        printf(\"%lu\\n\",res.tv_nsec);\r\n    }\r\n\r\n    return(0);\r\n}<\/pre>\n
A while<\/em> loop repeats 20 times, each iteration calling the clock_gettime()<\/em> function and outputting only the nanosecond value. Here’s the output:<\/p>\n
196619560
\n196697155
\n196699640
\n196700626
\n196701428
\n196702211
\n196702990
\n196703773
\n196704726
\n196705564
\n196706410
\n196707235
\n196708010
\n196708788
\n196709578
\n196710346
\n196711292
\n196712156
\n196713012
\n196713861<\/code><\/p>\n
These values aren’t consecutive because it takes the computer longer than a nanosecond to output text to the terminal. But if you modify the code to output more values, you see the nanosecond numbers “flip over” with each passing second. Doing so generates a lot of output.<\/p>\n
In next week’s Lesson, I continue my fascination with the clock_gettime()<\/em> function and its nanosecond values.<\/p>\n","protected":false},"excerpt":{"rendered":"
The time()<\/em> function is nice, but the clock_gettime()<\/em> function is more precise. Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[2],"tags":[],"class_list":["post-5862","post","type-post","status-publish","format-standard","hentry","category-main"],"_links":{"self":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5862","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=5862"}],"version-history":[{"count":4,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5862\/revisions"}],"predecessor-version":[{"id":5873,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5862\/revisions\/5873"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=5862"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=5862"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=5862"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}