{"id":5882,"date":"2023-05-20T00:01:18","date_gmt":"2023-05-20T07:01:18","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=5882"},"modified":"2023-05-27T08:27:55","modified_gmt":"2023-05-27T15:27:55","slug":"your-own-time-delay-function","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=5882","title":{"rendered":"Your Own Time Delay Function"},"content":{"rendered":"

I’ve written about my own delay()<\/em> function<\/a>, which uses processor clock ticks to calculate a time delay in milliseconds. Thanks to the clock_gettime()<\/em> function, I can write a new time-delay function that operates from nanoseconds — or close to it. I hope.
\n
\nLast week’s Lesson<\/a> demonstrated how the clock_gettime()<\/em> function returns a nanosecond value. The call is precise, but measuring the interval between calls is subject to a delay. Therefore, a nanosecond delay function won’t be accurate to a specific nanosecond value. This limitation doesn’t mean I won’t write the thing anyway.<\/p>\n

Off the top of my head, I can’t think why someone would want to delay a program for a single nanosecond. And I’m not going to dive into the quagmire of how many clock cycles each CPU instruction takes per nanosecond. Regardless, this Lesson is just for fun.<\/p>\n

2023_05_20-Lesson.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <time.h>\r\n\r\n#define MAX_N 999999999L\r\n\r\nvoid nano_delay(long ns)\r\n{\r\n    struct timespec res;\r\n    long end;\r\n\r\n    \/* avoid out of range values *\/<\/span>\r\n    if( ns<1 || ns>MAX_N )\r\n        return;\r\n\r\n    \/* obtain the current value *\/<\/span>\r\n    clock_gettime(CLOCK_REALTIME,&res);\r\n    \/* calculate end time *\/<\/span>\r\n    end = res.tv_nsec + ns;\r\n    \/* wait for the end time *\/<\/span>\r\n    while(1)\r\n    {\r\n        clock_gettime(CLOCK_REALTIME,&res);\r\n        if( res.tv_nsec > end )\r\n            break;\r\n    }\r\n}\r\n\r\nint main()\r\n{\r\n    long delay;\r\n\r\n    for( delay=10; delay<MAX_N; delay*=10 )\r\n    {\r\n        printf(\"Delay = %ld\\n\",delay);\r\n        nano_delay(delay);\r\n    }\r\n\r\n    return(0);\r\n}<\/pre>\n
The nano_delay()<\/em> function accepts a long int<\/em> as its argument, a delay in nanoseconds. An if<\/em> test immediately weeds out values less than one and greater than the value of MAX_N<\/code> or 999,999,999.<\/p>\n
After the clock_gettime()<\/em> function fetches the current time, variable end<\/code> is assigned the current nanosecond value plus the delay value (ns<\/code>). An eternal while<\/em> loop spins until the current nanosecond value fetched from clock_gettime()<\/em> is greater than the end<\/code> value.<\/p>\n
In the main()<\/em> function, the nano_delay()<\/em> function is called using powers of ten values: 10, 100, 1000, and so on. Here’s output from a sample run:<\/p>\n
Delay = 10
\nDelay = 100
\nDelay = 1000
\nDelay = 10000
\nDelay = 100000
\nDelay = 1000000
\nDelay = 10000000
\nDelay = 100000000<\/code><\/p>\n
I doubt anyone can notice any pauses inthe output, given that it takes the computer longer to write to the terminal than the delay. The final value may show a barely noticeable pause.<\/p>\n
This code contains a flaw that could disrupt the timing. For example, if the current nanosecond value stored in res.tv_nsec<\/code> is 750,000,000 (commas added for readability). And the delay is 500,000,000, the value of variable end<\/code> is 1,250,000,000. This value is out of range for res.tv_nsec<\/code>. The if<\/em> test in the endless while<\/em> loop will never be true.<\/p>\n
I’ve run the code a few times and not had any issues, though the math puzzles me. I’ll try to address this issue in next week’s post<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"
One way I put the clock_gettime()<\/em> function to work is to create a delay function that operates on nanoseconds. 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-5882","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\/5882","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=5882"}],"version-history":[{"count":4,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5882\/revisions"}],"predecessor-version":[{"id":5896,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5882\/revisions\/5896"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=5882"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=5882"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=5882"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}