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Practical and useful coding projects in JavaScript, Python and C. Dates and Times in C. Every time I need to work with dates and times in C I find myself scrabbling around trying to
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Practical and useful coding projects in JavaScript, Python and C. Dates and Times in C. Every time I need to work with dates and times in C I find myself scrabbling around trying to refresh my memory on how the data structures and functions provided by time.h work. I have therefore put together a short program to demo the most important functionality, mainly as a reference for myself but I hope you find it useful as well. Dealing with dates and times is a pain. The units are incredibly messy - not only do we have to deal with the various units being split up in different ways, we have to deal with leap years, daylight saving time, time zones and even the occasional leap second. Fortunately every mainstream language or framework provides us with off-the-shelf functionality for handling dates and times but even these are not entirely straightforward. This is particularly true with C so I thought I would provide myself with a few bits of sample code which I can copy and paste whenever I need, and also share with anybody else who might find it useful. time.h. The C standard library includes time.h which provides us with a struct and an integer type to represent dates and times. These are combined, there are no separate types for dates and times. This makes sense when you think about it, as the separation between the two is entirely arbitrary and has no real meaning. These are the two types used to represent dates and times. Types used to store a date and time time_t This is an integer type, usually long, which holds the datetime as the number of seconds before or after midnight on 1 Jan 1970. tm A struct with nine members, all ints, to represent the various parts of a datetime. Of course the two types show above both represent what is essentially the same information, albeit in very different forms. The standard library therefore provides us with functions for obtaining a variable of each type from the other. Converting between time_t and tm gmtime Creates a tm struct from a time_t using GMT (Greenwich Mean Time) localtime Creates a tm struct from a time_t using local time mktime Creates a time_t variable from a tm struct. Finally let's look at a few functions to provide human-readable strings from time_t variables or tm structs . Creating strings from date and time variables asctime Creates a string from a tm in a fixed ddd mmm dd hh:mm:ss yyyy" format ctime Creates a string from a time_t in a fixed "ddd mmm dd hh:mm:ss yyyy" format strftime Creates a string from a tm according to the specified format. The Project. For this project I will write three functions to illustrate the types and functions listed above. Get the current time as a time_t , use it to create tm structs in both GMT and local time, then print out the members of the local time struct Create and initialize a tm struct, then use mktime to both normalize the tm and get a time_t Get the local time and print it in various formats using strftime. The project consists of just one file called datetime.c. You can download it as a zip or clone/download the Github repository if you prefer. This is the first part. Source Code Links. datetime.c part 1. The main function merely calls the other three functions. I don't usually include a lot of comments unless something unusual is going on which needs an explanation, but as this code is intended mainly as a reference I have included comprehensive descriptions. Therefore I won't include details of the code in the text. When you have read the code and comments compile the program and run it: Compile and Run. gcc datetime.c -std=c11 -lm -o datetime ./datetime. This is the output. Program Output - current_time(), ------------------- | codedrome.com | | Dates and Times | ------------------- Current GMT time is: Wed Jul 24 12:41:00 2019 Current local time is: Wed Jul 24 12:41:00 2019 The members of the local tm struct are: tm_sec 0 tm_min 41 tm_hour 12 tm_mday 24 tm_mon 6 0-based tm_year 119 from 1900 tm_wday 3 0-based from Sunday tm_yday 204 0-based tm_isdst 1 -1 = not known, 0 = no, 1 = yes. Note that tm_mon , tm_wday and tm_yday are zero-based, and that the year counts up or down from 1900. Now let's look at create_time . datetime.c part 2. void create_time(void) When creating a tm I have used 7 as the month but subtracted 1, and 2019 as the year but subtracted 1900. This emphasises that they start at 0 and 1900 respectively. After printing out the value of the time_t variable both as a number and a human-readable string I have printed tm_wday and tm_yday to illustrate that the mktime function sets these. The mktime function also adjusts any values which exceed their maximum. For example if you create a tm struct with a date of 31 June then mktime will change it to 1 July. For this reason it is important to set all six date/time members even if you aren't interested in them. If you do not they will contain garbage values which mktime will use to adjust the members which you did set. (You don't have to look far to find complaints about this "bug"!) In main comment out current_time(), , uncomment create_time(), and build and run again. This is the output. Program Output - create_time(), ------------------- | codedrome.com | | Dates and Times | ------------------- The value of the time_t variable is: 1562919330 which represents: Fri Jul 12 09:15:30 2019 The mkdir function set these members of the tm struct: tm_wday 5 0-based from Sunday tm_yday 192 0-based. The fixed string format provided by asctime and ctime is OK for demonstration or perhaps writing to a log but isn't very flexible. The strftime function allows us to specify a format string so we can create a string with the various members of a tm struct in any order or format we want. strftime takes the following arguments: char* restrict n - a buffer to hold the formatted date size_t n - the maximum number of characters to write const char* restrict format - a string representing the format const struct tm* restrict timeptr - the tm to create a string representation of. The format string can contain any number of specifiers consisting of the % sign followed by an upper case or lower case letter. I won't list all the valid letters but you might like to read this http://man7.org/linux/man-pages/man3/strftime.3.html.
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