{"id":4590,"date":"2021-02-13T00:01:43","date_gmt":"2021-02-13T08:01:43","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=4590"},"modified":"2021-02-20T09:19:39","modified_gmt":"2021-02-20T17:19:39","slug":"parsing-integer-values-into-a-time_t-value","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=4590","title":{"rendered":"Parsing Integer Values Into a time_t<\/em> Value"},"content":{"rendered":"

Pulling a date from a string involves slicing the string into substrings, which are then converted into values for year, month, and day. Last week’s Lesson<\/a> demonstrated such code. This week, the final step is accomplished, converting year, month, and day integers into a time_t<\/em> value.
\n
\nI can think of two ways to create a time_t<\/em> value. My default method, which I’ve written about before on this blog<\/a>, is to populate a tm<\/em> structure which can then be converted into a time_t<\/em> value. It’s kind of a backwards way to obtain the value, but I know it works.<\/p>\n

As a review, the time_t<\/em> value represents the Unix epoch, or the number of seconds passed since January 1, 1970. The tm<\/em> structure contains members that reference individual parts of a date, such as the year, month, day of the month, and so on. Both of time_t<\/em> and the tm<\/em> structure are defined in the time.h<\/code> header file.<\/p>\n

In my earlier post, I populated a tm<\/em> structure with the current time, then updated specific members to reflect the time_t<\/em> value I wanted. Here’s a snippet from the code:<\/p>\n

\r\ntime(&today);\r\nbirthday = localtime(&today);\r\nbirthday->tm_mon = month-1;\r\nbirthday->tm_mday = day;\r\nbirthday->tm_year = year-1900;\r\n\r\nbday = mktime(birthday);<\/pre>\n
The time()<\/em> function sets the current time’s time_t<\/em> value into variable today<\/code>.<\/p>\n
The localtime()<\/em> function extracts from the time_t<\/em> value individual members of the tm<\/em> structure birthday<\/code>.<\/p>\n
The month, day, and year members of tm<\/em> structure birthday<\/code> are updated with values from int<\/em> variables month<\/code>, day<\/code>, and year<\/code>.<\/p>\n
Finally, the mktime()<\/em> function converts the data from tm<\/em> structure birthday<\/code> into the time_t<\/em> value bday<\/code>.<\/p>\n
I wondered if you really need to pre-populate a tm<\/em> structure to make this process work? So I ran a test and filled various members of the structure directly. Alas, the results were inconsistent, so I retained my approach from the earlier blog post.<\/p>\n
Here’s the final version of the filename-to-time_t<\/em> code:<\/p>\n
2021_02_13-Lesson.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n#include <time.h>\r\n\r\n\/* copy and convert the digits *\/<\/span>\r\nchar *convert( char *s, int size )\r\n{\r\n    int x;\r\n    static char buffer[5];\r\n    char c;\r\n\r\n    for( x=0; x<size; x++ )\r\n    {\r\n        c = *(s+x);\r\n        if( c=='.' || c=='\\0' )\r\n        {\r\n            fprintf(stderr,\"Malformed filename\\n\");\r\n            exit(2);\r\n        }\r\n        buffer[x] = c;\r\n    }\r\n    buffer[x] = '\\0';\r\n\r\n    return(buffer);\r\n}\r\n\r\nint main(int argc, char *argv[])\r\n{\r\n    char *filename;\r\n    int year, month, day;\r\n    time_t now,file;\r\n    struct tm *date;\r\n    \r\n    \/* check for filename argument *\/<\/span>\r\n    if( argc<2 )\r\n    {\r\n        \/* output error message to standard error *\/<\/span>\r\n        fprintf(stderr,\"Filename option required\\n\\n\");\r\n        \/* leave with exit code 1*\/<\/span>\r\n        exit(1);\r\n    }\r\n    \/* assign to pointer for convenience *\/<\/span>\r\n    filename = argv[1];\r\n\r\n    \/* code to confirm that the file exists goes here *\/<\/span>\r\n    \/* ... *\/<\/span>\r\n    \r\n    \/* extract integers. *\/<\/span>\r\n    year = strtol(convert(filename+0,4),NULL,10);\r\n    month = strtol(convert(filename+4,2),NULL,10);\r\n    day = strtol(convert(filename+6,2),NULL,10);\r\n\r\n    \/* fill the tm structure with the current date\/time *\/<\/span>\r\n    time(&now);\r\n    date = localtime(&now);\r\n    \/* backfill some specific items in the date structure *\/<\/span>\r\n    date->tm_sec = 0;\r\n    date->tm_min = 0;\r\n    date->tm_hour = 0;\r\n    date->tm_mday = day;\r\n    date->tm_mon = month - 1;\r\n    date->tm_year = year - 1900;\r\n    \/* convert the structure to a time_t value *\/<\/span>\r\n    file = mktime(date);\r\n\r\n    \/* output results *\/<\/span>\r\n    printf(\"%4d %2d %2d\\n\",year,month,day);\r\n    printf(\"time_t value is %ld\\n\",file);\r\n    printf(\"%s\\n\",ctime(&file));\r\n\r\n    return(0);\r\n}<\/pre>\n
Line 54 generates a time_t<\/em> value in variable now<\/code>. The tm<\/em> structure date is filled at Line 55.<\/p>\n
From Lines 57 through 62, the tm<\/em> structure date<\/code> is updated with new values. The month<\/code> and year<\/code> values must be modified to fit into the tm<\/em> structure properly; months are numbered from zero and years start at 1900.<\/p>\n
Sample output for the argument 20210115.txt<\/code>:<\/p>\n
2021  1 15
\ntime_t value is 1610697600
\nFri Jan 15 00:00:00 2021<\/code><\/p>\n
And for 2021okay.txt<\/code><\/p>\n
2021  0  0
\ntime_t value is 1606723200
\nMon Nov 30 00:00:00 2020<\/code><\/p>\n
With the output above, you see that bogus data is generated due to the bogus input. The code doesn’t validate input to the degree necessary to detect this issue. I leave a solution up to you.<\/p>\n
In next week’s Lesson<\/a>, I cover how to manually create a time_t<\/em> value.<\/p>\n","protected":false},"excerpt":{"rendered":"
The final step in parsing the filename string is to convert the year, month, and day integer values into a time_t<\/em> “Unix epoch” value. 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-4590","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\/4590","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=4590"}],"version-history":[{"count":12,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4590\/revisions"}],"predecessor-version":[{"id":4636,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4590\/revisions\/4636"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=4590"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=4590"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=4590"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}