The surprising struggle to get a Unix Epoch time from a UTC string in C or C++

Post by rstuart4133 » Mon, Jan 20, 2025, 11:17 PM UTC

chikere232 wrote: post #32

rstuart4133 wrote: post #31

For those skimmimg the problem is mktime() returns local time, and they want it in UTC. So you need to subtract the timezone used, but the timezone varies by date you feed mktime() and there is no easy way to determime it. If you are happy for the time to perhaps be wrong around the hours timezone changes, this is an easy hack: import time def time_mktime_utc(_tuple): result = time.mktime(_tuple[:-1] + (0,)) return r…

That is not really the problem. mktime() parses the time string which lacks any information on time zones then the article uses timegm() to convert it to unixtime on the assumption that it was in UTC also it's about C

Post by [deleted] » Tue, Jan 21, 2025, 7:32 PM UTC

rstuart4133 wrote: post #92

chikere232 wrote: post #32

Earlier quoted context omitted.

That is not really the problem. mktime() parses the time string which lacks any information on time zones then the article uses timegm() to convert it to unixtime on the assumption that it was in UTC also it's about C

> mktime() parses the time string No, mktime() doesn't parse a string. Parsing the string is done by strptime(). mktime() takes the output of strptime(), which is a C structure or the equivalent in Python - a named tuple with the same fields. > the time string lacks any information on time zones Not necessarily. Time strings often contain a time zone. The string you happen to be parsing doesn't contain a time zone yo…

Post by kazinator » Wed, Jan 22, 2025, 5:23 AM UTC

paxcoder wrote: post #22

johnisgood wrote: post #16

Earlier quoted context omitted.

It is not. int main(void) { struct tm tm = {0}; const char *time_str = "Mon, 20 Jan 2025 06:07:07 GMT"; const char *fmt = "%a, %d %b %Y %H:%M:%S GMT"; // Parse the time string if (strptime(time_str, fmt, &tm) == NULL) { fprintf(stderr, "Error parsing time\n"); return 1; } // Convert to Unix timestamp (UTC) time_t timestamp = timegm(&tm); if (timestamp == -1) { fprintf(stderr, "Error converting to timestamp\n"); retur…

I can't find `timegm` neither in the C99 standard draft nor in POSIX.1-2024. The first sentence of your link reads: >The C/Unix time- and date-handling API is a confusing jungle full of the corpses of failed experiments and various other traps for the unwary, many of them resulting from design decisions that may have been defensible when the originals were written but appear at best puzzling today.

Post by 1vuio0pswjnm7 » Fri, Jan 24, 2025, 5:02 AM UTC

1vuio0pswjnm7 wrote: post #47

johnisgood wrote: post #16

Earlier quoted context omitted.

It is not. int main(void) { struct tm tm = {0}; const char *time_str = "Mon, 20 Jan 2025 06:07:07 GMT"; const char *fmt = "%a, %d %b %Y %H:%M:%S GMT"; // Parse the time string if (strptime(time_str, fmt, &tm) == NULL) { fprintf(stderr, "Error parsing time\n"); return 1; } // Convert to Unix timestamp (UTC) time_t timestamp = timegm(&tm); if (timestamp == -1) { fprintf(stderr, "Error converting to timestamp\n"); retur…

"Mon, 20 Jan 2025 06:07:07 GMT" I thought the default output of date(1), with TZ unset, is something like Mon Jan 20 06:07:07 UTC 2025 That's the busybox default anyway

    int fileno (FILE *);
    FILE *f;
    int printf(const char *__restrict, ...);
    #include 
    char *strptime(const char *s, const char *f, struct tm *tm);
    struct tm t;
   a (Mon|Tue|Wed|Thu|Fri|Sat|Sun)
   b (Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec)
   d [0-2][0-9]|3[01]
   H [0-2][0-9]
   M [0-5][0-9]
   S [0-5][0-9]
   Y [1-9][0-9][0-9][0-9] 
   %option nounput noinput noyywrap
   %%
   {a}[ ]{b}[ ]{d}[ ]{H}:{M}:{S}[ ]UTC[ ]{Y} {
    strptime(yytext,"%a %b %d %H:%M:%S UTC %Y",&t);
    printf("%ld\n",mktime(&t));
    }
   .|\n
   %%
    int main(){yylex();exit(0);}
   
    flex -8Cem date.l
    cc -O3 -std=c89 -W -Wall -pipe lex.yy.c -static -s -o yydate

    date|yydate

     echo "Thu Jun 31 01:59:26 UTC 2024"|yydate

Post by kazinator » Thu, Jan 30, 2025, 1:21 AM UTC

johnisgood wrote: post #99

kazinator wrote: post #96

Earlier quoted context omitted.

Here is some of my code that works around not having timegm. It is detected in a configure script, so there's a #define symbol indicating whether it's available. https://www.kylheku.com/cgit/txr/tree/time.c

Regarding "detected in a configure script", one could use m4conf[1] which is lightweight and does the job without messy configure scripts. [1] https://zolk3ri.name/cgit/m4conf/about/ Nice job though.