Parse and create HTTP format timestamps in C

This is an example C program demonstrating two routines, one for parsing and one for producing HTTP format dates as described by RFC 1123 and RFC 2616. These routines deliberately do not use strptime and strftime so that the user's locale will not affect the input and output.

#include <stdio.h>
#include <string.h>
#include <time.h>
#include <stdlib.h>

static const char * wdays[7] = {
    "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat",
};

static const char * months[12] = {
    "Jan", "Feb", "Mar", "Apr", "May", "Jun",
    "Jul", "Aug", "Sep", "Oct", "Nov", "Dec",
};

static void
make_http_time (time_t t, char * buf, int buf_len)
{
    int bytes;
    struct tm g = {0};
    gmtime_r (& t, & g);
    if (g.tm_wday >= 7 ||
	g.tm_mon >= 12) {
	fprintf (stderr, "g.tm_wday = %d, g.tm_mon = %d.\n",
		 g.tm_wday, g.tm_mon);
	exit (EXIT_FAILURE);
    }
    bytes = snprintf (buf, buf_len,
		      "%.3s, %02d %.3s %4d %02d:%02d:%02d GMT",
		      wdays[g.tm_wday], g.tm_mday, months[g.tm_mon],
		      g.tm_year + 1900, g.tm_hour, g.tm_min, g.tm_sec);
    if (bytes >= buf_len) {
	fprintf (stderr, "bytes = %d, buf_len = %d.\n",
		 bytes, buf_len);
	exit (EXIT_FAILURE);
    }
}


static void
parse_http_time (const char * buf, time_t * t_ptr)
{
    struct tm g = {0};
    char M[4];
    time_t t;
    int i;

    sscanf(buf, "%*[a-zA-Z,] %d %3s %d %d:%d:%d",
	   & g.tm_mday, M, & g.tm_year,
	   & g.tm_hour, & g.tm_min, & g.tm_sec);
    for (i = 0; i < 12; i++) {
	if (strncmp (M, months[i], 3) == 0) {
	    g.tm_mon = i;
	    break;
	}
    }
    g.tm_year -= 1900;
    t = timegm (& g);
    * t_ptr = t;
}

const char * p = "Thu, 03 Feb 1994 00:20:00 GMT";

int main ()
{
    int buf_len = 100;
    char buf[buf_len];
    time_t t;
    time_t r;
    t = 300000;
    make_http_time (t, buf, buf_len);
    printf ("%s\n", buf);
    parse_http_time (buf, & r);
    printf ("%d\n", r);
    parse_http_time (p, & r);
    printf ("%d\n", r);
    make_http_time (r, buf, buf_len);
    printf ("%s\n", buf);
    return 0;
}

(download)

The output of the example looks like this:

Sun, 04 Jan 1970 11:20:00 GMT
300000
760234800
Thu, 03 Feb 1994 00:20:00 GMT

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Copyright © Ben Bullock 2009-2018. All rights reserved. For comments, questions, and corrections, please email Ben Bullock (benkasminbullock@gmail.com) or use the discussion group at Google Groups. / Privacy / Disclaimer