weekday.c

/**
 * @example weekday.c
 * This example demonstrates how the library makes it easy to concisely express
 * repetitive errors.
 */
 
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
 
#include "../err.h"
 
void
usage(void)
{
        fputs("usage: weekday YYYY-MM-DD\n", stderr);
}
 
int
main(int argc, char *argv[])
{
        struct tm tm = {0};
        char *temp;
        int year, month, day;
        char linebuf[64];
 
        program_invocation_name = argv[0];
 
        if (argc != 2) {
                usage();
                return EXIT_FAILURE;
        }
 
        /* Note that when errno == 0, strerror(errno) is not included in
         * the output of err(). */
 
        if ((temp = strtok(argv[1], "-")) == NULL)
                err("bad date: no year");
        else if ((year = atoi(temp)) == 0)
                err("bad date: invalid year");
 
        if ((temp = strtok(NULL, "-")) == NULL)
                err("bad date: no month");
        else if ((month = atoi(temp)) == 0)
                err("bad date: invalid month");
 
        if ((temp = strtok(NULL, "-")) == NULL)
                err("bad date: no day");
        else if ((day = atoi(temp)) == 0)
                err("bad date: invalid day");
 
        /* See ctime(3) */
        tm.tm_year = year - 1900;
        tm.tm_mon  = month - 1;
        tm.tm_mday = day;
 
        if (mktime(&tm) == (time_t)-1)
                err("mktime failed");
 
        strftime(linebuf, sizeof(linebuf), "%Y-%m-%d: %A", &tm);
        puts(linebuf);
 
        return EXIT_SUCCESS;
}