asctime_r.c

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#define _GNU_SOURCE
#include <assert.h>
#include <langinfo.h>
#include <stdio.h>
#include <time.h>

char* asctime_r(const struct tm* restrict tm, char* restrict buf)
{
    if(snprintf(buf, 26, "%.3s %.3s%3d %.2d:%.2d:%.2d %d\n", nl_langinfo(ABDAY_1 + tm->tm_wday),
                nl_langinfo(ABMON_1 + tm->tm_mon), tm->tm_mday, tm->tm_hour, tm->tm_min, tm->tm_sec,
                1900 + tm->tm_year) >= 26)
    {
        /* ISO C requires us to use the above format string,
         * even if it will not fit in the buffer. Thus asctime_r
         * is _supposed_ to crash if the fields in tm are too large.
         * We follow this behavior and crash "gracefully" to warn
         * application developers that they may not be so lucky
         * on other implementations (e.g. stack smashing..).
         */
        assert(0 && "tm fields are too large for buffer");
    }
    return buf;
}